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- Aberration
- Chromatic:
An optical image distortion conditional on the varying refraction of light rays of different
wavelengths on a lens. Thus light rays of shorter wavelengths have longer focal distances than light rays of longer wavelengths.
- Spherical:
An optical image distortion conditional on the varying distance of paraxial light rays of the same
wavelength from the optic axis. Light rays that travel through outer lens zones have shorter focal distances than rays that travel through the lens center (optic axis). The corrector lens on the front of a
Schmidt-Cassegrain, for example, is an aspheric lens which corrects the aberration from the spherical primary mirror.
- ABG - Anti-Blooming Gate
- An electronic drain structure on a CCD chip which assures that electrons/voltage exceeding the full-well capacity of a pixel do not spill over to adjacent pixels.
- Absolute Zero
- The lowest possible temperature, attained when a system is at its minimum possible energy. The
Kelvin temperature scale sets its zero point at absolute zero (-273.15 on the Celsius scale, and -459.67 on the Fahrenheit scale).
The idea of a true minimum temperature has been confirmed by many experiments. Given the concept of temperature as molecular energy it follows that there must be a point at which no further energy can be extracted from a system.
Although it is possible to approach ever closer to absolute zero, the "third law" of thermodynamics holds that it is impossible to attain absolute zero in a system.
The present temperature of the cosmic background radiation is about 2.7 degrees above absolute zero. If the universe expands forever, this temperature will asymptotically approach absolute zero. - AC - Alternating Current
- Electrical current that reverses (or alternates) at regular intervals.
- Achromat (achromatic objective)
- Describes a correction class for objectives. The chromatic aberration for two
wavelengths is corrected for objectives of this type. Usually an objective of this type is corrected to a wavelength below 500nm and above 600nm. Furthermore, the sine condition for one wavelength is met. The image curvature aberration is not corrected.
- Accretion Disk
- A disk of gas which accumulates around a center of gravitational attraction, such as a white dwarf, neutron star, or
black hole. As the gas spirals in, it becomes hot and emits light or even X-radiation.
- Age of the Universe, The
- An expanding universe must have been smaller in the past, and in fact the distance between any two points approaches zero roughly 13 billion years ago. This moment of ultra-high density is called the
Big Bang, and marks the birth of the universe (at least to all intents and purposes).
The age of the universe is therefore about 13 billion years. For about half a million years after the Big Bang, the universe was opaque to electromagnetic radiation. This sets a maximum distance that we can see: radiation emitted just as the universe became transparent (the cosmic microwave background), that is reaching us now, has
traveled 13 billion light years.
- AGN - Active Galactic Nucleus [Plural: Active Galactic Nuclei, (also
AGN)].
- To try to summarise what we know about AGN is to step into a minefield. There is a "standard model" which everyone agrees is at least partly wrong, but every expert has his or her own proposal for either fixing it up, or replacing it with something completely different. To avoid getting too bogged down in these controversies, the following description is deliberately vague in many places.
AGN are exclusively found in the centres of large galaxies; the galaxy containing an AGN is said to be its host. The central light-year or so of an AGN contains an enormous mass, equivalent to at least a million suns, and sometimes ranging up to a few billion suns. This region also contains something that shines brightly in the part of the electromagnetic spectrum from the ultraviolet through to X-rays. There is pretty good (but disputed) evidence that all this is actually concentrated on a much smaller scale, sometimes less than a few light-days across. Certainly the X-rays originate in an astronomically tiny volume, smaller than our own solar system. To put this into perspective, within a light-year of us there is just one sun (ours), and the host galaxies are typically 100 thousand light-years across or more.
The thing (if it is one thing) at the heart of an AGN is often called the "monster". If general relativity is correct it seems almost inevitable that nearly all the mass of the monster is contained in a spinning black hole. Black holes don't radiate, by definition, so the radiation from an AGN is believed to come from gas clouds falling into the hole. In a way, the monster works as a sort of engine, fuelled by matter falling in (accreting, in the jargon). To be more precise, the gravitational potential energy of the accreted stuff is ultimately converted to radiation, and to kinetic energy in the form of jets. We don't know just how this happens, although there are dozens of competing theories.
At larger distances from the center swirl clouds of gas and dust which are lit up and heated by the central heart, producing the characteristic emission lines and infra-red radiation. The outermost regions can be imaged from Earth in some of the nearest AGN, but mostly AGN appear just as points of light.
- ADC - Analog-Digital-Converter
- An electronic device, often an integrated circuit, that converts an analog
voltage to a digital value. All digital instruments use an A/D converter to
convert the input signal into digital information. The output signal does
only change at special times and can only take special values - quantization.
Compare with DAC
- ADU - Analog-to-digital Unit
- ADUs are employed as a measurement of pixel value or brightness. Pixel voltages (numbers of electrons) stored during
CCD integrations are converted to ADU integers representing the measured voltage compared to maximum (full pixel) voltages in terms of the full Base 2 dynamic range of the CCD system (12 bit = 2^12th, 16 bit = 2^16th, etc.).
- Afocal
- A photographic method that uses an eyepiece in the telescope focused normally (you look through it) and a camera with its lens focused at infinity. You then just point the camera into the eyepiece. The camera can be on a separate tripod or attached to the telescope with a bracket or attached with a threaded adapter or even hand held. It is a great method to use for photographing the moon and planets.
- Ah
- Abbreviation for "ampere-hour". Designates an amount of electric
charge. Used for accumulators to denominate their capacity. Because the
voltage of an accumulator is nearly constant, you can calculate the stored
energy from the given Ah-rating, e.g. 12Vx100Ah=1.2kWh.
- Airy Disc
- The Airy disc refers to the inner, light circle (surrounded by alternating dark and light diffraction rings) of the
diffraction pattern of a point light source. The diffraction discs of two adjacent object points overlap some or completely, thus limiting the spatial resolution capacity.
- Algorithm
- In CCD imaging, an algorithm usually refers to a software procedure, often for image processing. An algorithm is the mathematical function which tells the computer what to do with an image.
- Aliasing
- An image distortion caused by a sampling frequency that is too low in relation to the
signal frequency.
- Where the sampling rate is less than twice the input signal's highest
frequency content.
- Alt-Azimuth (also called Alt-Az)
- Short for Altitude-Azimuth . Telescopes which are mounted so that they move up-down and left-right (as opposed to equatorially) are called
alt-az. This is a convenient mounting configuration for visual observing as the eyepiece is always in a convenient position. However, an equatorial
wedge must be used for photography or CCD imaging.
- Amplifier
- A device that uses an active component to increase the voltage or power of
a signal without distorting its waveshape.
- analog
- A continuous, non-digital representation of phenomena. An analog voltage,
for example, may take any value. Opposite to "digital".
- AND Gate
- A gate whose output is ON only if all input signals are ON.
- Angle of Incidence
- Angle between the incident ray of light and a normal drawn to the point
of reflection. I.e. The angle between the optical axis of the light incident on the surface of a filter and the axis normal to this surface.
- Angle of Reflection
- Angle between the ray of light and the normal drawn to the point of refraction.
- Angle of Refraction
- Angle between the refracted ray of light and the normal drawn to the
point of reflection.
- Angle of view
- The amount of a scene that can be recorded by a particular lens;
determined by the focal length of the lens; also field of view FOV.
- Angular deviation
- A shift in the direction of light beam from the true optical axis of the system, measured in units of angle such as
arcminutes (1/60 of a degree) or arcseconds (1/60 of an
arcminute).
- Angstrom (Å)
- A unit of length. 1/10,000 of a micrometer (10-4µm).
- Anti-Blooming
- This feature is added to a CCD chip to prevent pixel blooming. This feature generally reduces sensitivity,
well-depth, and linear response. For these reasons, non-anti-blooming chips are popular, and there is even software available to remove blooming streaks from CCD images
- Antimatter
- Particles with certain properties opposite to those of matter. Each matter particle has a corresponding antiparticle. The antiparticle has exactly the same mass and the opposite electric charge as its partner. An example is the electron (negative charge) and its antimatter version the positron (positive charge).
When a particle and its antiparticle collide, both are annihilated and converted into photons. Similarly two photons with sufficient energy can combine to form a particle-antiparticle pair.
The universe is made almost entirely out of matter. This means that in the big bang there was an excess of matter over antimatter so that when matter and antimatter combined and annihilated, some matter was left over.
- Apparent Field of View
- A characteristic of eyepieces. The apparent field of view is the angle through which your eyeball rotates when you look through an eyepiece and transfer your gaze from one edge of the field to the other.
- Aperture
- The lens opening formed by the iris diaphragm inside the lens. The size of
the hole can be made larger or smaller by the auto focus system or a manual
control. The size is indicated as a 'f-number' or 'f-stop' i.e. f/4, f/5.6,
or f/8.
- Aperture diaphragm
- An adjustable diaphragm located in the illumination optics, which controls the
numerical aperture of the illuminating beam and affects the brightness of the beam.
- Aperture, maximum
- The largest size of the hole though which light enters the camera.
- Aperture, numerical
- The aperture is the sine of the angle under which light enters into the front lens of a microscope objective; its symbol is NA. The aperture influences both the light gathering capacity and the resolution capacity of an objective. Since various media can be present between specimen and objective lens (such as the embedding medium for the specimen), the numerical aperture (NA = n * sin a) is usually applied as the unit of measurement for the light gathering capacity and the resolution capacity.
- Aperture Synthesis
- The technique of combining the signals from a collection of individual antennas or telescopes to provide an image with a resolution equivalent to a single telescope with a size roughly equal to the maximum distance between the individual antennas. This may be quite large, e.g. 217 km for MERLIN, and up to the size of the Earth for VLBI.
