1:1 Pixel Resolution - Where one pixel's recorded data on the sensor equals one pixel's data in the output image.
480p - Standard-definition video usually recorded at 640 pixels horizontal by 480 pixels vertical resolution with progressive scan.
720p - High-definition video recorded at 1280 pixels horizontal by 720 pixels vertical resolution with progressive scan.
1080p - High-definition video recorded at 1920 pixels horizontal by 1080 pixels vertical resolution with progressive scan.
Acclimation - To adapt to a different climate. Cooling a telescope to the ambient temperature.
Afocal - An optical setup where a camera with a camera lens attached to it shoots through a telescope with an eyepiece in it.
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.
Albedo - The ratio of the light reflected by a planet compared to the amount of light it that illuminates it.
Albedo Feature - A dark or light marking on the surface of a planet.
Algorithm - An algorithm is a series of mathematical instructions to solve a problem or produce a result in a step-by-step procedure.
Altazimuth, 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.
Altitude - Elevation above the horizon.
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.
Angular Resolution - The angular size of detail that is resolved in an image.
Annular Eclipse - A type of solar eclipse where a ring or annulus of the surface of the Sun is not completely covered by the Moon when it passes in between the Sun and the Earth.
Aperture - The size of the opening that lets light into a camera lens, which can usually be adjusted and changed with an iris diaphragm inside the camera lens. 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 3,504 x 2,336 pixels, or 8,185,34 total pixels.
Artifacts - Artificial, spurious defects in an image that were not part of the original image or data.
Asteroid - A rocky body, smaller than a planet, in orbit around the Sun, usually between the orbits of Mars and Jupiter.
Atmospheric Prismatic Dispersion - When light from a celestial object is broken into its constituent colors by the prismatic effects of the Earth's atmosphere.
Aureole Effect, Venus - A very thin line or arc around the edge of Venus off the limb of the Sun just before second, and after third contact during a transit of Venus across the Sun. It is caused by refraction of sunlight through the thick atmosphere of Venus and is usually only visible during good seeing with excellent optics.
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.
AVI - Audio Video Interleaved. A video file format by Microsoft.
Azimuth - The arc of the horizon measured from north.
Bailey's Beads - Brilliant points of the Sun's photosphere visible between mountain peaks and valleys on the edge of the Moon at second and third contact of a total solar eclipse.
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.
Barges - Brown oval storms in the North Equatorial Belt of Jupiter.
Barlow - a diverging optical lens named after its creator, Peter Barlow, which increases the focal length of the optical system.
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.
Belts - Large dark bands across a planet.
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 212 and equals 4,096 levels. 16-bits 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.
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 4,095 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.
Cassini's Division - A large 4,800 km (3,000 miles) wide division between the A and B Ring of Saturn, discovered by Giovanni Cassini in 1675.
Catena - A crater chain on the Moon.
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) - 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. 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.
Chromosphere - The thin layer of the Sun's atmosphere just above the photosphere.
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.
Clouds, Discrete - Clouds that form on Mars over Libya, Chryse and Hellas regularly. Of particular interest is the Syrtis Major Blue Cloud that forms over Libya and Syrtis Major and makes the surface look blue.
Clouds, Dust - Clouds on Mars that can cover small areas or the entire planet. Dust storms form when the Sun heats the Martian air, causing winds to blow, lifting dust from the surface.
Codec - A mathematical algorithm that compresses and decompresses, or encodes and decodes data into a digital format.
Collimation - Collimation is the precise aligning of the optical elements in a telescope so that it performs as well as it can.
Color 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):
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.
Comet - A small body comprised of ice, dust and rocky material in orbit around the Sun. Upon close approach to the Sun, the gas and ice can sublimate, forming a thin atmosphere around the comet called a coma, and gas and dust tails can form following behind it. Small rocky particles left behind in the comet's orbit can later crash into the atmosphere and burn up, causing meteors.
Composite - In digital photography, to 1.) combine different individual images 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'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.
Corona - The Sun's outer atmosphere.
Coronal Mass Ejection (CME) - A CME is a release of electrons, protons and heavier particles that are ejected from the Sun.
