Additive Color - A method of creating color by combining red, green and blue lights. White is created by adding by adding all of these colors, each at maximum intensity. Black is created by a total absence of these colors.

A/D Converter - Analog to Digital Converter. The CCD or CMOS sensor in the camera outputs an analog voltage after detecting photons. The A/D converter turns this continuous voltage information into discrete steps of digital data.

ADU - Analog to Digital Unit. A number that represents the CCD or CMOS sensor's digital output. The number of electrons in each ADU is determined by the system gain.

Aesthetic - Relating to artistic, non-tangible, qualitative, non-quantitative, aspects of a thing or concept that evoke a pleasant appreciation or perception of beauty by the viewer without regard to monetary value or utility.

Afocal - Using a camera with its lens attached to take a picture through a telescope's eyepiece.

Airy Disk - The Airy Disk is a disk of light from a star that forms at the focal plane of an optical system instead of a point. The stars we see in the night sky are so far away that they are essentially point sources. But the image that forms at the focal plane is not a point, because the light that passes through the circular aperture opening of the telescope suffers from the effects of diffraction. Diffraction occurs because of the wave nature of light. It is called the Airy disk, after British Astronomer Royal Sir George Airy. It is also called the diffraction or spurious disk.

Algorithm - An algorithm is a series of mathematical instructions to solve a problem or produce a result in a step-by-step procedure.

Aliasing - Diagonal lines that appear stair-stepped or jagged.

Alt-azimuth - Altitude-Azimuth. A telescope that moves in two axes, one left-to-right around a turntable base, and one up and down. Any part of the sky can be located with movement in these two axes.

Amp Glow - A red glow in the corners and edges of a long exposure image caused by electrons in the metal-oxide semiconductor field-effect transistors (MOSFET) that are in the readout amplifier of the camera's sensor. Amp glow is not caused by heat, it is caused by electroluminescence. If not too severe, it can be removed by subtraction of a dark frame.

Analog - Continuously varying information that represents an infinite number of values. Analog data is usually sampled in a discrete, finite number of steps before it is turned into digital data.

Aperture - The size of the opening that lets light into a camera lens, which can usually be adjusted and changed. In a telescope, the aperture is usually fixed, and is defined by the size of the primary mirror or objective lens.

Array - A grid or rectangular arrangement of elements. In a digital camera it refers to arrangement of individual pixels in a sensor. The array size is specified by the number of pixels in the x and y dimensions of the chip, for example, the Canon 20Da has an array size of 3504 x 2336 pixels, or 8,185,34 total pixels.

Artifacts - Artificial, spurious defects in an image that were not part of the original image or data.

Autoguider - A separate CCD or Webcam that is used to send corrections to the telescope's mounting to guide or follow the stars with high accuracy to compensate for inaccuracies in the mount's tracking.

Averaging - adding together a number of frames and dividing by the total. Averaging many frames together increases the signal-to-noise ratio in an image by the square root of the total number of images used. It works because the signal, which is constant, is reinforced, while the noise, which is random, is averaged out.

Banding (Posterization) - Visible steps in an area that is supposed to be even-toned or smoothly varying, such as a gradient of brightness in a blank sky that fades from light to dark. Banding is usually caused by an insufficient number of digital steps of tone.

Bayer Array - A CCD or CMOS sensor that has an individual red, green or blue filter over each pixel to synthesize color information from a grayscale sensor. The "Bayer" pattern is also called a Color Filter Array (CFA). It is named after Bryce E. Bayer, the Kodak engineer who invented it. The colored filters are arranged so that out of each four-pixel group, two will be green, one red, and one blue. Bayer found that because human perception was more sensitive to luminance (brightness) information in the green portion of the spectrum, increasing luminance resolution by using more green pixels meant more perceived sharpness in the final image. Color for each individual pixel is created by examining the color of the pixels around it and then processing this information through a sophisticated mathematical algorithm.

Bias - A low-level charge that is applied to a CCD or CMOS sensor in the form of a fixed offset voltage value.

Bias Frame - A zero second exposure that records the bias signal present in every frame. Used in advanced calibration.

Binary - 1.) Made of two components. 2.) Using binary numbers in the base 2 number system. All computers are binary because the transistors of which they are made have only two states, on and off, which represent the numbers 0 and 1, the two numbers in the base 2 number system. 3.)Two stars that are gravitationally bound together and orbit around a common center of mass.

Bit - A contraction of "binary digit", representing the number 1 or 0 in the base 2 number system that computers work with.

Bit Depth - Describes the number of steps of tonal resolution, or brightness levels, that the dynamic range is divided into when it is quantized by the Analog to Digital converter. Bit depth is specified in base 2 notation. Eight bits is 2⁸ or 2 raised to the 8th power. 8 bits equals 256 steps or levels of information. 12-bits is is 2¹² and equals 4096 levels. 16-bits is is 2¹⁶ and equals 65,536 levels.

Bitmap - A bitmap is an image that is made of a grid of pixels. Each pixel's brightness and color information is described in terms of bits, and this information is mapped to a specific location.

Black Point - The darkest area of an image that is mapped to level 0 (completely black) when setting the dynamic range during a levels adjustment in image processing.