- Apochromat (apochromatic objective)
- Describes a correction class for objectives. The chromatic aberration for three
wavelengths is corrected for objectives of this type (usually 450nm, 550nm and 650nm) and the sine condition for at least two colors is met. The image curvature aberration is not corrected.
- Artifacts
- Erroneous pixels created during the capture phase of imaging, caused by electrical interference or physical barriers such as dust.
- Aspheric Surface
- A lens or mirror surface that is altered slightly from spherical to reduce
aberrations.
- Aspect ratio
- The relationship of the X and Y scales of a 2-dimensional grid. Non-square
CCD pixels are represented as square by video monitors and other output devices, yielding an aspect ratio not in accord with true sky coordinates unless the images are
resampled to an aspect ratio that, in effect, squares the pixels.
- I.e. The ratio between the width and height of an image or image
sensor.
- Astronomy
- The science that studies the natural world beyond the earth.
- Asynchronous
- In hardware, it is an event that occurs independent of other events; it is
not synchronized with a clock signal.
In software, it refers to a function that begins an operation and returns to
the calling program prior to the completion or termination of the operation.
- Atom
- The smallest component of matter which retains its chemical properties. An atom consists of a nucleus composed of at least one
proton, some number of neutrons, and at least one
electron.
The atomic number of an atom corresponds to the number of protons present in the nucleus of an atom. This determines its elemental identity. The number of neutrons determines the
isotope of the atom.
- Attenuation Level (also Blocking level)
- A measure of the out-of-band attenuation of an optical filter, over an extended range of the spectrum. The attenuation level is often defined in units of optical density
- Aurora
- Spectacular array of light in the night sky, caused by charged particles from the
Sun hitting the Earth's upper atmosphere. The aurora borealis is seen in the north of the Northern hemisphere; the aurora australis in the south of the Southern.
- Autoguiding / Guiding
- Telescope tracking controlled by feedback from real-time sensing of star movements within the field of view (FOV). Movement may be sensed by an electro-optical device, such as a
CCD chip, or by the human eye comparing star movement to a eyepiece reticle intersection or a
reticle grid. Autoguiding refers to automatic feedback to telescope drives provided by electronic devices, while manual guiding is accomplished by human feedback intervention using slow-motion controls on telescope drives.
- Automatic exposure
- A mode of camera operation in which the camera automatically adjusts the aperture,
shutter speed, or both for proper exposure.
- AutoStar
- A brand name for Meade's hand-held computerized controller.
- Average transmission
- The average calculated over the useful transmission region of a filter, rather than over the entire spectrum. For a
bandpass filter, this region spans the FVMM of the transmission band.
- Averted Vision
- When you look squarely at something, you are using a part of the retina of your eye that is not as sensitive to low light levels as the parts that are off to the side. Thus to see faint objects, don't look straight at them. Center them in the field of view of your telescope, but fix your stare part way out to the edge of the field.
- Azimuth
- a) Directional bearing around the horizon, measured in degrees from north (0°).
- b) Angular distance from the north point eastward to the intersection of the celestial horizon with the vertical circle passing through the object and the zenith.
-
- Bandpass
- An optical filter that has a well-defined short wavelength
cut-on and long wavelength cut-off. Bandpass filters are denoted by their center
wavelength and bandwidth.
- Bandwidth
- Also FWHM. For optical bandpass filters, typically the separation between the cut-on and cut-off
wavelengths at 50% of peak transmission. Sometimes a bandwidth at, for example, 10% of peak transmission is specified.
I.e. The highest frequency signal component that can pass through input
amplifiers and/or filters without being attenuated.
- Barlow
- An extra lens you can add to an eyepiece to amplify the magnification. Usually a 2 times multiplier.
- Base
- The control portion of a bipolar transistor. In an NPN transistor, the
P-type material forms the base.
- Bayer pattern
- A pattern of red, green, and blue filters on the image
sensor's photosites.
There are twice as many green filters as the other colors because the human
eye is more sensitive to green and therefore green color accuracy is more
important.
- Bias Signal
- The electrons and subsequent ADU generated by the voltage maintained over the
CCD array during integration.
- Big Bang
- The state of extremely high (classically, infinite) density and temperature from which the universe began expanding. The beginning point of time and space for the universe.
- Big Crunch
- One hypothesized future for the universe in which the current expansion stops, reverses, and results in all space and all matter collapsing together; a reversal of the
big bang.
- Binary
- A system of numbers using 2 as a base, in contrast to the decimal system
which uses 10 as a base. The binary system requires only two symbols: 0 and
1.
- Binary Star
- A system of two stars orbiting around a common center of gravity. Visual binaries are those whose components can be resolved telescopically (i.e., angular separation > 0'.5) and which have detectable orbital motion. Astrometric binaries are those whose dual nature can be deduced from their variable proper motion; spectroscopic binaries, those whose dual nature can be deduced from their variable radial velocity. At least half of the stars in the solar neighborhood are members of binary (or multiple)
systems.
- Binning
- Binning involves combining pixels on a CCD chip to create larger pixels. For example, taking a 2x2 square of pixels and creating one pixel that is twice the width and four times the area of the original pixel. This is done to increase sensitivity or to match a long
focal length telescope to a CCD camera with small pixels.
- Binocular Viewer
- A set of prisms that allows you to use two eyepieces on a telescope. Using both eyes is particularly good for the moon and planets.
- Bipolar
- 1. An analog signal range that includes both positive and negative values.
2. An electronic device whose operation depends on the transport of both
holes and electrons.
- Bipolar Transistor
- BJT (bipolar junction transistor). Most important
semiconductor device,
foundation for integrated circuits. You may think of it being an
electrically controlled valve or current amplifier. Modern bipolar
transistors work up to the three-digit GHz-range.
- Bipolar Switch
- An electronic switch which is able to control bipolar signals. The switch
consist of two identical in/outputs and a control line which does open and
close the switch.
There are usually in 2 or 4 switches packed in one IC e.g. the CMOS IC
4016 contains four.
- Bit
- A binary digit. A bit is the smallest unit of storage in a digital
computer, and is used to represent one of the two states in the binary
number system.
- Bit Depth
- A measurement of the number of bits used to create a single pixel in a digital image. A 24-bit RG8 image is created from a palette of
16.7 million colors.
- Bit-mapped
- Images formed from pixels with each pixel a shade of gray or color. Using
24-bit color, each pixel can be set to any one of 16 million colors.
- Black Hole
- An object so dense that its escape velocity exceeds the speed of light. According to general relativity, such an object must collapse to an infinitely dense point, a singularity. The singularity is surrounded by a surface called the event horizon, within which objects and information can only move inwards, quickly reaching the singularity (and being crushed to a point in the process, of course). Therefore nothing can escape from a black hole. A technical exception is Hawking radiation, a quantum mechanical process first described by Steven Hawking, but this is unimaginably weak for the massive black holes of interest to astronomers.
- Blocking range
- The range of wavelengths over which an optical filter maintains a specified attenuation level.
- Blooming
- Each photosite of a CCD chip can contain a certain amount of electric charge. This amount is determined by the well depth of the
CCD. When the well-depth is exceeded, electric charge "bleeds" out of the photosite appearing in an image as a bright streak extending vertically from a bright source in the image (usually a star). This effect can be minimized or eliminated by using a CCD with an
anti-blooming gate.
- Blueshift
- A shift in the frequency of a photon toward higher energy and shorter
wavelength. Blueshifts can be produced by relative motion of the emitter toward the observer
(doppler blueshift), light falling in a gravitational field from the emitter to the observer (gravitational blueshift), or in a contracting universe
(cosmological blueshift). For further details, see Redshift.
- Boolean Algebra
- A logical calculus named for mathematician George Boole, using alphabetic
symbols to stand for logical variables, and 0 and 1 to represent states. AND,
OR, and NOT are the three basic logic operations in this algebra.
NAND
and NOR are each combinations of two of the three operations.
- Brightness
- The business of observational astronomy boils down to measuring the brightness of celestial objects. Unfortunately, the English word "brightness" covers three quite different concepts, each of which covers several subtle variations.
that is: Luminosity, Flux Density
and Intensity (or Surface Brightness)
- Brown Dwarf
- A low-mass substellar object that is near the minimum mass for nuclear fusion reactions to occur in its core. Brown dwarf objects are a possible source of baryonic
dark matter. Brown dwarfs are possible dark matter halo objects.
- Byte
- A group of eight bits (in the telecommunication field also octet).
Multipliers are Kilobyte (1024 bytes), megabyte (1024x1024 bytes) etc.
- Calibration
- 1. Generally, normalizing a system to a set of standards or constants.
- 2. Specific to CCD imaging, to eliminate unwanted signal and reduce noise components by subtracting a
dark frame and dividing by a flat-field frame.
- Capacitance
- 1. The capability of storing electrical charge. Unit of measure is the
Farad (F).
2. In a capacitor or system of conductors and dielectrics, the property that
permits the storage of electrically separated charges when potential
differences exist between the conductors. Capacitance is related to charge
and voltage as follows: C = Q/V, where C is the capacitance in farads, Q is
the charge in coulombs, and V is the voltage in volts.
- Cassegrain
- Telescope devised by Cassegrain in which an auxiliary convex mirror reflects the magnified image, upside down, through a hole in the center of the main objective mirror - i.e., through the end of the telescope itself.
- Catadioptric
- Any of a number of compromise telescope designs, using both a lens and mirrors. Examples are the
Schmidt-Cassegrain and Maksutov-Cassegrain. Because the light path is folded twice, the telescope is very compact.
