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Stargazing June 15th ✦ 6pm-10pm
Stargazing June 15th ✦ 6pm-10pm


Astronomy Glossary

The following list gives definitions for many of the terms used in this website and throughout any other astronomical literature you may read. Definitions are listed in alphabetical order and many are linked from other pages so you may quickly refer to an unfamiliar term from elsewhere on the Guide to CCD Imaging website.

Clicking on the letters below will quickly link you to terms beginning with that letter.

A | B | C | D| E | F | G | H | I| J | K | L | M | N | O| P | Q | R | S | T| U | V | W | X | Y | Z


A/D Converter
The A/D Converter changes the analog signal from the CCD camera into a digital signal which is read by the computer. How finely the signal is divided digitally is determined by the bit depth of the A/D Converter.

An ADU is an Analog/Digital Unit. This is the unit of measure for the values in a CCD image. If you measure a value of 2500 ADU for a particular pixel in an image, you can calculate how many electrons were actually collected by the corresponding photosite if you know the system gain of the CCD, which determines the conversion from electrons to ADUs.

Afocal Imaging
Afocal imaging involves taking a picture through both a telescope and eyepiece. As opposed to prime focus imaging, where the camera is attached directly to the telescope without any eyepiece, afocal imaging requires an eyepiece. This is used with CCD, DSLR, and film cameras for planetary imaging (called eyepiece projection imaging). This is also the only way to image using digicams which do not have removable lenses. The camera is attached directly to the eyepiece.

In CCD imaging, an algorithm usually refers to a software procedure, often for image processing. An algorithm is the mathematical function which tells the computer how to modify the image.

Short for Altitude-Azimuth (also called Alt-Az). Telescopes which are mounted so that they move up-down and left-right (as opposed to equatorially) are called alt-az. This is a convenient mounting configuration for visual observing as the eyepiece is always in a convenient position. However, an equatorial wedge must be used for photography or CCD imaging.

This feature is added to a CCD chip to prevent pixel blooming.This feature generally reduces sensitivity, well-depth, and linear response. For these reasons, non-anti-blooming chips are popular, and there is even software available to remove blooming streaks from CCD images.

Apparent Size
The apparent size of an object is how large the object looks in the night sky. This measurement is called an angular size since it measures how much of an angle in the sky the object subtends. It is usually given in degrees, arcminutes, or arcseconds, depending on the type of object. For example, the moon's apparent size is 1/2 degree. Jupiter's apparent size is around 45 arcseconds.

Array refers to the actual layout of photosites(or pixels) on a CCD chip. Chips are often referred to by their array size, or number of pixels. For example a KAF-1600 chip has an array that is 1530 pixels wide by 1024 pixels high.

Astrometry is the measurement of the position of an object, relative to known fixed positions. For example, astrometric measurements of an asteroid or comet as it moves can be used to determine the object's orbit.

An autoguider is a small CCD camera which is used to send guiding corrections to the telescope's mount to increase tracking accuracy. An autoguider is used in conjunction with a film camera or CCD camera. Some CCD cameras have built-in autoguiders (see self-guiding).


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Binning involves combining pixels on a CCD chip to create larger pixels. For example, taking a 2x2 square of pixels and creating one pixel that is twice the width and four times the area of the original pixel. This is done to increase sensitivity or to match a long focal length telescope to a CCD camera with small pixels.

Bit Depth
A CCD camera takes an analog signal which must be converted to a digital signal for the computer. This conversion is done by the A/D Converter. Each pixel has a value which is assigned a numerical value in the computer. The bit depth determines how finely each pixel value can be divided, meaning a greater bit depth can provide more information. A 12-bit A/D converter divides the signal into 4096 levels of information. A 16-bit converter provides 65,536 levels.

Each photosite of a CCD chip can contain a certain amount of electric charge. This amount is determined by the well depth of the CCD. When the well-depth is exceeded, electric charge "bleeds" out of the photosite appearing in an image as a bright streak extending vertically from a bright source in the image (usually a star). This effect can be minimized or eliminated by using a CCD with an anti-blooming gate. Anti-blooming filters are available for image processing software to remove blooming streaks from images.