Blooming - Electrons that spill over into adjacent wells when a potential well is full cause blooming, which appears as vertical spikes from bright stars that are overexposed.

Brightness Value (Gray Level, Gray Value, Pixel Value, Digital Number) - The brightness of a pixel described as a number. Brightness levels range from 0 for black to 255 for white in an 8-bit image. They can range from 0 to 4095 for a 12-bit image, and 0 to 65,535 for a 16-bit image.

Bulb - The name given to the exposure setting where the shutter stays open for a long time exposure until it is closed by the photographer. Derives from the ancient days of film photography when an air bulb was used to pneumatically open a leaf shutter in a camera lens.

Byte - A basic computer unit of data made up of 8 bits.

Calibrate - 1.) To remove unwanted fixed signals (such as thermal current and bias), and to correct signal modifications (such as vignetting) so that the Raw image accurately represents the intensity of light incident on the sensor during the exposure. 2.) To adjust a computer monitor into compliance with a pre-defined standard.

CCD - Charged Coupled Device. A light-sensitive solid-state silicon sensor used in digital cameras to record light intensities.

Celestial Equator - The Earth's equator projected onto the celestial sphere.

Celestial Poles - The Earth's axis of rotation projected onto the celestial sphere. The celestial poles on the celestial sphere correspond to the North and South rotational poles on the Earth.

Celestial Sphere - The three-dimensional sky seemingly projected onto the inside two-dimensional surface of an imaginary, infinite sphere surrounding the Earth. The Sun, Moon, Planets and stars appear to be located on this celestial sphere. A celestial coordinate system of right ascension (similar to longitude) and declination (similar to latitude) allows location of objects in the sky.

CFA (Color Filter Array) - A CFA image is a monochrome (black and white and grayscale) image containing data from the Bayer array in the CCD or CMOS sensor in the DSLR camera that has not yet been interpolated into color. It presents the data from each individual filter, which has a red, green or blue color filter over it, as a grayscale tone with a brightness value.

Charge - The amount of electrical energy measured by the quantity of electrons stored. In digital imaging, the charge created by electrons generated by photons by the photoelectric effect that are stored in a pixel's potential well.

Clip, Clipping - To lose, cut off, or saturate data at either end, black or white, of the dynamic range by overexposure or incorrect manipulation of data.

Color Filter Array (CFA) - An arrangement of individual color filters over individual pixels in a CCD or CMOS detector designed to synthesize color from a grayscale sensor. A Bayer array using red, green and blue pixels is the most common type of CFA.

Colour Depth - The number of steps or levels of tone that each primary color of the total dynamic range is divided into. In 24-bit color, 8 bits of color depth are assigned to each color channel of red, green and blue is represented by 256 steps. This yields more than 16 million total colors (256 * 256 * 256 = 16,777,216).

Color Gamut - A unique range of colors that a device can capture or represent.

Color Management System (CMS) - Color management is a way of trying to keep colors consistent across a range of input, display and output devices by the use of software.

Color Space - A color space is a three-dimensional model for mathematically representing colors with numbers in a coordinate system. A color space is defined by a set of three primaries and a white point. In a given color model, we can have various different color spaces. For instance, in the RGB color model, we can have these color spaces (among many others):

• Adobe RGB (1998)
• sRGB
• Apple RGB
• ColorMatch RGB

Combine - To merge individual images into a "master" image by adding, averaging or other mathematical operations, or to join individual frames into a single picture that is a larger mosaic.

Composite - In digital photography, to 1.) combine different individual images sub frames of the same exposure into a single master image. 2.) to create an image with an extended dynamic range by combining exposures of different lengths through the use of masks, or functions such as Photoshop CS2's High Dynamic Range (HDR) command.

Compression - Compacting digital data so that it takes up less space. There are two kinds of compression: lossy and lossless. Lossy compression throws away original data. JPEG is an example of Lossy compression. Lossless compression does not throw away any of the original data. LZW is an example of Lossless compression.

Continuous-Tone - A smooth transition from one tone to another without any visible steps or banding. Continuous-tone usually refers to analog representations which have an infinite series of tones.

Convolution - Convolution is when one function modifies another, such as blurring from poor seeing modifying the point spread function of a star.

CMOS - Complimentary Metal Oxide Semiconductor. A solid-state electronic chip in a camera that can act as a sensor to detect photons, convert them into electrons and then a voltage that can be measured and digitized.

CMYK - A subtractive color printing system where the primary colors are comprised of Cyan, Magenta, Yellow and Black components. The pigments in inks of these colors subtract from white light. All of these primary colors mixed together equally in maximum intensity result in black.

CRW - Canon Raw File format. A proprietary file format with the .CRW file extension for storing the original data from a Canon digital camera.

CR2 - The second generation of Canon's Raw File Format with a .CR2 file extension.

Dark (Thermal) Current - Signal that is generated by electrons created in the silicon substrate of a CCD or CMOS sensor by heat. Dark current increases as temperature goes up. This happens even when the sensor is not exposed to light, hence the name "dark current". This signal is more properly called thermal current signal because it is present in all frames, light and dark. Thermal current signal can be removed by the proper subtraction of a dark frame.

Dark (Thermal) Current Noise - Noise that is generated because of statistical variations in thermal (dark) current, equal to the square root of the thermal current. Because it is random, it cannot be removed from an image.