- CCD - Charge Coupled Device
- A semiconductor device used for signal filters or as sensor elements.
Electronic cameras get their picture from a CCD-sensor. The inner consists
of a matrix of light sensitive elements, converting light into current.
Because of the huge amount of elements, a direct wiring is not suitable.
Therefore the charge packets generated during light exposure are passed by
electric fields until they reach a connection point. I.e. "charge
coupled". It operates by storing charge on capacitors and selectively
moving that charge through the device by manipulating voltages on its
electrodes.
- Usually, CCDs are micromanufactured into two-dimensional grids of rows and columns, each intersection comprising a pixel several microns in both
dimensions.
- CCD raw format
- The uninterpolated data collected directly from the image sensor before
processing.
- Cepheid Variable
- A type of luminous giant star whose luminosity varies in a periodic fashion. Cepheids are characterized by a rapid rise in
luminosity followed by a slow decline. The period of the cycle is related to the luminosity of the Cepheid by the Period-Luminosity relationship. The more luminous the Cepheid, the longer the period. This property makes Cepheids useful for obtaining distances. One determines the pulsation period and uses the relationship to get the luminosity. The apparent
brightness of the star then gives you the distance. Cepheids come in two types, Type I which are metal rich and Type II which are metal poor. Type I Cepheids are more luminous than Type II.
- Circuit Layout
- The physical arrangement of all the circuit elements on the surface of the
device.
- Chrominance
- The color parameters of an image. Usually represented by hue and saturation.
- Clear Aperture
- The surface area of an optical filter which is free of any defects or obstructions. On interference filters the clear aperture is often delimited by an annulus of metal or opaque material.
- CG-5
- A style of GEM made by Celestron.
- CMM - Color Management Modules
- This is software, such as Apple's Color Sync and Kodak's Color Management System, that attempts to regulate the display of color.
- CMOS - Complementary Metallic Oxide Semiconductor
- Circuit technique initiating the high integration level of today's
integrated circuits. Nearly all modern microprocessors are produced in CMOS-technology.
Conceptual not the fastest design technique, but allowing for complex
circuits and automatic layout.
- CMOS image sensor
- An image sensor created using CMOS technology. It requires more light but
it is cheaper than an CCD sensor.
- CMY - Cyan, Magenta, Yellow.
- Named the subtractive colors of the human visual spectrum, since cyan = white - red, magenta = white - green, and yellow = white - blue.
- CMYK- Cyan, Magenta, Yellow, Key
- Cyan, Magenta, Yellow, and Black (historically called the key color, hence K) are the standard inks used in the lithographic printing industry to reproduce color images.
- Collector
- The region of a bipolar transistor that "collects" the emitted
electrons and then passes them on through a conductor, completing the
electrical circuit.
- Collimated Light
- Light in which the rays are parallel.
- Collimation
- This refers to how correctly the optics are pointing towards each other. If a telescope is out of collimation, you will not get as clear an image as you should.
Refractors generally have fixed optics, so you don't have to collimate them.
Reflectors and catadioptrics usually have screws that you turn to collimate.
- Color balance
- The overall accuracy with which the colors in a photograph match or are
capable of matching those in the original scene.
- Color depth
- The number of bits assigned to each pixel in the image and the number of
colors that can be created from those bits. True Color uses 24 bits per
pixel to render 16 million colors.
- Color Space
- Describes the mode used to represent color such as RGB,
CMYK, or Lab. Each space has its own unique limitations.
- Color Temperature
- A stellar temperature determined by comparison of the spectral distribution of the
star's radiation with that of a blackbody.
- Coma
- This refers to the blurring of objects at the edge of the field of view, most common in short
focal ratio Newtonian telescopes (at f/10 and longer, Newtonians are very well corrected for coma).
- CompactFlash
- One of three competing memory card formats used in digital cameras. Holds
between 2Mb and 128Mb of picture data.
- Compression
- The process of reducing the size of a file. Compression techniques are
distinguished by whether they remove detail and color from the image. 'Lossless'
techniques compress image data without removing detail; 'lossy' techniques
compress images by removing detail.
- Compression, lossless
- A file compression scheme that makes a file smaller without degrading the
image. This method is generally less effective than lossy methods in terms
of resulting file size, but retains the entire original image.
- Compression, lossy
- A file compression scheme that reduces the size of a file but degrades it
in the process so it can't be restored to its original quality. Once deleted
the data cannot be recovered. The higher the compression the more noticeable
the artifacts of the compression.
- Conductor, Electrical
- A material capable of carrying (conducting) electricity. Silver is the
best electrical conductor. Copper, gold, and aluminum are also popular
conductors. Aluminum is the conductor most commonly used in IC fabrication.
- Confocal (Confocality)
- While the optical design of conventional microscopes results in the detection of both
focused and unfocussed image components, the confocal principle suppresses the structures outside of the focal plane of the microscope objective. To achieve this pinholes are implemented in optically conjugated locations in the optical path. They function as point light source (excitation pinhole) and point detector (detection pinhole). The diameter of the detection pinhole, along with the
wavelength and numerical aperture of the objective being used, determines the axial extension of an optical section.
- Constellation
- Precisely defined area of the celestial sphere, associated with a grouping of stars, that the International Astronomical Union has designated as a constellation.
- Continuum Emission
- Any type of electromagnetic emission which produces radiation over a relatively wide range of frequencies. c.f.
line emission.
- Coordinates
- Quantities which provide references for locations in space and time. A typical coordinate system consists of a point of reference (the origin), a set of directions (axes) that span space, and a set of labels that indicate how points are related to the origin. Coordinates in and of themselves are user defined and arbitrary, although certain simple, regular coordinate systems (e.g. Cartesian coordinates) are widely used.
A Coordinate Singularity is a location at which a particular coordinate system fails, such as the Schwarzschild metric coordinates at the Schwarzschild radius of a
black hole, or lines of longitude at the North pole. This failure doesn't indicate a breakdown in the underlying geometry. It is merely a failure of the coordinate system to give a unique well-defined label to a point in that geometry.
- Corrector Plate
- Thin lens-like optical piece which removes certain optical aberrations.
- Corrosion
- An unwanted electrochemical process that affects device (e.g., semiconductor, telescope-mounts etc.)
reliability.
- Cosmic Background Radiation (CBR)
- The Cosmic Background Radiation (CBR) consists of relic photons left over from the very hot, early phase of the
Big Bang. It now peaks in the microwave band, corresponding to blackbody radiation with a temperature of about 2.7 degrees
Kelvin. The CBR is also sometimes called the Microwave Background, or the Cosmic Microwave Background (CMB).
- Cosmic Rays
- Cosmic rays are really charged particles such as protons, alpha-particles (i.e. helium nuclei) and electrons,
traveling at almost the speed of light, c. From the theory of special
relativity, cosmic rays carry a very high energy (tending to infinity as the speed tends towards c). This energy is much larger than their rest-mass energy (mc ²), and so they are also known as high-energy or relativistic particles.
- Cosmological Constant, the
- A physical constant that appears in the theory of general relativity. It corresponds to a force between particles which increases with separation, and only has an effect over cosmological distances, hence the name. Although the constant fits well into the equations, they are more elegant when the constant is dropped (set to zero). Einstein originally put the constant in because without it he found that general relativity predicted an expanding or contracting universe, and he assumed that the universe must be static. By doing this he missed the chance to claim the expansion of the universe as a prediction of GR. To rub salt into this wound, it later turned out that Einstein's model of a static universe held balanced by the cosmological constant could never have worked: the balance is precarious and the slightest disturbance would send the universe into accelerating contraction or expansion.
Not surprisingly, Einstein called the cosmological constant `my greatest blunder'.
- Cosmological Distance Ladder, The
- Distances to galaxies are found by a long chain of arguments called the cosmological distance ladder.
1. A scale model of the solar system can be constructed from observations of the motions of the planets in the sky. All distances are known in terms of the radius of the Earth's orbit, the Astronomical Unit, (AU). Copernicus made the first roughly accurate solar system model, using data taken in ancient times, in his famous De Revolutionibus (1543). Modern models are exquisitely accurate.
2. The actual distances to most of the planets can be measured by radar, and since we also know the distance to them in AU, the length of the AU can be found (to nine significant figures!).
3. Distances to nearby stars can be found by various geometrical methods. The simples is via their annual parallax, i.e. their apparent change of position in the sky caused by the motion of the observer on Earth around the sun. The best precision is now about three significant figures in a handful of cases. More usefully, the final results of ESA's Hipparcos satellite (released in April 1997) give the parallaxes of around 10,000 stars to within few percent. Before
Hipparcos, annual parallaxes were not as important as some more subtle geometric methods, but the new data will change the situation completely.
4. Stars of similar type have similar luminosities. Thus if we know a star's type (from its colour and/or spectrum) we can find its distance by comparing its apparent with its absolute magnitude; the latter derived from geometric parallaxes to nearby stars. Unfortunately nearby stars are not very bright in absolute terms, so we cannot see distant versions very far away (certainly not in other galaxies).
5. Distances to the super-bright stars that can be seen in other galaxies (especially Cepheid variables and RR Lyrae stars) are found by searching for distant star clusters in our Galaxy that contain both a Cepheid (say) and some fainter stars whose absolute magnitudes are known directly.
6. Distances to the nearest galaxies are found using Cepheids, RR Lyraes, etc. The Hubble Space Telescope is now finding Cepheids in galaxies about ten times more distant than was possible from the ground.