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CCD stands for Charge-Coupled Device. CCDs are silicon chips with receptors (photosites) to collect light. The light is turned into electric charge and read out to a computer where an image is displayed. Astronomical CCD cameras, digital cameras, and digital camcorders are all based on this same charge-coupled device.

Charge Transfer Efficiency
Pixels are read out one at a time into a register along one side of the CCD chip. The charge collected in each pixel is shifted down the CCD chip toward the register. Some charge is lost during this transfer. The ability of a CCD camera to keep as much charge as possible from being lost during readout is known as its Charge Transfer Efficiency.

Chromatic Aberration
This aberration is found in telescope systems with lenses, but is generally associated specifically with refractors where it is most noticeable. In a refractor, light passes through a lens and is bent to reach a focus point. Each wavelength of light is bent differently, so they do not all meet at the same point of focus. The result is an out-of-focus glow, usually purple in color since the violet light is least likely to meet focus with the other colors. Some refractors are specially corrected for this aberration. CCD cameras are far more sensitive to ultraviolet and infrared light than the human eye, so only the very best refractors are suitable for high-resolution CCD imaging.

CMOS stands for Complimentary Metal Oxide Semiconductor. CMOS chips are similar to CCD chips but have some differences in how they function. In a CCD, the charge from each pixel is read out to an output node where it is converted to a voltage. This voltage is then sent off chip to be digitized and sent to the computer. In a CMOS chip, each pixel outputs a voltage so there is no need to convert off chip. Digitization is done on chip, allowing for a less complex overall system (though the chip itself is more complex). CMOS sensors are used in some digital cameras and offer very low noise characteristics (although this is not necessarily an inherent characteristic of all CMOS chips).

Color Filter Wheel
Many CCD cameras are inherently black and white. In order to get a color image from such a camera, three images must be taken through red, green, and blue filters and then combined into a single color image. (This is called tri-color imaging.) A color filter wheel holds the color filters (and sometimes other optional filters) and is normally motorized to automate the process of selecting a filter.

Coma is an aberration seen in certain telescope designs. It is an off-axis aberration, meaning it is not seen in the center of the field but grows progressively worse toward the edge of the field. Newtonians and commercial Schmidt-Cassegrain telescopes suffer from coma, although it is most noticeable in Newtonian scopes. Coma correctors can be used to eliminate this aberration. Achromatic refractors also tend to suffer from some coma, although apochromatic refractors usually eliminate or minimize the aberration. Ritchey-Chrétien designs are specifically made to eliminate coma.

CMYK imaging is an alternative form of color imaging. UsuallyRGB or LRGB imaging is used, but CMY (cyan, magenta, and yellow) filters can be substituted for RGB filters, and the L (luminance) layer of and LRGB becomes the K (or black) layer in a CMYK image.


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Dark Current
Astronomical CCDs are so sensitive that they can detect electronic noise generated by the CCD chip itself. The amount of noise created by the CCD chip is known as Dark Current. Dark current is a function of temperature and cooling the CCD chip can remove much of the noise. However, not all noise is eliminated and so imagers rely on Dark Frames to remove much of the remaining noise.

Dark Frame
A dark frame is simply a CCD image taken with the camera's shutter closed. This image detects noise generated by dark current. An image of the same duration is taken with the shutter open, detecting both the noise and the object being imaged. Using image processing software to subtract the dark frame from the raw image leaves only the object. The dark noise is eliminated.

Dark Subtract
This is the process of removing noise (generated by dark current) from a CCD image. A dark frame is digitally subtracted from an image to eliminate dark noise.

DDP stands for Digital Development Processing. This an image processing routine used in MaxIm DL software. DDP processing allows both bright and dim parts of an astronomical object to be displayed at the same time. DDP essentially compresses washed-out regions of an object into a range that the computer can display. This process is especially useful on galaxies which have bright cores and faint spiral arms.

Download Speed
Also called Transfer Rate, or System Throughput, this describes how quickly the information in the CCD is transferred to the computer. There are several factors involved, including reading the data from the CCD chip, sending it to the computer, and then displaying it on the screen. The total download time is how long you wait from the end of the exposure until you see the image. For large-format CCDs using parallel-port interfaces this might be as much as 2 minutes, while for smaller chips with high-speed USB interfaces there might be less than 1 second of download time.