Dark Frame - A dark frame is a photo taken at the same exposure, ISO and temperature as a light frame, but with no light reaching the sensor. It is a picture of the camera's thermal current and bias current. Dark frames are used in calibration of digital image data in light frames. By subtracting a dark frame from a light frame, thermal current signal and bias signal are removed.

Dark Frame Subtraction - Removing thermal current signal by subtracting a dark frame. This can be done automatically as a custom function in some DSLR cameras, or manually in later processing.

DDP - Digital Development Processing is image processing that takes linear data and modifies it by gamma stretching and unsharp masking so that the shadow and highlight areas simulate traditional photographic images, which are non-linear.

Dead Pixel - An individual pixel in a sensor that does not respond to light and appears as black in an image.

Deconvolution - Deconvolution is a process that attempts to remove the effects of convolution. For example, deconvolution was used to attempt to restore the aberrated images in the Hubble Space Telescope that were caused by spherical aberration.

Diffraction - The bending of light waves as they pass through an aperture or go around an obstruction. Diffraction causes stars to appear as disks at the focal plane instead of points.

Digital - Represented by numbers.

Digitize - To convert continuous analog data or image to digital data.

DNG - Adobe's Digital Negative Raw format. Adobe's attempt to create a non-proprietary, cross-platform, cross-manufacturer Raw negative file format that any software can open.

DPI - Dots Per Inch. A measure of resolution that refers to the number of dots a printer can print in an inch of output. Higher resolution means more dots per inch. Often mistakenly used for PPI, or pixels per inch. It more correctly applies to output devices that print with dots, such as inkjet printers.

DSLR - Digital Single Lens Reflex. A camera that uses a mirror to intercept the light from the camera's lens and send it to a focusing screen for inspection by the photographer's eye. The reflex mirror swings up and out of the way when the picture is taken, allowing the light to reach the digital sensor.

Dust Donuts - Out of focus shadows that look like donuts that are caused pieces of dust on the sensor or cover glass in front of the sensor.

Dynamic Range - The range of brightness from light to dark in which detail can be recorded.

EOS, - Electro-Optical System. Canon's system where instructions for focus and aperture are sent to the control motors in the lens from the camera body by electrical signals instead of the traditional mechanical methods such as levers.

Equatorial Mount - A telescope mount designed with two axes, one of which (the Polar axis) is made parallel to the Earth's axis of rotation. Movement in this single axis allows celestial objects to be followed to compensate for the Earth's rotation. Any object in the sky can be found by a combination of movements in the two axes. The polar axis corresponds to right ascension, and the other axis to declination.

EXIF - EXchangeable Image File. A storage format that allows technical and other information about the camera, exposure, ISO, lens focal length, and many other settings to be stored along with the actual image data in a file. Other caption information can also be stored in the file. It is essentially a file "wrapper" around a JPEG or TIFF file.

Expose - To make an exposure, or to open the shutter to take a photograph.

Exposure - The length of time that the shutter is open and light is hitting the sensor in the camera.

Eyepiece Projection - A method of photography where the image is formed at the focal plane of the camera by projection by the eyepiece in a telescope. No camera lens is used on the camera, only the telescope's eyepiece is used in the scope.

Field of View (FOV) - The amount of a scene that is captured by a given focal-length lens. Wide-angle, short-focal length lenses capture a wide field of view. Telephoto lenses and telescopes capture a very narrow field of view. The field of view is usually specified as an angle that depends on the size of the sensor. For example, a lens or telescope of 500mm focal length will cover a field of view of 4 degrees, 17 arc minutes, 43 arc seconds by 2 degrees 51 arc minutes, 51 arc seconds with a Canon 20Da DSLR camera sensor that is 22.5 x 15mm.

File Format - The structure of a specific type of computer file. Different file formats are associated with different file types and programs. For example, JPEG and TIFF file formats are associated with image files. Their codes are JPG and TIF respectively. Files of a particular format are given a specific file extension in the form of a three letter code. The name of the file is separated from the file format code by a period, such as ORION.JPG, where ORION is the name of the file followed by a period and the extension code for the JPEG file format.

Fill Factor - The percentage of a photosite (pixel) that is actually sensitive to light. For example a CMOS sensor may only have a fill factor of 40 percent, so 60 percent of an individual photosite cannot collect photons. Designers sometimes use a microlens to collect photons over a wider area to increase the fill factor by sending the photons to the light sensitive portion of the photosite.

Filter - 1.) A piece of glass or gelatin placed in the optical path that modifies the wavelength or light that ultimately reaches the sensor. An example would be a hydrogen-alpha filter that only allows the light of the hydrogen-alpha wavelength to pass. 2.) A piece of software that performs particular algorithms on digital data. An example would be a Gaussian blur filter in Photoshop that blurs an image.

Firmware - Instructions and software stored permanently in read-only memory, usually for control of a specific device such as a digital camera or CD-R drive. The firmware for most digital cameras can be updated to correct a problem when a manufacturer finds a bug in the way the camera operates.

FITS - Flexible Image Transport System. A file format specifically developed for scientific images that are designated by the .FIT file extension. Non-image data can also be stored in a FITS file.