7. For more distant galaxies, we need objects even brighter than Cepheids. Examples are supernova explosions, planetary nebulae, and globular star clusters. The absolute magnitudes for such things can't be easily found in our own Galaxy, so they are measured in nearby galaxies (or clusters of galaxies) with Cepheid or similar distances.
8. At the furthest limits, only whole galaxies are detectable. Galaxies come in a very wide range of luminosities, so we need a way to find their luminosity before we can get their distance. Various methods exist. For instance galaxy luminosity is related to the speed of internal motions; most radio galaxies seem to have similar luminosities; the range of brightnesses in clusters of galaxies do not vary much from one cluster to another.
At every step of the distance ladder, errors and uncertainties creep in. Each step inherits all the problems of the ones below, and also the errors intrinsic to each step tend to get larger for the more distant objects; thus the spectacular precision at the base of the ladder degenerates into an uncertainty of a factor of several at the very top.
To find Hubble's Constant, the ratio of the cosmological recession speed to the distance, we need to go up to Step 7 of the ladder. This is because we can only measure the sum of the recession speed and the random motion of a galaxy, and so we need to go far enough away that the the random motions are small compared to the recession speed.
More details of individual methods are given in Ned Wright's The ABC's of Distances. A good book describing the Distance Ladder in detail is Rowan-Robinson (1985) (although a lot has happened since it was written).
| Object |
Distance (pc) |
Method |
| Sun, Solar System |
10-6 |
Radar, Orbits |
| Alpha Centari |
1 |
parallax |
| Hyades Cluster |
40 |
Hipparcos parallax |
| Galaxy |
104 |
Cepheids, Main Sequence Fitting |
| Andromeda |
105 |
Cepheids, Supernovae, OB star |
| Virgo Cluster |
107 |
HST Cepheids, OB stars, SN |
| Beyond |
108 and up |
Brightest Galaxies, Tully-Fisher |
- Counter Weights
- Many telescope motors are not very powerful, if you don't balance the system (telescope & camera) the motors cannot drive accurately.
- CPU - Central Processing Unit
- see Processor
- Crayford
- A style of eyepiece focuser that does not use gears; very smooth motion.
- Critical Angle
- Angle of incidence for which the angle of refraction is 90° when light
goes from one medium of high index of refraction into one of lower index.
- CRT - Cathode Ray Tube
- The CRT is a vacuum tube used as a display screen in a monitor or television set. The inner surface of the CRT is coated with phosphors, which glow and produce light when hit by an electron beam.
- Curvature Constant (k)
- A constant (k) appearing in the Robertson-Walker metric which determines the curvature of the spatial geometry of the universe. The three standard Friedmann models have
k > 1 for positive curvature (spherical geometry)
k < 1 for negative curvature (hyperbolic geometry)
k = 1 for zero curvature (flat geometry)
- Curvature of image field
- The curved surface to which a microscopic image is to be clearly and distinctly mapped is described as image curvature
aberration. It is conditional on the convex shape of the lens and makes itself apparent as an error due to the short focal distances of microscope objectives. Here the object image is not in focus both in the center and at the periphery at the same time. Objectives that are corrected for image curvature aberration are called plane objectives (plane = flat image field).
- DAC - Digital-Analog-Converter
- Opposite of the ADC. An electronic device, often an integrated circuit,
that converts a digital value to an analog voltage. D/A converters are used
in many instruments to convert digital reading information into an analog
signal for analog output. A time- and value-discrete signal is converted
into a time-continuous, value-discrete signal.
- Dark Current
- The electronic signal generated by the thermal characteristics of the CCD even in the absence of impinging light.
- Dark Frame
- An image of the dark current and camera readout and
bias signal made by integrating an image while keeping the
CCD array in total darkness.
- Dark Matter
- Term used to describe any astronomical mass that does not produce significant light and hence is hard to observe. Examples of dark matter include planets,
black holes, white dwarfs (because they are low luminosity) and more exotic things like weakly interacting particles.
- Dark Subtract
- This is the process of removing noise (generated by dark current) from a
CCD image. A dark frame is digitally subtracted from an image to eliminate dark noise.
- Darlington Amplifier
- An amplifier in which the collectors are tied together, and the
emitter of
the first directs current to the base of the second.
- DC - Direct Current
- The flow of electrons only goes in one direction.
- DDP - Digital Development Processing.
- This an image processing routine. DDP processing allows both bright and dim parts of an astronomical object to be displayed at the same time. DDP essentially compresses washed-out regions of an object into a range that the computer can display. This process is especially useful on galaxies which have bright cores and faint spiral arms.
- Deconvolution
- An iterative image processing filter that uses Fourier transform mathematics to restore a blurred image as nearly as possible to an unblurred state.
- Density
- The density of an object is equal to the mass of that object divided by its volume. Substances (like lead, water, iron, granite) have a certain density under normal pressures. In such cases the density of a substance can also be used to determine how much mass will be present given a certain volume of the substance. For example, water has a density of 1 gram per cubic centimeter
(gm/cm3) so a cube of water 10 centimeters on a side weighs 1000 gm (1 kilogram). Some substances (like gases) are compressible and have different densities depending on how much pressure is exerted upon them. The
Sun is composed of compressible (and hot!) gases and is much denser at its center than near its surface.
- Depth of field
- The distance between the nearest and farthest points that appear in
acceptably sharp focus in a photograph. Depth of field varies with lens aperture, focal length, and camera-to-subject distance.
- Depth of focus
- The focal length of a lens system to maintain a precise image size.
- Dew Heater
- An electric strap that wraps around the front of a telescope and heats up when plugged in to a power supply. Keeping the front lens warm prevents dew condensing.
- Dew Shield
- A cylinder extending out from the front of refractors, SCTs, and Maks to prevent stay light from entering the telescope and to capture air in front of the lens to delay the formation of dew.
- Dichroic
- Dichroic filters are interference filters at an angle of incidence of light of 45°. The transmissivity and reflectivity of dichroites depend on a specific
wavelength of light. For an RSP 510 filter (reflection short pass), for example, the excitation light below 510 nm is reflected and the excitation light above this value is transmitted. The transmission values are generally between 80% and 90% and the reflection values between 90% and 95%.
- Dielectric
- An insulating layer. A material that has high resistance. This term is
usually used when the insulating layer separates the plates of a capacitor.
- Diffraction
- Spreading or bending of a wave upon passing around an obstacle or through
a narrow opening.
- digital
- A method of representing information in an electrical circuit by switching
the current ON or OFF. Only two output voltages are possible, usually
represented by "0" and "1." In clocked circuits a
digital signal may change its logic state once during a clock cycle.
Opposite to analog.
- Digital Circuit
- A circuit that operates like a switch and can perform logical functions.
Used in computers or similar logic-based equipment.
- DIL - Dual In-Line
- See DIP below
- Diode
- Simple semiconductor element, comparable to a valve. The ideal diode is
blocking electric current in one direction and conducting it in the other.
The real diode causes a voltage drop and energy loss in the forward
direction, therefore power electronics sometimes use active diodes.
- DIP - Dual In-line Package also DIL
- The most common type of IC package; circuit leads or pins extend
symmetrically outward and downward from the long sides of the rectangular
package body. Usual package form for ICs in the past, where the pins lie in
two rows with 2.54mm (1/10") distance. The pins are put through holes
in the circuit board and are soldered from the back side. Nowadays in series
fabrication the SMD-package is used.
- Discrete Device
- A semiconductor containing only one active element, such as a
transistor
or a diode.
- Dispersion
- a) The separation of a beam of light into the individual wavelengths
of which it is composed by means of refraction or
diffraction.
- b) Resolution of white light into its component wavelengths, either by refraction or by diffraction.
- DMM - Digital Multi Meter
- An electronic instrument that measures voltage, current, resistance, or
other electrical parameters by converting the analog signal to digital
information and display. The typical five-function DMM measures DC volts, DC
amps, AC volts, AC amps, and resistance.
- Dobsonian (Dob)
- A very simple and stable mount usually used with reflectors; especially very large (greater than 10-inches) telescopes. There are usually no motors or tripods. A simple rotating base
and a tilting tube make for an easy to push or pull viewing session.
- Doppler Effect
- The change in frequency of a wave (light, sound, etc.) due to the relative motion of source and receiver. Things moving toward you have their
wavelengths shortened (blueshift). Things moving away have their emitted wavelengths lengthened (redshift).
- Doppler Shift
- Change in the apparent wavelength of radiation (e.g., light or sound) emitted by a moving body. A star moving away from the observer will appear to be radiating light at a lower frequency than if at rest; consequently, lines in the star's spectrum will be shifted toward the red (lower frequency) end of the spectrum. The existence of a direct relationship between the
redshift of light from galaxies and their distances is the fundamental evidence for the expansion of the universe.
- Dot pitch
- Describes the distance between the perforations on the monitor's shadow mask- The better displays usually have a dot pitch under 0.28mm. As with fine halftone screens, the smaller the dots, the sharper the image.
- Double Star
- A "system" of two stars that appear - because of coincidental alignment when viewed from Earth - to be close together; it is, however, an optical effect only, and therefore not the same as a binary star system (although until the twentieth century there were few means of distinguishing double and
binary stars).
- Doublet
- Two simple lenses used in combination, placed close together or in contact. If they are cemented together, they constitute a "cemented doublet". If they are merely closely adjacent, they are a "separated doublet".
- DRAGN (Double Radiosource Associated with
Galactic Nucleus)
- i.e. extragalactic radio sources - are clouds of radio-emitting plasma which have been shot out of active galactic nuclei
(AGN) via narrow jets.
- The very best example is Cygnus A, the brightest DRAGN in the sky.