Crater - A circular depression, usually caused by an impact.
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.
Dawes Criterion - Derived by William Rutter Dawes, this empirical measure of the resolving power of a telescope for two stars of similar brightness observed in a modestly sized telescope. The formula is θ = (1.02 * λ / D) * 206,265 where θ is the angular separation in arcseconds, λ is the wavelength in millimeters, and D is the diameter of the aperture of the telescope in millimeters.
Declination - Part of a coordinate system along with Right Ascension for locating objects in the sky. Declination lines are similar to lines of geographic latitude projected onto the celestial sphere.
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.
Detail Smearing - High resolution detail on planets that are smeared in long exposures by planetary rotation.
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.
Dispersion - When light from a celestial object is broken into its constituent colors by the prismatic effects of the Earth's atmosphere.
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.
Dobsonian - An altazimuth mounted Newtonian telescope popularized by the amateur astronomer John Dobson.
Dorsum - A Ridge on the Moon.
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.
Dwarf Planet - A dwarf planet, or Plutoid, is a large trans-Neptunian body in the Kuiper belt. The Kuiper belt is the region of the solar system beyond the orbit of Neptune, at a distance of 30 to 50 astronomical units from the Sun. A dwarf planet is larger than an asteroid, but smaller than an official planet.
Dynamic Range - The range of brightness from light to dark in which detail can be recorded.
Earthshine - Sunlight reflected off the Earth that illuminates the side of the Moon not in direct sunlight.
Eclipse - When one solar system body moves in front of another and obscures it, or when one body passes into the shadow of another body.
Encke's Gap - A 325 km (202 miles) gap in the outer portion of Saturn's A ring. The gap is named after Johann Encke, who did not observe it, but was named in his honor for his ring observations. The gap was actually discovered by James Edward Keeler in 1888.
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.
Extended Detail - Detail on an extended object such as the Sun, Moon or a planet.
Extended Object - An object that is larger than a point source that contains detail.
Extinction - Extinction causes the brightness of celestial objects to diminish. Extinction is caused by absorption and scattering of light in the atmosphere.
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.
Faculae - Bright areas in the photosphere of the Sun that sometimes precede or follow regions of activity in time.
Festoons - On Jupiter, festoons are blue finger-like protrusions from belts or zones into the adjacent zone or belt, usually from the Northern Equatorial Band south into the Equatorial Zone.
Filaments - Prominences viewed in front of the disk of the Sun. They appear as dark lines, strings or dark clouds.
Flares - The sudden release of magnetic energy heating surrounding gasses in the Sun's chromosphere which accelerates electrons, protons and heavier ions to high velocities. Most occur in active regions around sunspots.
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 500 mm focal length will cover a field of view of 4 degrees, 17 arcminutes, 43 arcseconds by 2 degrees 51 arcminutes, 51 arcseconds with a Canon 20Da DSLR camera sensor that is 22.5 x 15 mm.
Field Rotation - Rotation of the field of view around the center of the field in an altazimuth mount tracking the stars, or around a guidestar tracking the stars on a misaligned equatorial mount.
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.
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.
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.
Flatten - A term used in Photoshop that means to merge separate layers of corrections or masked data information in an image.
Frame - 1.) A single still image that is part of a movie or video. Movies and videos are comprised of a series of still images. 2.) 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 astrophotography, you can have light frames, dark frames, bias frames and flat-field frames. 3.) 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.
Frames Per Second (fps) - The number of frames that are recorded or played back per second.
Framing Rate - The number of frames that are recorded per unit of time.
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 1,000 / 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 - The point at which rays of light converge after being refracted or reflected. In imaging, focus occurs when the focal plane of an optical system is coincident with the focal plane of a 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 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.
Go To - A computerized telescope mount with a database of objects that will automatically point the scope to an object when selected.
Granules - Small cells on the Sun's surface caused by convection.
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.
Great Red Spot - A large storm system in Jupiter's upper atmosphere.
Greatest Elongation - The largest angle between the Sun and an inferior planet as viewed from Earth.
High Definition Video - "High Definition" video is usually video recorded with a resolution of 1920 x 1080 pixels, or 1280 x 720 pixels.