Fixed-Pattern Noise: Fixed-pattern noise is pattern that repeats in the same location in an image from frame to frame. It can be removed with proper calibration procedures. Technically it should be called fixed-pattern signal since noise is random and does not repeat.

Flatten - A term used in Photoshop that means to merge separate layers of corrections or masked data information in an image.

Flat-Field Frame - A calibration frame taken of an evenly illuminated subject used to correct for vignetting, uneven illumination, dust on the sensor's cover glass, and uneven pixel response.

Frame - 1.) Used as a noun, a frame is an image or an exposure. Derived from the days of film where images were taken on a roll of film and each individual image was called a frame. In digital astrophotograhy, you can have light frames, dark frames, bias frames and flat-field frames.  2.) Used as a verb, to frame means to compose the subject inside of the viewfinder. For example, you want to "frame" the Orion Nebula so that none of the faint outer nebulosity gets cut off.

F/Stop - The designation marker on a lens that indicates the focal ratio that is being created by stopping down the aperture of the lens with an internal diaphragm inside of the lens. The f/stop is same thing as the focal ratio or f/ratio. It is defined as the ratio between the aperture and focal length of an optical system. For example, a 125 mm aperture telescope with a focal length of 1,000 mm has a focal ratio of 1000/125 or f/8.

Focal Length - The distance from the lens or mirror in an optical system and the focal plane where the light is focused.

Focal Ratio (F/ratio) - The ratio between the aperture and focal length of a lens or telescope. For example, a 125 mm aperture telescope with a focal length of 1000 mm has a focal ratio of 1000/125 or f/8.

Focal Reducer (Telecompressor) - An optical component made of a lens or glass elements that decrease the focal length (and focal ratio) of a telescope.

Focus - Focus is when the focal plane of an optical system is coincident with the focal plane of an eyepiece or digital camera. An optical system can be considered focused when the maximum amount of light from a star is concentrated into the smallest possible area at the photodetector sensor surface, yielding the smallest possible star size.

Full Well Capacity - The number of electrons that can be stored in a potential well in a photosite (pixel) in a CCD or CMOS sensor. Full well capacity goes down as the ISO goes up in a digital camera.

Full Width Half Maximum (FWHM) - A measurement of the diameter of a star where the intensity is 50 percent of the star's maximum brightness value.

Gain - Gain defines how many electrons are represented by each Analog to Digital Unit (ADU). A gain of 4 means that the A/D converter has digitized the signal so that each ADU corresponds to 4 electrons. DSLR cameras can change ISO by changing the gain.

Gamma - In photography, gamma refers to the midtone contrast of an image. Technically, gamma refers to the relationship between input voltage and output intensity, where gamma is the exponent in a power-law relationship between input values and output displayed brightness, such as in a computer monitor.

Gamut - The range of colors that can be detected, recorded or displayed by an imaging device. Each device has a unique gamut.

Gaussian Blur - A mathematical function that is applied to an image to blur it based on a bell-shaped curve or Gaussian distribution, named after the famous German mathematician Karl Friedrich Gauss.

Gegenschein - A relatively large, but very subtle oval brightening in the night sky several degrees across located at the anti-solar point. This brightening can only be seen from extremely dark skies with no light pollution or moonlight. The Gegenschein, which is German for "counter-glow" is caused by sunlight scattered by fine interplanetary dust particles in orbit around the Sun. It is connected to the Zodiacal light by the Zodiacal bridge or band.

GIF - Graphic Interchange Format, an image file format with a .GIF file extension that supports only 256 colors that is useful for non-pictorial images, such as line drawings and diagrams.

Grayscale - Containing no color, only various shades of gray in a black and white image.

Gray Level (Gray Value, Brightness Value) - The brightness level of a monochrome (not color) pixel indicated by a number. Gray levels run from 0 to 255 for an 8-bit image.

Guiding - Manually or automatically following a star by making corrections in right ascension and declination to produce higher tracking accuracy.

High Dynamic Range (HDR) - 1.) An image with a larger dynamic range than usual. 2.) A method of combining different exposures to extend the dynamic range recorded in an image, such as Photoshop CS2's HDR function.

Highlight - The brightest areas of an image that contain detail.

Histogram - An image histogram is simply a bar graph that shows the number of pixels at each brightness level in an image. A histogram runs from pure black with a brightness value of 0 on the left to pure white with a brightness value of 255 on the right hand side of the graph.

Hot Pixel - A pixel that registers a brightness value much higher than it should based on the incident light that hit it.

Hydrogen Alpha - A specific emission line of ionized hydrogen at 656.3 nanometers. Hydrogen-alpha emissions are responsible for the red color in emission nebulas.

Image - 1.) Used as a noun, an image is simply a picture. For example, "That is a very nice image of the Orion Nebula." 2.)Used as a verb, to image is to take a picture. For example, "Tonight I am going to image the Veil Nebula."

Image Scale - The size on an image formed by a lens or telescope based on the magnification of the optical system. Image scale is usually measured in a digital camera as arc seconds per micron, or arc seconds per pixel.

Infrared (IR) - Long wavelengths of light beyond the visible portion of the spectrum, typically between 770 nanometers and 1 millimeter.

Integration - Collecting photons for a given exposure time to accumulate a charge or signal in a digital sensor. integration time is essentially equivalent to exposure time.