- DRAM - Dynamic Random Access Memory
- A semiconductor read/write memory chip, in which the presence or absence
of a capacitive charge represents the state of a binary storage element
(zero or one). The charge must be periodically refreshed.
- Driver
- Program operating as an interface between an application and a special hardware. A graphics card driver has to insure, that every program can
display itself independently from the actual graphics board by means of a
special command set.
- DSP - Digital Signal Processor
- A versatile all purpose chip, used in cameras to handle basic contrast and
brightness adjustment and image compression.
- Dust
- Tiny grains of stuff, e.g., carbon grains (soot) and silicate grains (sand) that are about 0.1-1.0
micron in size. Dust grains are a major component of the interstellar
medium. Dust blocks visible light causing interstellar extinction. Dust scatters incident starlight, particularly the blue
wavelengths of light (blue light has a wavelength comparable to the dust grain's size) causing interstellar reddening. The dust itself is cold, and cools even further by giving off infrared emission.
- Dynamic Range
- The ratio of a CCD pixel's full-well capacity to the readout noise. Useful in determining the appropriate number of digitization levels that the
analog-to-digital conversion system should use.
- Earth
- Third planet from the Sun. First forms of life appeared about 3.2 to 3.5 ×
109 years ago (Homo sapiens appeared as a species about 105 years ago).
- Eclipse
- The obscuration of a celestial body caused by its passage through the shadow cast by another body.
- EEPROM OR EPROM. Electrically-Erasable Programmable Read-Only
Memory.
- Similar to PROM, but with the capability of selective erasure of
information through special electrical stimulus. Information stored in
EEPROM chips is retained when the power is turned off.
- Electromagnetic Spectrum
- The distribution of light separated in order of some varying characteristic such as
wavelength or
frequency. The "electromagnetic spectrum" refers to the full range of possible frequencies and wavelengths of light. If we "take a spectrum" of a star we analyze its light according to wavelength or frequency by, say, passing the light through a prism. A "spectral line" refers to emission or absorption at a particular wavelength of light.
- Electron
- A negatively charged particle revolving round the nucleus of an atom.
- An elementary particle (of the type known as a lepton) with a negative charge. One of the components of atoms, the electrons orbit around the nucleus, and the distribution and number of electrons determine the chemical properties of an element.
- Electronic Shutter
- Electronic shuttering is the process of controlling the exposure period of
the CCD by electronic methods (compared to conventional
shuttering, which involves an electro- mechanical shutter
system that opens, allowing light to fall momentarily on the CCD, and then
closes). In most interline transfer CCDs the process of electronic
shuttering involves "dumping" accumulated charge from the imaging photosites
to the substrate for a predetermined amount of time and then stopping the
"dumping" process for the actual exposure
period. This is followed by the normal charge transfer during vertical
blanking and then readout during the next field.
- Element
- A particular type of atom, with specific atomic number and chemical properties. The smallest unit into which matter may be broken by chemical means.
- EMF - Electromagnetic Force
- The force between charged particles, which accounts for electricity and magnetism. One of the four fundamental forces of nature, it is carried by
photons, and is responsible for all observed macroscopic forces, except for gravity.
- Emission Filter
- Also Barrier filter, Emitter. A color filter that attenuates all of the light transmitted by the excitation filter and very efficiently transmits any fluorescence emitted by the specimen.
- Emission Spectrum
- The bright lines seen against a darker background, created when a hot gas emits
photons characteristic of the elements of which the gas is composed.
- Emitter
- The region of a bipolar transistor that serves as a source or input end
for carriers. N-type for NPN, P-type for PNP.
- Encke's Division
- A region of decreased brightness in the outermost ring of Saturn.
- Energy
- Energy is usually defined as "the capacity to do work" but just what does that mean? Work is defined in physics as the exertion of a force over some distance, e.g., lifting a rock up against the gravity of the Earth. You probably have a pretty good colloquial grasp of the idea of "work" as something that takes effort. Energy is also something that is conserved within a closed system. This means that it is neither created nor destroyed but simply moved about (possibly changing from one form of energy to another). Light is basically a form of energy, one that radiates through space. So the
Sun can release nuclear energy, creating light which travels through space to the Earth, where it can be absorbed by, say, a
photocell, which in turn permits a motor to run propelling a solar-powered car forward.
- Entrance Pupil
- The apparent size of the limiting aperture of a lens or lens system (properly that of the diaphram), as seen from the object plane. This can shift and become a complex matter in some circumstances.
- Entropy
- A quantitative measure of the disorder of a system. The greater the disorder, the higher the entropy.
- Equatorial (Eq)
- a) A special kind of telescope mount that has its axes tilted up to match the latitude of your observing site and is pointed at the North (or South below the equator).
- b) The classic type of telescope mount with one axis parallel to the Earth's polar axis (i.e. pointing at the celestial pole) and the other at right angles. Once the object is located, only the polar axis need be driven by a motor to counteract the Earth's rotation.
- See also GEM.
- Equilibrium
- A balance in the rates of opposing processes, such as emission and absorption of
photons, creation and destruction of matter, etc. so that there is no net change.
- Escape Velocity
- The outward velocity required to leave the surface of a body mass M and radius R and escape to infinity (not fall back). The formula for the escape velocity is (2GM/R)
1/2.
- Euclidean Geometry
- Flat geometry based upon the geometric axioms of Euclid.
- Event
- A point in four-dimensional spacetime; a location in both space and time.
- Event Horizon
- An event horizon is a lightlike surface in spacetime which divides spacetime into two regions: that which can be observed, and that which cannot.
In the case of a black hole, the event horizon is that surface surround the region out of which light itself cannot escape. No signal or information from within the event horizon can reach the outside universe. For a nonrotating black hole, the horizon is located at the
Schwarzschild radius, corresponding to
Rs = 2GM/c2
- Excitation Filter
- Also Exciter. A color filter that transmits only those wavelengths of the illumination light that efficiently excites a specific dye. See Emission
filter.
- Exit Pupil
- The exit pupil of a lens system is an image of the entrance pupil (hence conjugate to it) and normally should be the image of the limiting diaphram. In both the microscope and the telescope it is the eyepoint where the beam has its smallest cross-section. It is also called the Ramsden circle (q.v.) or eyepoint.
- Experiment
- A controlled trial for the purpose of collecting data about a specific phenomenon.
- Explosive
- A chemical that causes a sudden, almost instantaneous release of pressure,
gas, and heat when subjected to sudden shock, pressure or high temperature.
- Exposure
- 1. The act of allowing light to strike a light-sensitive surface.
2. The amount of light reaching the image sensor, controlled by the
combination of aperture and shutter
speed. In photographic terms this is the
product of the intensity of light and the time the light is allowed to act
on the sensor, or the film. In practical terms, the aperture controls the
effective diameter of the hole that allows light through, and shutter speed
controls the length of time the shutter is open.
- Extinction
- As light from a star travels through interstellar space it encounters some amount of
dust. This dust scatters some of the light, causing the total
intensity of the light to diminish. The more dust, the dimmer the star will appear. It is important to take this effect into account when measuring the
apparent brightness of stars.
The dark bands running across portions of the milky way in the sky are due to extinction by copious amounts of dust in the plane of our
Galaxy.
- Eyepiece
- The lens system used in an optical instrument for magnification of the image formed by the objective.
-
- Eyepieces come in various types. Every eyepiece has a focal length. The magnification that results when a given eyepiece is used with a given telescope, is equal to the focal length of the telescope divided by the focal length of the eyepiece. Thus if the telescope has a focal length of 1000 mm and the eyepiece has a focal length of 25 mm, the magnification will be 1000 / 25, or 40.
-
- There are several types of eyepiece designs. The most popular are:
- Ramsden
- The Ramsden is a very old design, with a rather narrow apparent field of view -- perhaps as little as 30 degrees. Ramsdens do not work well at focal ratios shorter than about f/9, but good ones make surprisingly nice eyepieces for Lunar, planetary, and double-star observation, at longer focal ratios. Ramsdens often have prominent ghosts.
The simplest form of Ramsden consists of two identical simple lenses, each flat on one side and convex on the other, facing each other with convex sides inward, spaced apart by a distance equal to or slightly less than their focal length.
Kellner
- In essence, the Kellner is an achromatized Ramsden. It has a slightly larger apparent field of view than the Ramsden, and works at slightly faster focal ratios. Kellners tend to have rather prominent ghosts.
Kellner eyepieces consist of a small achromat -- a cemented doublet -- near your eye, and a simple lens at the far end of the eyepiece.
Orthoscopic
- Orthoscopics have moderate apparent fields of view -- 40 or 45 degrees -- and work well at fast focal ratios. Many consider them the best eyepieces for Lunar, planetary, and double-star work.
There are actually several designs called "orthoscopic". The most common kind has a simple lens nearest your eye, and a cemented triplet further away. Another kind resembles a Plossl.
- Erfle
- The Erfle has a rather wide apparent field of view -- perhaps 68 degrees or more. The image quality at the edges of the field, at small focal ratios, is not as good as for more modern wide-field eyepieces.
Erfles are generally composed of five or six simple lenses, grouped into two
doublets and a singlet, or three doublets.
Plossl
- Plossls have moderate apparent fields of view -- 50 degrees is typical -- and work well at fast focal ratios.
Plossls consist of four simple lenses, grouped as two cemented doublets.
- Ultra Wide
- Ultra Wide Angle is a "house brand" of Meade. These eyepieces are well corrected, with very large apparent fields of view, of 84 degrees.
Ultra Wide Angle eyepieces are reported to be generally similar in design to
Naglers.
- Koenig
- Koenigs have a rather wide apparent field of view -- perhaps as much as 70 degrees.