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.
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 in relation to the sky. Image scale is usually measured in a digital camera as arcseconds per micron, or arcseconds per pixel.
Image Size - The size of an image at the focal plane of a telescope or camera lens.
Inferior Conjunction - When the two planets lie in a line on the same side of the Sun, the planet closest to the Sun is in inferior conjunction and the outer planet is at superior conjunction, relative to each other.
Inferior Planet - One of the planets inside of the orbit of another planet. Mercury and Venus are inferior planets to the Earth.
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.
Jet Stream - A fast moving, narrow current of air found in the upper atmosphere near the tropopause on Earth.
JPEG - A file format with a .JPG extension that compresses 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.
Keeler's Gap - A 42 km (26 mile) gap located 250 km (155 miles) from the outer edge of the A ring of Saturn. It was discovered by the Voyager space probe and named after James Edward Keeler who discovered Encke's gap.
Kilobyte - 1,024 bytes.
Kuiper Belt - A region at the edge of the solar system beyond Neptune containing bodies of ice and rock that is believed to be the origin of some short period comets.
Lacus - A small plain on the Moon.
Lagnaippe - Something extra given at no extra cost to a consumer out of the kindness and generosity of the merchant.
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 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.
Limb Clouds - Clouds that are visible on both the eastern and western limbs of Mars caused by dust and carbon dioxide particles in the atmosphere scattering light.
Linear Resolution - The linear size of detail that is resolved in an image.
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.
Live View - The display of a near real-time video feed from the sensor of a camera to an LCD screen
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.
Lucky Imaging - Shooting many frames in a video in the hope of getting lucky and capturing high-resolution planetary detail in moments of good seeing. The frames are then graded for quality, and "stacked" or averaged together either as a whole frame, or with multiple alignment points to produce an image with higher quality than a single image.
Magic Time - The time during twilight when the exposure for the sky matches the exposure for Earthshine on the Moon.
Magnification - To enlarge an image with an optical system.
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 arcsecond, as if starlight from a point source was spread out over an area of one square arcsecond.
Mare - A large circular plain on the Moon.
Mask - An overlay that blocks certain portions of an image so that other portions can be selectively manipulated. 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. An aperture mask used with a telescope is an opaque disk with a hole in it that reduces the scope's native aperture to a smaller aperture.
Megapixel - A CCD or CMOS sensor that has one million pixels. A 5 megapixel camera has 5 million pixels.
Meteor - A meteoroid (small rocky body) that enters the Earth's atmosphere and starts to burn up.
Meteoroid - A small rocky body in orbit around the Sun that is smaller than an asteroid.
Meteorite - A meteor that survives re-entry into the Earth's atmosphere and impact with the Earth's surface.
Micron (Micrometer) - One millionth of a meter. Also equal to one thousandth of a millimeter. Abbreviated µm.
Millimeter - One thousandth of a meter. Abbreviated mm. There are 25.4 millimeters in one inch.
Mons - Mountains.
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.
MOV - An Apple Quicktime movie file format.
Movie - A series of still images that when played back in rapid succession give the appearance of continuous motion.
Movie Crop Mode - A special high-definition movie recording mode found on Canon T2i (550D), 60D and 60Da DSLR cameras that records the central 640 x 480 pixels in the camera sensor at 60 frames per second.
Multi-Point Alignment - Selecting the best parts of many frames and aligning and stacking them to improve the sharpness and detail in an image.
Native Resolution - The natural or optical resolution of a sensor or image where the original number of pixels is not changed.
Narrowband Solar Imaging - The use of narrowband filters, which pass only selective wavelengths of light, to take an astrophotograph of the Sun. For example, a narrowband Hydrogen-Alpha filter passes only a narrow window of wavelengths, usually several tenths of an angstrom wide, centered around 6562.8 angstroms, the Hydrogen-Alpha wavelength. By filtering out the rest of the spectrum, much more contrast is gained in the wavelength of interest. Calcium K is another narrowband filter used in solar imaging.
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".
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.
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.