Interpolation - A mathematical procedure for increasing resolution by up-sampling, or decreasing resolution by down-sampling. Up-sampling creates new data from existing data and increases file sizes. It is not real data though, it is the algorithm's best guess at what the real data would have been if it had actually existed. Down-sampling lowers resolution and decreases file size by throwing away real data in existing pixels and creating new pixels.

ISO - An international standard published by the International Organization for Standardization. In the field of photography, the term ISO is used as a shorthand name for the standard defined by the specification for determining the sensitivity to light of film or a digital camera sensor. In film, a higher ISO number means the film is more sensitive to light. Digital camera sensors really only have one sensitivity to light though. Changing the ISO on a digital camera changes the gain in the camera, seemingly changing the sensitivity.

JPEG - A file format with a .JPG extension that compresses image image data according to a standard algorithm as defined by the Joint Photographic Experts Group (JPEG). JPEG compression is lossy, it throws image data away, but in a method that reduces the visual impact at reasonable compression ratios.

Kilobyte - 1024 bytes.

Lab Color - Technically called L*a*b* color, Lab Color is a color model that encompasses all of the colors that the human eye can see, defined mathematically in a device-independent method that is perceptually uniform. The L* channel contains the luminance information. Two other channels are the chromatic channels that contain the color information. The a* channel contains the red-green color axis and the b* channel contains the blue-yellow axis.

Layers - A separate channel from the color channels used to store information in a photo-editing program, such as Photoshop. Layers can be used for color and tonal adjustments without altering the original data in the file until the layer is merged or "flattened" with the other channels. Layers also allow the creation of masks for use in an image.

Levels - Individual steps of brightness in an image. The Levels command in Photoshop allows adjustment of an images black and white points and mid-tone gamma in each individual color channel.

Light - Light is a form of radiant energy that we can see with our eyes, and record with film and CCD cameras. Visible light (400 nm to 770 nm) is the portion of the electromagnetic spectrum of energy that can be detected by the rods and cones in the retina of our eyes and that causes the sensation of vision in our mind. The nature of light is not completely understood. It is a complicated subject because it involves two of the deepest mysteries of which we are now aware - human consciousness and quantum mechanics.

Light Box Flat - A flat-field frame created with a light box with an artificial light source.

Light Frame - An exposure to the light from the subject through a lens or telescope.

Light Year - A measure of the distance (not time!) that light travels in vacuum in one year of Earth time, equal to 9.46 trillion kilometers or 5.88 trillion miles.

Linear Response - A response in a digital sensor where the output directly corresponds to the input signal. Most CCD and CMOS sensors have a linear response to light, that means that a doubling of exposure time results in a doubling of brightness in the recorded image.

Lossless Compression - A compression method, such as LZW, where the original data is completely preserved and no information is thrown away. Lossless compression usually only results in a modest saving in file size for images.

Lossy Compression - A compression method, such as JPEG, where data is thrown away to gain increased compression ratios and smaller file sizes.

LRGB - A method of creating an image where high-resolution black and white Luminance data is combined with lower-resolution RGB color data to decrease total exposure time. The L in LRGB stands for Luminance.

Magnitude - A scale for measuring the brightness of a celestial object. Each magnitude varies by a factor of 2.512. The brightest star in the sky is Sirius at magnitude -1.4. The apparent magnitude of an object is how bright the object seems from the Earth. With two objects of the same intrinsic luminosity or absolute magnitude will appear with different apparent magnitudes if they are located at different distances from the Earth with the nearer one appearing brighter. On the magnitude scale, the lower the number the brighter the object. Diffuse objects are measured in magnitudes per square arc second, as if starlight from a point source was spread out over an area of one square arc second.

Mask - An overlay that blocks certain portions of an image so that other portions can be selectively manipulated.

Master - In Photoshop and other image processing programs, a mask is used to block out certain areas of an image so that corrections and filters can be selectively applied. Masks range from black, where no effect is applied, to white where 100 percent of the effect is applied. Shades of gray in a mask allow varying percentages of the effect to get through in proportion to the brightness of the mask.

Median Combine - A method of combining images mathematically where the middle value is used out of a distribution of different values, above and below which lie an equal number of values. A median combine is useful for combining a group of images to remove renegade pixels that might result from cosmic ray strikes in different locations in each image.

Megapixel - A CCD or CMOS sensor that has one million pixels. A 5 megapixel camera has 5 million pixels.

Micron (Micrometer) - One millionth of a meter. Also equal to one thousandth of a millimeter. Abbreviated µm.

Microlens - An extremely small lens that goes over a photosite (pixel) to direct photons to the light sensitive photodiode that comprises only a part of the total area of the entire photosite. This increases the sensitivity of the detector and improves the fill factor.

Millimeter - One thousandth of a meter. Abbreviated mm. There are 25.4 millimeters in one inch.

Mosaic - 1.) In digital sensors, an arrangement of non-overlapping tiles or pixels that constitute the sensor array, such as a Bayer pattern in a DSLR CCD or CMOS sensor. 2.) In an astrophotographic image, a mosaic is a wider-angle picture made up of a series of narrower-angle pictures. Each individual tile in the larger picture is shot so that there is some overlap with the tile next to it so that individual images can be correctly aligned. When the individual images are put together like a puzzle to form the larger image, the edges of the tiles that overlap are blended together seamlessly so that the edges are invisible. This creates a higher-resolution images with wider fields of view than would normally be possible.