The various eyepieces commonly labeled "Koenig" contain anywhere from four to seven simple lenses, grouped into various combinations of cemented doublets and singlets.
- Speers-Waller
- Noted for a very wide apparent field of view -- almost 80 degrees -- and for excellent correction at fast focal ratios.
Speers-Waller eyepieces are reported to be similar to Naglers.
- Naglers
- Nagler is a "house brand" of Tele Vue. These eyepieces are noted for a very wide apparent field of view -- 82 degrees -- and for excellent correction at fast focal ratios. Naglers are big, heavy, and expensive, and consist of seven or eight simple lenses grouped together into four singlets or doublets.
- Eye Relief
- The distance from the surface of the rearmost lens of the eyepiece, to the exit pupil. When the eyepiece is in use, that distance should be the distance from the rearmost lens of the eyepiece to the iris of the observer's eye. The remaining distance is the space between the observer's eye and the
eyepiece.
- It is the clearance available for moving the observer's head without bumping the telescope, and is also the place where the observer's spectacles must fit, if they are worn while observing.
- Fastar
- Fastar is Celestron's high-speed CCD imaging system. Fastar involves removing a Schmidt-Cassegrain telescope's secondary mirror and placing the CCD camera at the front of the telescope. This provides a wide field of view and a vary fast imaging system. A telescope which is said to be "Fastar compatible" has a removable secondary mirror. The actual accessories needed for Fastar imaging are sold separately.
- FET - Field Effect Transistor
- A solid-state device in which current is controlled between source and
drain terminals by voltage applied to a gate terminal, which is insulated
from the semiconductor substrate.
- Fermi Mechanism
- A process hypothesised by Enrico Fermi whereby a small fraction of the charged particles in space can be boosted up to the high energies of cosmic rays. The mechanism works on particles which are already distinctly more energetic than average. Fermi imagined the high-energy particles scattered from dense clouds in the interstellar gas, or more correctly from the magnetic field embedded in the clouds. In these collisions the particle gains energy if the cloud is moving towards it, and loses if the cloud is moving away, but Fermi realised that the first case happens slightly more often than the second, so the particles slowly gain energy.
In a strange coincidence, in 1977 four independent groups of researchers realised that in shock waves the high-energy particles find that every scattering is nearly head-on, giving much more efficient acceleration (called "first-order Fermi acceleration"). According to simplified calculations, the particles end up with a "power-law" distribution of energies, very like that of the electrons which actually produce cosmic synchrotron radiation. It therefore seems quite likely that the first-order Fermi mechanism is responsible, although things don't agree so nicely when the calculations are done more carefully.
- FFT - Fast Fourier Transform.
- FFT filters are image processing algorithms applied to
CCD images, usually to sharpen or smooth and image. FFT filters have the same function as kernel filters, but are applied differently so the processing is faster. Applying an FFT to an image changes the image so that applying a kernel filter is easier. The FFT function is then reversed and a filtered image results.
- Field
- A mathematical representation of a quantity describing its variations in space and/or time.
- Field Rotation
- Rotation of the FOV over time. With an Alt-Az system that does not control for field rotation, if tracking is otherwise perfect, the stars and other objects in the FOV will pivot around the center of the FOV during a
CCD or film exposure or during a visual observing session. Equatorially mounted and driven systems will suffer from field rotation if polar alignment is imperfect.
- FIFO - First In, First Out
- Electronic pendant of the "stand in a queue". Used to
synchronize data sources with different speed or unsynchronized data
streams.
- Filter Wheel
- Black and white CCD cameras use color filters to separate the three primary colors (red, green, and blue) in order to produce full color images. Usually an automated mechanical wheel holds the filters in front of the camera and rotates the appropriate filter into place before an exposure.
- Filtering
- 1. Using color-dyed or interference-layered glass inserted into the optical path to restrict the passage of full-spectrum
wavelengths.
- 2. Applying a mathematical function to the pixels in an image array that modifies each pixel's value according to the values of an assigned set of neighboring pixels. Primarily used to blur or sharpen localized aspects of an image.
- Firewire
- Apple's name for the communication standard IEEE
1394.
- FITS - Flexible Image Transport System.
- The standard data file format for astronomical CCD images. FITS images use file extension names of FTS, FIT, or FITS.
- FLASH MEMORY
- It is a non volatile memory technique with fast access times; rewriteable
many times and uses a block erase technique as opposed to EEPROM, which
erases one bit at a time.
- Flat-Field Frame
- A CCD image of the irregularities in the optical system and the CCD chip. The image is integrated while the optical/CCD system is pointed at a wide-field, evenly illuminated source, such as that provided by a specially manufactured light box, the inside wall of an observatory dome, a large poster board positioned in front of the telescope, or a twilight sky.
- Flip-Flop
- An electrical circuit having two stable states: on and off. A basic logic
circuit component.
- Fluorite Objectives
- Describes a correction class for objectives. Fluorite lenses are semi-apochromatic, meaning their degree of correction lies between the
achromatic and apochromatic.
- Flux
- A flux is the rate at which something is transferred through a surface, like 10 flies per minute through the 1 square inch hole in the busted screen door. In astronomy flux is used to express the amount of energy radiated per second across an area like a square centimeter.
- Flux Density (see also
Brightness)
- When we describe Sirius as the brightest star in the sky, we are talking about flux density. This is a measure of how bright things seem to us from our platform on Earth. It depends both on the
luminosity and the distance, since distant objects appear fainter. Radio astronomers measure this in units of Jansky, optical astronomers use apparent magnitude.
- Frequency
- This is a property of a wave, and it is the number of wave crests that pass a given point per second. Frequency is is measured in units of inverse time (e.g.,
"cycles per second''). A cycle per second is the unit of frequency and it is known as a
"Hertz.'' Since light moves at the constant speed of light, the frequency of a light wave is related to the
wavelength: the frequency is given by the number of wavelengths that go by per second at the speed of light, hence frequency is wavelength (distance) divided by speed (c). The higher the frequency of light the greater its energy.
- It designates the incidence of a change in signal or state. Given in Hz, that
is number per second. FM-radio works in the MHz-region (M=Mega=106), fast
integrated circuits in the GHz-region (G=Giga=109), as does the mircowave
oven
- Focal Length
- The distance from the optical center of the lens to the image sensor when
the lens is focused on infinity. The focal length is usually expressed in
millimeters (mm) and determines the angle of view (how much of the scene can
be included in the picture) and the size of objects in the image. The longer
the focal length, the narrower the angle of view and the more that objects
are magnified.
- Focal Ratio
- The effective focal length of an optical system divided by the diameter of the primary optical component.
I.e. The ratio between the focal length of a telescope and the aperture.
- A telescope with an 8" aperture and 80" (2000mm) focal length has a focal ratio of f/10. Smaller focal ratios equate to shorter exposure times. An f/4 system is faster than an f/6 system, for example.
- Focal Reducer
- An optical component or system for changing the image scale of a telescope to achieve a better match between the seeing disk and the pixel size.
- Focus
- The process of bringing one plane of the scene into sharp focus on the
image sensor.
- Fork Mount
- A fork mount is a type of mount where the telescope is held by two arms, and swings between them. A fork mount can be either
alt-azimuth or equatorial (through the use of a
wedge). Fork mounts are most commonly used with Schmidt-Cassegrain telescopes, and are almost always equatorial.
- FOV - Field of View
- The size of an imaged or visual scene in terms of sky dimensions, usually stated in degrees, arcminutes, or
arcseconds. The approximate width of the FOV in arcseconds can be calculated by dividing the width of the
CCD chip in microns by the focal length of the optical system in millimeters and multiplying the result by 206.
- Frame Grabber
- A device that lets you capture individual frames out of a video camera or
off a video tape.
- Frame Rate
- The number of pictures that can be taken in a given period of time.
- f-stop
- A numerical designation (f/2, f 2.8, etc.) indicating the size of the
aperture (lens opening). A small f-number ( large aperture diameter) gives a
small depth of field, and a large f-number gives a large depth of field.
- Full Well Capacity
- Each photosite on a CCD chip can contain a certain number of
electrons. This number is called the Full Well Capacity. Within a certain chip full well capacity is a function of
binning, so that binning 2 times, for example, quadruples the full well capacity. Of course, 2x binning also makes the CCD effectively 4 times more sensitive and so the wells will fill in the same amount of time. Chips with
anti-blooming generally have lower full well capacity, but will not bloom when the well is filled
- Future
- Those events which could be influenced by a given event. All events located within the future light cone.
- FWHM - Full Width Half Maximum
- A measurement of the size of a point source image, such as a star, in terms of the width of the 50% peak value circumference. Usually stated in units of
arcseconds or pixels.
- GaAs - Gallium-Arsenide
- A compound semiconductor material in which active devices are fabricated.
GaAs has a higher carrier mobility than silicon, thus it has the capability
of producing higher speed devices.
- Gain
- The factor by which an incoming signal is multiplied.
- Galactic Equator
- The primary circle defined by the central plane of the Galaxy.
- Galaxies
- Galaxies are often described as huge collections of stars. This is about as accurate as describing a city as a huge collection of street-lights: stars are certainly the most obvious feature of galaxies when see far away in the dark, but they represent a fairly small fraction of the total mass, 90% or so of which consists of the famous dark matter, whose nature is one of astronomy's biggest mysteries. As well as stars and dark matter, galaxies contain gas and dust in the inter-stellar medium between the stars. The may also contain an active galactic nucleus at their
center.