Oort Cloud - A spherical shell of comets and icy and rocky bodies in the far reaches of the solar system that lies beyond the Kuiper Belt up to a distance of a light year from the Sun.
Opposition Surge - The brightening of an object when the Sun lies directly behind the observer at opposition. Also known as the Seeliger effect.
Orographic Clouds form over mountains and volcanoes from Martian air being lifted over these structures and moisture condensing when the gases in the atmosphere cool due to lower pressure at higher elevations. These are frequently seen over Tharsis and Elysium. The famous W-shaped cloud that forms over Olympus Mons and other volcanoes on Tharsis are an example of orographic clouds.
Ovals - Smaller storms in the upper atmosphere of gas-giant planets.
Oversample - To use more resolution than is necessary to correctly sample the high-resolution detail in an image.
Partial Eclipse - When a celestial body is only partly hidden. When the Moon does not completely cover the Sun a partial solar eclipse occurs, and when the Moon does not pass completely through the umbra shadow of the Earth, a partial Lunar eclipse occurs.
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.
Photosphere - The visible surface of the Sun.
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.
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. 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.
Plage - A bright region in the Sun's chromosphere found near sunspots and related to faculae in the photosphere below.
Plutoid - A Plutoid, named after the former planet Pluto, is dwarf planet, or large trans-Neptunian body in the Kuiper belt. The Kuiper belt is the region of the solar system beyond the orbit of Neptune, at a distance of 30 to 50 astronomical units from the Sun. A Plutoid is larger than an asteroid, but smaller than an official planet.
Poncet Platform - An equatorial platform that turns an altazimuth mount, usually on a Dobsonian, into an equatorial mount.
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 occurring 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.
Polar axis - The axis of a telescope mount that is aligned with the celestial pole.
Polar Cap - These polar caps grow and recede, depending on the season on Mars. Both have a permanent cap made of water ice as well as a seasonal one made of carbon dioxide that forms in the winter.
Polar Hoods are clouds or ice fog that form over the polar cap in the autumn season for their respective hemispheres. They are caused by cooling temperatures when water vapor condenses into clouds.
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.
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.
Prismatic Dispersion - When light from a celestial object is broken into its constituent colors by the prismatic effects of the Earth's atmosphere.
Promontorium - A cape on the Moon.
Prominences - Hydrogen gas projected from the limb of the Sun into the chromosphere or corona.
Qualitative Analysis - Evaluation based on subjective judgment.
Quantitative Analysis - Evaluation based on objective, measurable quantities.
Raw - Unprocessed data directly from the sensor.
Rayleigh Criterion - Derived by Lord Rayleigh (John William Strutt), The Rayleigh criterion says that two stars are resolved when the brightest part of the center of one star (the maxima) is centered over the first interspace (the minima) between the disk and the first ring of the other star. The formula is θ = (1.22 * λ / D) * 206,265 where θ is the angular separation in arcseconds, λ is the wavelength in millimeters, and D is the diameter of the aperture of the telescope in millimeters.
Reddening - Objects in the sky close to the horizon appear more red because of Rayleigh scattering.
Refraction - The change in direction of a wave due to a change in the medium through which it is traveling. In astronomy, refraction is the angular amount which the apparent altitude of a celestial body is increased by refraction in the Earth's atmosphere
Resampling - Resizing an image by mathematical algorithms that examine neighboring existing pixels and create new ones based on this analysis.
Resolution - 1.) Producing separate images of close objects such as stars or fine details in a subject. 2.)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.
Resolution, Angular - The angular size of detail that is resolved in an image.
Resolution, Linear - The linear size of detail that is resolved in an image.
Resolving Power - The ability to resolve fine detail in an image.
Right Ascension - The angular east-west coordinate along the celestial equator of a celestial body measured from the vernal equinox, measured in hours, arcminutes and arcseconds.
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.
Rima - A rille on the Moon. A rille is a long, narrow depression or channel in the lunar surface.
Rupes - A scarp on the Moon. A scarp is like a cliff on the Earth.
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.
Satellite - A body that orbits around another body, such as the Moon around the Earth or the Earth around the Sun. Artificial satellites are man-made devices that orbit the Earth.