Narrowband Imaging - The use of narrowband filters, which pass only selective wavelengths of light, to take an astrophotograph. For example, a narrowband Hydrogen-Alpha filter passes only a narrow window of wavelengths centered around 656.3 nm, the Hydrogen-Alpha wavelength of red emission nebula. By filtering out the rest of the spectrum, much more contrast is gained in the wavelength of interest, at the cost of increase exposure time. Some typical filters for narrowband imaging are Hydrogen-Alpha, Oxygen-III, and Sulfur-II.

NEF - Nikon Electronic Format. A proprietary Raw file format with an .NEF extension for Nikon Digital cameras.

Noise - Technically, random and non-repeatable signal in an image. In common use in digital photography, any unwanted or undesirable signal that does not convey useful information. For example, a dark frame is composed of thermal current signal, thermal signal noise (and bias). Thermal and bias signals are technically not noise because they are consistently repeatable, and this is how we are able to remove them by subtraction with a calibration frame. Thermal signal noise is random and cannot be removed. However many people refer to thermal current as "noise".

Non-Linear Processing -

Normalization - Applying a mathematical function like multiplication to data from one image to make it match another. For example, multiplying each pixel's brightness value by 2x in a 30 second exposure to make it match the pixel values in a 1 minute exposure.

Nyquist Sampling Theorem - A theorem in communications theory, formulated by Harry Nyquist in 1928, that says when converting an analog wave form to digital data, the sampling must be at two times the highest frequency of the original to preserve all of the information in the original. The theorem can also be applied to spatial information such as high-resolution detail in an image that is sampled by the pixels in a digital sensor.

Offset (Bias) - A low-level charge that is applied to a CCD or CMOS sensor in the form of a fixed offset voltage value.

Offset (Bias) Frame - A zero second exposure that records the offset signal present in every frame. Used in advanced image calibration.

One-Shot Color - Color that is created in one exposure, such as with a Bayer array on a CCD or CMOS camera. Since these sensors are really monochrome grayscale devices, creating color with individual filters would usually require three separate exposures, one for each of the red, green and blue filters. A Bayer array places a pattern of red, green and blue filters over the pixels in a sensor and through interpolation creates color for each pixel location in a single exposure.

Periodic Error - An error in tracking that repeats with a regular period in an equatorial mount, usually the period of revolution of the worm gear. It is caused by imperfections in the gears which are used to move the telescope at the same rate that the Earth rotates.

Photon - A quantum of electromagnetic energy usually associated with light. Photons appear to be both waves and particles simultaneously.

Photodiode - An photodetector that converts light (photons) into an electric charge (electrons) through the photoelectric effect. The photodiode is the heart of the sensor in a digital camera.

Photoelectric Effect - The ejection of electrons from the surface of a substrate such as silicon caused by the energy contained in photons, such as in a photodiode in a CCD or CMOS sensor. The Photoelectric effect was explained by Einstein in 1905, for which he won a Nobel prize for Physics in 1921. It is the fundamental reason that silicon sensors can be used for digital photography. The energy of the photons that hit a photodiode release electrons that are stored in a potential well. These electrons form an electric current that is measured as a voltage. This analog voltage is amplified and then turned into a digital value (simply a number) by the Analog to Digital converter. The amount of current is proportional to the number of photons that hit the detector.

Photoelectron - An electron that is released through the photoelectric effect when a photon is absorbed in the silicon substrate of a photodetector in a digital camera.

Photon (Shot) Noise - Noise that is created because of the statistical variation of photon emission by a light source over time.

Photon Noise Limited - An image is photon-noise limited when the exposure is of sufficient length so that photon noise is the major source of noise in the image instead of read-out noise. Short exposures do not record many photons and therefore do not have much photon noise. The noise in short exposures is predominantly read-out noise and these short exposures are called read-out noise limited.

Photosite - An individual square in a sensor array that contains a photodiode and storage area for electrons. Also frequently called a pixel. In a CMOS chip a photosite may also contain additional electronics such as an amplifier, noise reduction circuitry and an analog to digital converter.

Piggyback - Mounting a camera and lens on top of a telescope on a mounting. Piggyback photography is usually used for long-exposure wide-angle astrophotography where the camera and lens take the picture while riding on top of a telescope on an equatorial mount that is polar aligned and tracking the stars to compensate for the Earth's rotation.

Pixel - A "Picture Element". In a digital camera, it refers to an individual photosite on the CCD or CMOS sensor. In the image it refers to the smallest building block out of which the image is made of. A pixel on the sensor corresponds one to one with a pixel in the final image.

Pixel Array - A grid or rectangular arrangement of pixels in the CCD or CMOS sensor in a digital camera.

Pixel Size - The physical size of the pixel in the pixel array in the sensor. Usually measured in microns. For example, the size of each individual pixel in the Canon 20Da camera is 6.4 microns square.

Pixel Well - An area in a photosite where electrons are stored that are released from the silicon surface by the energy of impacting photons through the photoelectric effect. Also called a potential well.

Pixelization - Pixelization occurs when an image is enlarged so much that individual pixels become visible.