Galaxies come in a wide range of sizes (meaning diameter, brightness, and mass, all of which roughly go together), ranging from tiny dwarfs with only a few million stars, through normal galaxies like our own Milky Way, with a hundred billion stars, to the trillion-star cD galaxies that sit at the
centers of the great clusters of galaxies. As is often the case in nature, the smallest are the most common; on the other hand the size of the larger galaxies more than makes up for their rarity, so that a typical star is likely to be in a galaxy with a size approaching that of the Milky Way or larger, and large galaxies are also the easiest to find in the sky, due to Malmquist bias.
Normal galaxies come in two basic types: spiral and elliptical.
- Gamma
- The logarithmic brightness value assigned to video monitors to allow replication of the logarithmic visual range of the eye.
- Gate
- 1. A signal which in the active state enables an operation to occur; and
when in the inactive state inhibits an operation from occurring.
2. The basic digital logic element - where the binary value of the output
depends on the values of the inputs.
3. The primary control terminal of a field effect transistor.
- GEM - German Equatorial Mount
- A telescope mount that can easily counteract the Earth's rotation and track the stars.
- General Relativity (GR)
- Einstein's theory of gravity. Einstein describes a gravitational field in terms of the "curvature" of space. J. H. Wheeler boiled the theory down to the slogan
Space tells matter how to move; matter tells space how to curve.
Because of the curvature of space, the various ways we use to define length end up giving different answers when general relativistic effects are important, for instance in cosmology, or near a black hole.
More...
When Einstein proposed the theory in 1916, there was very little evidence in its
favor. The situation is very different today: the predictions of General Relativity have been tested with high precision in many ways, in laboratories, in the Solar System, and by observations of distant pulsars. The results agree very well with General Relativity, showing that it must be, at the least, very close to the right answer.
... A full appreciation of general relativity involves mastering some difficult mathematics (unlike
special relativity). To be honest, even professional astronomers often never get around to learning GR properly!
- Geocentric
- Taking the Earth to be the center, e.g., of the solar system, or of the universe.
- Geodesic
- In geometry, that path between two points/events which is an extremum in length. In some geometries, such as
Euclidean, the geodesics are the shortest paths, whereas in others, such as in the spacetime geometries appropriate to
general relativity, the geodesics are the longest paths.
- Giant Molecular Cloud
- A region of dense interstellar medium that is sufficiently cold that molecules can form. They are
very cold (10-20K) with relatively high densities (trillion particles per cubic meter), and huge. Even though the
temperatures are very cold the molecules in these molecular clouds emit radio radiation which can be detected on Earth. These regions are believed to be where new stars can form.
- GIF - Graphics Interchange Format
- An image file format used heavily used on the Web (256 colors, patented by
UNISYS)
- Globular Cluster
- A tightly packed, symmetrical group of thousands of very old (pure Population II) stars. The stellar density is so great in the center that the nucleus is usually unresolved. The stars within a globular cluster orbit each other because of their mutual gravity.
- GoTo
- A motorized & computerized telescope.
- Gravitational Lens
- A massive object which causes light to bend and focus. This occurs because light falls in a gravitational
field.
- Graviton
- A hypothetical massless carrier boson which is the carrier of the gravitational force.
- Gravity
- The weakest of the four fundamental forces; that force which creates the mutual attraction of masses.
- Gray scale
- A series of shades of gray ranging from pure white to pure black.
- The linear array of brightness values assigned to a monochrome image represented in black and white, where 0 = black and the maximum array value = white. For example, in an 8-bit dynamic range, 0 = black, 256 = white, and medium gray = 128.
- Ground
- A common reference point for an electrical system.
- Guiding
- No telescope mount can track perfectly, yet for CCD imaging it is necessary to very accurately track the object being imaged. This is done by guiding on a
star to make small corrections to the mount to accurately follow the star. This makes up for any errors in the telescope's drive system. Guiding can be done manually by watching a star through a crosshair eyepiece, or, more commonly, by using an autoguider to automatically guide. See also,
Unguided Exposure, Autoguider, Self-Guiding, and Track & Accumulate.
- H-alpha Solar Filter
- A very special filter that allows a very small portion of the sun's total spectrum through the telescope. It is on the order of 1.7 to 0.7
Angstroms. It allows you to view solar prominences.
- Hadron
- A class of particles which participate in the strong interaction. Hadrons consist of those particles (baryons, mesons) which are composed of quarks.
- Halftone
- A method of printing a continuous tone image, such as a photograph, using a screen of tiny dots - Used in inkjet and lithographic printers only four
CMYK process colors are used. Once printed and when viewed from a distance, the human eye perceives these tiny dots as a continuous tone.
- Hardware
- Generic term for electronic devices, that is, things you can touch.
Contrary to software. (My definition: throw it out of the window, if it is
broken it was hardware....)
- Hawking Radiation
- Emission of particles, mostly photons, near the event horizon of
black holes due to the quantum creation of particles from the gravitational energy of the black hole.
- Heat Sink
- An assembly that serves to dissipate, carry away, or radiate into the
surrounding atmosphere heat that is generated by an active electronic
device.
- Heliocentric
- Taking the Sun to be the center, e.g., of the solar system.
- Histogram
- A histogram is a graph of number of pixels versus pixel value.
- Pixel values run from lowest (displayed as black) to highest (displayed as white). A bar is plotted for each pixel value showing the number of pixels in the image with that value. An astronomical image typically has more bars toward the lower (darker) end of the histogram since most astroimages contain are large amount of dark sky around a brighter (but small) object.
- HSL - Hue, Saturation, Luminance
- A particular conformation of color theory.
- HST - Hubble Space Telescope
- Hubble Constant
- The constant of proportionality (designated H) between recession velocity and distance in the
Hubble law. It is a constant of proportionality but not a constant in time, because it can change over the history of the universe.
The Hubble constant is defined as the rate of change of the scale factor with respect to time divided by the scale factor.
Measuring the Hubble constant is difficult and remains and important task for astronomers. Present best values lie between approximately 50 km/sec/Mpc and 100 km/sec/Mpc, with a value around 70 km/sec/Mpc favored.
- Hubble's law
- In 1929 Edwin Hubble showed that the further a galaxy is away from us, the faster it is moving away; this is now called Hubble's law, and it means that the universe is expanding.
What Hubble actually measured was the redshift, usually written z , of the spectrum of the galaxies: he found that the spectrum was `stretched out' so that a feature that should have been
at a certain wavelength was actually detected at a wavelength longer by a factor (1 + z). The usual cause of such a redshift is the Doppler effect, and by assuming this it is possible to calculate the recession velocity of the galaxy. In the cosmological case, it turns out that the Doppler formula is only an approximation; the deeper meaning of the redshift is that the factor (1 + z) gives the ratio of the size of the universe at the time the light was emitted, to the size it is today.
This expansion is quite a subtle concept. It does not mean that everything (galaxies, planets, people, ants, atoms) is getting bigger; if that were the case, we would never know, because our tape measures and rulers would be growing at the same rate. The idea is clearest for a toy universe consisting of `dust' spread evenly through space, with individual particles not interacting at all with each other. Expansion means that the distance between any two particles is getting larger. Now think of a row of particles: since each is moving away from its immediate neighbours (at the same rate, from homogeneity), the relative motion must be proportionately larger for particles separated by larger distances: Hubble's law. Of course the real universe is lumpy on `small' scales; instead of being a smooth soup, matter is condensed into galaxies and stars and all the rest (just as well for us!). Within these objects the original expansion has been overcome by gravity. This is a runaway process. Start with a small region in the early universe with a slightly higher-than-average density. Its gravity will slow down both its own expansion and that of surrounding matter, so both the size of the region and its relative over-density increase. Eventually the expansion near the centre is stopped altogether. The size of the region affected continues to grows as outlying particles have time to move in. At present the largest regions in which the expansion has been stopped are the clusters of galaxies a few million light years across. For points with larger separations, the motions induced by local density peaks are small compared to the relative motion from the overall expansion.
- Hue
- Distinct color or shade.
- Hybrid Circuit
- Mounting technique, where several elements are placed on a substrate
(Al2O3-ceramics) by special pastes by silk-screen print, followed by a
burning process. Semiconductors are bonded "naked" (without
package) or soldered in SMD packages. Complete modules are often founded in
synthetic resin.
- Hyperbolic Geometry
- A geometry which has negative constant curvature. Hyperbolic geometries cannot be fully visualized, because a two-dimensional hyperbolic geometry cannot be embedded in the three-dimensional
Euclidean space. However, the lowest point of a saddle, that point at which curvature goes both
"uphill'' and "downhill,'' provides a local representation.
- Hypothesis
- A proposed explanation for an observed phenomenon. In science, a valid hypothesis must be based upon data and must be subject to testing.
- IC - Integrated Circuit
- consists of several (up to some million) transistors,
resistors and
capacitors (sometimes planar inductors, too). These are built in a numerous
process steps on a ultra-clean silicon disk (the wafer). These processes
include the fabrication of "masks", used for structuring a light
sensitive varnish. After etching the enlightened parts of the varnish, so called
dotants are brought in by diffusion of ion implantation. This modifies the
electrical behavior of the silicon.
- ICC profiles - International Color Consortium
- A profile is a list of characteristics that describes a particular color space
Pixel such as the space of an Apple 13-inch monitor. ICC profiles are interpreted by
Color Management Modules, when your image is viewed on a monitor in a different workstation.
- IEEE - Institute of Electrical and Electronics Engineers
- Abbreviation for Institute of Electrical and Electronics Engineers. Also a
standardization organization.
- IEEE 1394
- A port on the computer capable of transferring large amounts of data very
fast. (Firewire)
- i.Link
- Sony's name for IEEE 1394.