Satellite Flare - The brightening of a satellite as seen from the ground due to the reflection of sunlight. Iridium satellites in particular are known to flare intensely because they have mirrored antennae that reflect direct sunlight.
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.
Scars, Comet Crash - Darker spots caused by upwelling from material below the upper atmosphere.
Scattering - The diffusion of light in the atmosphere caused by particles such as atoms and molecules smaller than the wavelength of light scattered. It can be caused by atmospheric gasses as well as smoke and other particulates
Scintillation - Rapid changes in the brightness of a celestial object due to atmospheric turbulence.
Seeing - The steadiness of the atmosphere from atmospheric turbulence 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.
Seeliger Effect - The brightening of an object when the Sun lies directly behind the observer at opposition. Also known as the opposition surge.
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.
Shadow Transit - The movement of the shadow from a planetary moon across the face of a planet. Shadow transits are usually seen on Jupiter and Saturn. They are seen on the cloud tops in the atmospheres of these gas giants, appearing as small black dots. They are not part of the atmosphere itself.
Shoot - To take a picture or capture or record an image.
Signal - In a digital camera, the 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.
Sinus - A bay or small plain.
Smearing - The loss of high resolution detail on planets caused by planetary rotation in long exposures.
Sparrow Criterion - Derived by C. M. Sparrow for the resolution of double stars, the Sparrow criterion that said an observer could detect duplicity when two equally bright stars were elongated. The formula is θ = (0.95 * λ / D) * 206,265 where θ is the angular separation in arcseconds, λ is the wavelength in millimeters, and D is the diameter of the aperture of the telescope in millimeters.
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.
Spicules - Jets that project above the chromosphere of the Sun that look like fur around the edge.
Spot Size - The size of the Airy disk.
Stack, Stacking - To align and average multiple images to improve detail and the signal-to-noise ratio in the resulting image. Derived from the days of film when two pieces of film were physically stacked on top of each other.
Standard Definition - A video term that usually means 640 x 480 pixel resolution.
Stretching - Redefining the black or white points in an image to increase the contrast.
Solar Cycle - The waxing and waning of solar activity such as Sunspots, solar flares and coronal mass ejections over an 11-year period.
Solar Wind - The stream of charged particles emanating from the Sun.
Superior Conjunction - When the two planets lie in a line on the same side of the Sun, the planet closest to the Sun is in inferior conjunction and the outer planet is at superior conjunction, relative to each other.
Sufficient Sampling - Having enough pixels at a size smaller than the detail to be recorded so that all of the detail present is captured.
Summing - Mathematically adding together individual shorter-exposure images to create the equivalent of a longer-exposure image.
Sunspot - Areas of the photosphere of the Sun that appear darker and that are cooler than nearby regions. They are associated with strong magnetic fields on the Sun.
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 1,000 mm focal length f/8 optical system into a 750 mm 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 1,000 mm focal length f/8 optical system into a 2,000 mm f/16 optical system.
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.
Total Eclipse - When an object is completely covered or hidden by another object or the shadow of another object. A total solar eclipse occurs when the Moon completely covers the disk of the Sun. A total lunar eclipse occurs when the Moon moves completely into the Earth's umbra.
Tracking - Following a star with a telescope to compensate for the Earth's rotation.
Transit - 1.) The passage of a celestial body across the meridian of a given location. 2.) The passage of a celestial object across the disk of another.
Transparency - The clarity of the atmosphere.
Ultraviolet [UV] - The short wavelength region of the spectrum below blue and violet from about 10 nanometers to 380 nanometers.
Undersample - To not have enough pixels to record all of the fine detail present in an object.
USB - Universal Serial Bus. A protocol and hardware system for transferring data from peripherals to a computer over cables.
Video - The electronic recording and transmission of a series of still images that when played back give the illusion of motion.
Vignetting - Light falloff in the corners of an image due to optical, geometrical or mechanical reasons.
Vallis - Valley.
Wavelets - Wavelets break an image down by frequency information so that details can be independently sharpened based on their scale, or size, in the image.
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.
White Spots - Storms or clouds smaller than ovals or barges
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.
Zones Large light bands across a planet.
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