Point Spread Function (PSF) - A mathematical description of how the light from a theoretical point source like a star is spread out by seeing, diffraction, optical quality, tracking accuracy, and the resolution of the sensor.

Poisson distribution - A mathematical probability function that describes the distribution of a randomly occuring event over a specific time interval. For example, photons emitted by a source of constant intensity are not output at a perfectly constant rate over time. In one minute, 100 photons may be counted. In the next minute, 110 photons may be counted. The minute after that, only 90 photons may be counted. The actual intensity of the source can be known only to the square root of the total number of photons that are measured. The result, over time, is a Poisson distribution which looks like a bell-shaped curve. A Poisson distribution is named after French mathematician Simeon-Denis Poisson, who developed the math.

Polar Alignment - Making the polar axis of an equatorial mount parallel to the Earth's axis of rotation by pointing it accurately at the North Celestial Pole in the Northern Hemisphere, or the South Celestial Pole in the Southern Hemisphere.

Posterization (Banding) - Visible steps in an area that is supposed to be even-toned or smoothly varying, such as a gradient of brightness in a blank sky that fades from light to dark. Banding is usually caused by an insufficient number of digital steps of tone.

Potential Well - An area in a photosite where electrons are stored that are released from the silicon surface by the energy of impacting photons through the photoelectric effect. Also called a pixel well.

PPI - Pixels Per Inch. A basic measurement of resolution. More pixels per inch yield higher resolution images.

Prime Focus - Prime focus describes a camera attached to a telescope without any other eyepieces or camera lenses in the optical path. The telescope then acts as the camera lens.

Qualitative Analysis - Evaluation based on subjective judgement.

Quantitative Analysis - Evaluation based on objective, measurable quantities.

Quantum Efficiency - The percentage of photons that hit a CCD or CMOS sensor that are detected and turned into photoelectrons. Quantum efficiency varies by the wavelength of light. It can also vary by the color of the filter over an individual pixel in a Bayer array.

Raster Data - In digital photography, data that is represented by a grid of pixels that make up an image. Usually used for normal pictorial images. Vector Data is a mathematical description of the data that allows unlimited scaling of the image, usually used for typefaces and graphic line images and illustrations.

Raw - Unprocessed data directly from the sensor.

Readout Amplifier - An electronic circuit that amplifies the signal after the charge in a digital sensor has been converted to a voltage and before it is sent to the analog to digital converter. The readout amplifier is the main source of readout noise in an image.

Read Noise (Read-Out Noise) - Electronic noise that is generated by the readout amplifier as the signal is read out, amplified, and sent to the analog to digital converter.

Read-Noise Limited - The majority of noise in a digital image comes from photon noise, dark current noise and read noise. Dark current noise and read noise dominate when the total number of photons being gathered is small. Photon noise dominates when a large number of photons are gathered in long exposures. Short exposures are usually called "read-noise" limited because the read-noise dominates.

Resampling - Resizing an image by mathematical algorithms that examine neighboring existing pixels and create new ones based on this analysis.

Resolution - Spatial Resolution is the number of pixels that we have in an image, and the size of the space that these pixels are contained in. Two parameters are necessary to specify resolution: the number of pixels per inch or centimeter and the total number of inches or centimeters. More pixels in a given space mean higher resolution. Tonal resolution specifies the number of steps of tone that the dynamic range is divided into.

RGB - Red, Green and Blue. These are the three primary colors, out of which all other colors can be created, in the additive color model.

Sampling - Measurement in discrete, regular intervals. Spatial sampling in a digital camera is done by the number of pixels in a given sized area sensor. Tonal sampling is determined by the bit-depth of the analog to digital converter. Correct spatial sampling in high-resolution astrophotography matches the sample size (pixel size) to the size of the Airy disk and seeing, based on the Nyquist sampling theorem.

Saturation - 1.) Tonal or pixel values on the bright end of the dynamic range that are maxed out and contain no detail. 2.) The purity or vividness of a color.

Scaling - 1.) Changing the black or white endpoints in image histogram to modify the data so that it changes its distribution in the dynamic range. 2.) Enlarging or reducing the size of an image.

Seeing - The steadiness of the atmosphere that allows fine details to be seen in celestial objects. If the seeing is good, detail is not blurred as much by atmospheric scintillation. Scintillation is what causes the stars to twinkle, which may be appreciated poetically, but usually means the seeing is not that good. Thermal gradients at different elevations in the atmosphere are usually responsible for the quality of the seeing. Seeing also usually deteriorates for objects closer to the horizon because the light has to pass through a greater air mass.

Sensor - Usually refers to the CCD or CMOS chip in the camera that senses photons of light and turns them into electrons which ultimately end up as digitized numbers that represent the light that hit the sensor.

Shadow Area - Dark areas in an image that contain detail.

Shoot - To take a picture.

Signal - In a digital camera, signal is an electric current or voltage, whose variations represent information. For example, the number of electrons released through the photoelectric effect from photons from a star forms a current that represents the brightness of the star. Signals can be interesting, such as those from astronomical objects, or not interesting, such as that from thermal current.

Signal-to-Noise Ratio - A measure of the quality of a signal, expressed as the ratio of the signal to the noise present.