- Image sensor
- A solid-state device containing a photosite for each
pixel in the image.
Each photosite records the brightness of the light that strikes it during an
exposure.
- Inch
- One inch (1") is 25.4mm. The inch is the commonly used measure in
electronics, mostly 1/10" or 1/20" of distance is found for pin
distances of components.
- Index of Refraction
- Ratio of the speed of light in a vacuum to the speed of light in a
particular substance; ratio of the sine of the angle of incidence to the
sine of the angle of refraction.
- Insulator
- A material that is a poor conductor of electricity - used to separate
conductors from one another or to protect personnel from electricity.
- Intensity (or Surface Brightness)
(see also Brightness)
- This is what we mean when we say that the center of a galaxy is brighter than the outer regions; an image is essentially a map of intensity. It measures the
flux density we receive, not from the object as a whole, but from each unit area of the sky (technically, solid angle). The flux density of an object is thus the product of its intensity times its solid angle.
- Integration
- Acquisition of electronic data from a CCD array. Synonymous with image acquisition or
exposure. Analogous to exposure in film terminology.
- Interference
- A wave is something that moves along and has high points (crests) and low points (troughs). If two (or more) different wave trains pass over one another the crests and troughs can add together to make bigger crests and troughs, and a crest and a trough can add together to produce zero. So if light is a wave phenomenon, then two light sources produce waves that in some places produce large amplitudes and other places produce zero. When to point sources of light are projected onto a screen this wave interference effect produces alternating light and dark spots. This demonstrates the wavelike nature of light.
- Interpolation
- In an image interpolation adds extra pixels. It's done with some zoom
lenses.
A process that uses software to add new pixels to the mosaic-like bitmap of an image or part of an image. The color of the new pixels is derived from the original adjacent pixels. Interpolation appears to increase the original resolution and quality of an image.
- Interstellar Medium
- The name given to the stuff that floats in space between the stars. It consists of gas (mostly hydrogen) and dust. Even at its densest the interstellar
medium is emptier than the best vacuum humanity can create in the laboratory, but because space is so vast, the interstellar medium still adds up to a huge amount of mass.
- Ion
- An atom which has gained or lost an electron and thereby acquired an electric charge. (Charged molecules are called radicals, not ions.)
- I/O - Input/Output
- The process of transferring data to and from a computer-controlled system
using its communication channels, operator interface devices, data
acquisition devices, or control interfaces. The computer may exchange
information in several ways, that is, it has to accept data or to provide
data. At lowest level, the circuit level, the computer has a set of so
called port addresses and memory addresses. A 8-bit port holds 8 binary
signals, for example.
- Ion
- This is what an atom becomes when an electron is separated from the atom,
leaving it with a net positive charge or, if an electron is added, leaving
it with a net negative charge.
- IRQ - Interrupt ReQuest
- With the help of interrupts a device can get the attention of the processor, e.g. because an error appeared or new data is ready. This way the
processor doesn't have to look at all available ports all the time, which is
called polling. IRQs provide flexibility and can enhance performance.
- ISO - International Standards Organization
- ISO is taken from the Greek word isos, meaning equal. The speed or light sensitivity of conventional photographic film and of the CCDs found in digital cameras, is measured in ISO values. Lower ISO numbers indicate slower films, or lower
CCD ratings which require less light to be correctly exposed.
- Isotope
- One of the forms in which an element occurs. One isotope differs from another by having different numbers of
neutrons in its nucleus. The number of protons determines the elemental identity of an
atom, but the total number of nucleons affects properties such as radioactivity or stability, the types of nuclear reactions, if any, in which the isotope will participate, and so forth.
- Jansky
- Unit of flux density, named after Karl Jansky, who first discovered extra-terrestrial radio waves (from the Milky Way) at Bell Telephone Laboratories in Holmdel, New Jersey, in 1932.
1 Jy = 10-26 Watt Hertz-1metre-2.
- Jets
- Perhaps the weirdest feature of AGN is that they can produce narrow jets of material streaming outwards from the centre. The jets are usually produced in pairs, pointing in opposite directions; occasionally there only seems to be one jet. These jets can sometimes be "seen" directly by radio telescopes, but more often we deduce their existence because far outside the galactic nucleus they produce a
DRAGN. In one sense, we should not be too surprised to find jets, since in every other case where accretion is thought to occur in astronomy, jets are also common. These include young stars forming out of gas clouds, and binary stars where matter is falling from one of the pair onto the other. Unfortunately, we don't know how jets are produced in any of these circumstances!
- JPEG - Joint Photographic Experts Group
- A very popular digital camera file format that uses lossy compression to
reduce file sizes. Developed by the Joint Photographic Experts Group. JPEG
compression provides the best results with continuous-tone images, such as
photographs, when the size of the file is an important factor.
JPEG images are stored with a file extension name of JPG or JPEG.
- Jupiter
- Fifth planet from the Sun. It is more massive than all other planets and satellites combined; if it were about 80 times more massive, it would become self-luminous due to fusion reactions. The heat flux to from the center to the surface is mainly convective. For both Jupiter and Saturn it is necessary to invoke a substantial source of internal heating (presumably gravitational contraction) to account for the surface temperature (Jupiter radiates about 2 1/2 times as much heat as it receives from the Sun). Jupiter's surface shows pronounced horizontal striations: the light layers (zones) are at a slightly higher altitude and about 15° cooler than the dark layers (belts). It is surrounded by a partial torus of atomic H in the orbit of Io. Thirteen satellites, the four outermost of which have retrograde motion, high eccentricity, and high inclination. (Jupiter XIII, discovered in 1974, has a period of 239 days; i = 26°.7, e = 0.147.)
- Kelvin Scale
- This is the temperature scale which uses the same size of degree as the Celsius or Centigrade system, but which begins at
absolute zero, the coldest temperature possible corresponding to the lowest possible energy state of a system. Temperature in degrees Kelvin gives a measure of the average energy of a system.
- Kepler's Laws
- The three laws of planetary motion discovered by Johann Kepler.
1. Planets orbit on ellipses with the Sun at one of the focii of the ellipse
2. Equal area is swept out by the planets motion as it moves around the ellipse (a planet moves fastest when it is nearest the
Sun),
3. The square of the period of the orbit is equal to the cube of the semimajor axis (half the long axis of the ellipse). This is written as
P2 = a3, where P stands for the period of the orbit (the sidereal period) and a is the size of the semimajor axis of the ellipse.
- Kernel Filters
- Kernel filters are image processing algorithms applied to a
CCD image, generally to smooth or sharpen an image. A kernel is a small grid which tells the computer how to change the value of a certain pixel based on the values of neighboring pixels. The most common types of kernel filters are low-pass ("smoothing" and "blurring" filters) and high-pass ("sharpening" filters).
- Kinetic Energy
- The energy associated with macroscopic motion. In nonrelativistic physics the kinetic energy is equal to one half the mass times the velocity squared, i.e., 1/2
mv2.
- Lab color mode
- Lab mode describes a theoretical color space where color and brightness are split into three different channels. two for color and one for brightness.
- LED - Light Emitting Diode
- Semiconductor, emitting light of a defined wavelength
when current flows.
Because of its high efficiency, its robustness and its fast reaction LED
have replaced bulbs for displays.
- Lepton
- A member of a class of particles which do not participate in the strong interaction (the force that binds atomic nuclei
together). The best-known lepton is the electron. Another example is the neutrino.
- Libration
- Any of several oscillations in the apparent aspect of the Moon as seen from Earth, which, when combined, enable Earth-based observers over a period of time to see altogether about 59 percent of the Moon's surface. Physical librations are angular motions about the center of mass due to gravitational torques on the Moon. Optical librations are the apparent rotations of the Moon, caused by our observing it from somewhat different directions at different times.
- Light
- Electromagnetic radiation that may or may not be visible to the human eye.
- Light Box
- An internally illuminated container that uses indirectly placed light sources to evenly illuminate a translucent diffuser screen on one side of the container. The screen is sized to cover at least the entire aperture of a telescope so that accurate
flat-field frames can be made.
- Line Emission
- An electromagnetic emission process which produces radiation at a number of specific frequencies, c.f. continuum emission. It is so-called because when the light is
analyzed with a spectrograph, line emission shows up as bright lines crossing the spectrum. --picture?-- Line emission is generally produced by fluorescence, in which atoms or molecules are "excited" to a high energy state by absorbing ultraviolet radiation, and then "decay" to their "rest state" via a series of quantum jumps. In each quantum jump, a photon is emitted with a specific frequency which is determined by the quantum properties of the atom. The frequency of the photon when received on Earth is affected by the Doppler effect because the emitting atom is moving relative to us. Of course we actually receive the accumulated radiation from large numbers of atoms moving with different speeds, so emission lines have a finite width, which are usually specified in terms of the equivalent range of speeds.
- Linear Circuit
- A circuit whose output is an amplified version of its input or whose
output is a predetermined variation of its input. See also OpAmp.
- Linear Response
- A CCD camera with a "linear response" has sensitivity such that doubling the
exposure time of an object of a certain brightness will result in an image twice as bright. There is a linear relationship between exposure time and brightness. This is especially useful for making
magnitude measurements of variable stars, comets, asteroids, or
supernovae. A camera with a nonlinear response is not suitable for making magnitude estimates since a star which appears twice as bright in an image is not necessarily twice as bright in actuality. CCD cameras equipped with an anti-blooming feature are generally nonlinear. For taking pretty pictures linear response makes no difference.
- LiOn
- Lithium ion battery
- Long-focal-length lens (telephoto lens)
- A lens that provides a na
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