Sky Flat - A flat-field frame that uses the clear sky as a light source.

Stack, Stacking - A generic term that means to combine images by any of several mathematical processes such as averaging, or addition. The term originates in the days of film astrophotography where images were literally stacked on top of one another to improve contrast, color and signal-to-noise ratio.

Stretching - Redefining the black or white points in an image to increase the contrast.

Spatial Resolution - The amount of detail contained in a given space. In digital imaging, spatial resolution is defined by the number of pixels per unit area.

Spectral Sensitivity - The wavelengths of light to which the CCD or CMOS chip is sensitive.

Sub Exposure - A sub exposure is one of many shorter exposures that are made and then added or averaged together to equal a single longer exposure. Same as a Sub Frame.

Sub Frame - A sub frame is one of many shorter exposures that are made and then added or averaged together to equal a single longer exposure. Same as a Sub Exposure.

Subtractive Color - A color model where color is created by absorption of light by ink or pigments, for example, CMYK color printing.

Summing - Mathematically adding together individual shorter-exposure images to create the equivalent of a longer-exposure image.

Support Frames - Frames that are used to calibrate a light frame image, such as darks, flat-fields, and bias frames.

System Gain - The total gain in a system that defines how many electrons are represented by each Analog to Digital Unit (ADU). A gain of 4 means that the A/D converter has digitized the signal so that each ADU corresponds to 4 electrons. DSLR cameras can change ISO by changing the gain.

System Noise - The total noise generated in the camera by various different sources such as interference, dark noise, photon noise and read noise.

Target - The astronomical object of interest.

Telecompressor (Focal Reducer) - An optical component made of a lens or glass elements that decrease the focal length (and focal ratio) of a telescope. For example, a 0.75x telecompressor will make a 1,000mm focal length f/8 optical system into a 750mm f/6 optical system.

Tele-extender - An optical component made of a lens or glass elements that increases the focal length (and focal ratio) of a telescope. For example, a 2x tele-extender will turn a 1,000mm focal length f/8 optical system into a 2,000mm f/16 optical system.

Thermal Current - Signal that is created from electrons released by the thermal energy in the sensor substrate, even when it is not exposed to light.

Thermal Frame - A calibration frame that is comprised of the thermal current present in a sensor at a given exposure, ISO and temperature. A thermal frame is created by subtracting a bias frame from a dark frame.

Thermal Noise - Noise that is generated because of statistical variations in thermal (dark) current, equal to the square root of the thermal current. Because it is random, it cannot be removed from an image.

TIFF - Tagged Image File Format. An image file format with the .TIF file extension. The TIFF file format has become a standard for storing uncompressed images.

Tonal Range - The range of tones present in an image from black to white. Also known as the dynamic range.

Tonal Resolution - The number of steps that the dynamic range is divided into as specified by the bit-depth of the analog to digital converter.

Tracking - Following a star with a telescope to compensate for the Earth's rotation.

Transparency - The clarity of the atmosphere.

Twilight Flat - A flat-field frame that uses the clear twilight sky as a light source.

Ultraviolet [UV] - The short wavelength region of the spectrum below blue and violet from about 10 nanometers to 380 nanometers.

Unsharp Mask - An image processing technique used to sharpen detail. The term derives from the graphic arts industry where a blurred positive copy of a negative image was registered with it and then contact printed, masking low-frequency information, allowing the contrast of the remaining high-frequency information to be increased, resulting in more apparent sharpness. The same effect is now accomplished in software in a somewhat analogous process.

USB - Universal Serial Bus. A protocol and hardware system for transferring data from peripherals to a computer over cables.

Vignetting - Light falloff in the corners of an image due to optical, geometrical or mechanical reasons.

Well Depth - How many electrons can be accumulated in an individual charge well in a photosite in a CCD or CMOS sensor. Well depth determines the available dynamic range once noise is factored in.

White Balance - Adjusting the color in an image to compensate for the color temperature of the illumination source.

White Point - The brightest area of an image that is mapped to the highest level available (pure white) based on the bit-depth of the image (Level 255 for an 8-bit image) when setting the dynamic range during a levels adjustment in image processing.

X-Rays - Short-wavelength, high-frequency electromagnetic radiation that falls between ultraviolet and gamma rays in the electromagnetic spectrum. X-rays have a wavelength of approximately 0,1 to 10 nanometers.

Year - The amount of time it takes for the Earth to make one revolution around the Sun. There are several different kinds of year based on the frame of reference for the Sun and the Earth. For example, the sidereal year is based on the Earth position against the stars from the viewpoint of the Sun. The tropical year is the time between two vernal equinoxes.

Zodiacal Light - A large triangular-shaped cone of light that is visible at a dark-sky location in the west after the end of evening twilight and in the east before the start of morning twilight. Centered on the ecliptic, it is caused by sunlight that is scattered off fine particles of dust in orbit around the Sun. The particles have diameters between a couple of micrometers and a few millimeters.

Canon Glossary of Film Terms

Digital Photo Review Photography and Imaging Glossary

Digital Dingus Glossary

Enchanted Learning Astronomy Glossary

Wikipedia

The Encyclopedia of Astrobiology, Astronomy and Spaceflight by David Darling

A Glossary for Telescope Buyers and Users by Jay Reynolds Freeman