Dynamic Range
Dynamic range describes the range of brightness that can be seen by a CCD camera. CCD cameras have a much greater dynamic range than does film. A CCD's ability to capture both faint and bright details in determined by its dynamic range.


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Fastar was Celestron's high-speed CCD imaging system. Fastar involves removing a Schmidt-Cassegrain telescope's secondary mirror and placing the CCD camera at the front of the telescope. This provides a wide field of view and a vary fast imaging system. A telescope which is said to be "Fastar compatible" has a removable secondary mirror. Celestron has discontinued Fastar. HyperStar is Starizona's improved version of the Fastar lens.

FFT Filters
FFT stands for Fast Fourier Transform. FFT filters are image processing algorithms applied to CCD images, usually to sharpen or smooth and image. FFT filters have the same function as kernel filters, but are applied differently so the processing is faster. Applying an FFT to an image changes the image so that applying a kernel filter is easier. The FFT function is then reversed and a filtered image results.

Filter Wheel
Black and white CCD cameras use color filters to separate the three primary colors (red, green, and blue) in order to produce full color images. Usually an automated mechanical wheel holds the filters in front of the camera and rotates the appropriate filter into place before an exposure. Filter wheels can also hold narrowband or photometric filters for different types of imaging.

Flat Field
Out-of-focus dust particles in the optical system and vignetting can lead to an unevenly illuminated CCD chip. By taking an image of an evenly illuminated source (such as the evening sky after sunset or the inside of an observatory dome) the differences in illumination across the field of view can be removed using image processing software.

Focal Length
This is the effective length of the path which the light must follow through a telescope. In a refractor or Newtonian design the focal length is equal to the actual length of the light path from the primary mirror to the focus point (where a CCD would be placed). In a Cassegrain type of telescope, the effective focal length is usually about 5 times longer than the actual length of the light path due to the magnifying effect of the secondary mirror.

Focal Ratio
This is the ratio between the focal length of a telescope and the aperture. A telescope with an 8" aperture and 80" (2000mm) focal length has a focal ratio of f/10. Smaller focal ratios equate to shorter exposure times. An f/4 system is faster than an f/6 system, for example.

Focal Reducer
A focal reducer is used to decrease the focal ratio (and focal length) of a telescope. A smaller focal ratio yields a faster optical system and thus a shorter exposure time. The trade off is reduced image scale, but this is usually acceptable in return for a wider field of view and shorter exposure. Focal reducers typically attach to the back of the telescope, just ahead of the camera in the optical path. They are made in a variety of focal reduction factors, from 0.18x to 0.8x. Popular focal reducers for SCTs are 0.33x and 0.63x reducers, often called f/3.3 and f/6.3 reducers since most SCTs have an inherent focal ratio of f/10 and thus the reducers yield these new focal ratios. A focal reducer is a highly recommended accessory for imaging.

Full Well Capacity
Each photosite on a CCD chip can contain a certain number of electrons. This number is called the Full Well Capacity. Within a certain chip full well capacity is a function of binning, so that binning 2 times, for example, quadruples the full well capacity. Of course, 2x binning also makes the CCD effectively 4 times more sensitive and so the wells will fill in the same amount of time. Chips with anti-blooming generally have lower full well capacity, but will not bloom when the well is filled.


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The system gain of a CCD camera tells you how many electrons are represented by each ADU. Gain is expressed in electrons/ADU. For example, a CCD camera with a gain of 1.3 converts 1.3 electrons into one ADU. So, if you were to measure a pixel value of 2500 ADU for some area of the image, that value corresponds to 2500 x 1.3 = 3250 electrons collected by that particular photosite. Equivalently, a camera with a gain of 2.5 would convert those same 3250 electrons into an ADU value of just 1300. This can be useful in calculations of optimum exposure times and photometric measurements. (Technically, most cameras add a 100 ADU pedestal, so ADU values are normally 100 less than measured.)

No telescope mount can track perfectly, yet for CCD imaging it is necessary to very accurately track the object being imaged. This is done by guiding on a star to make small corrections to the mount to accurately follow the star. This makes up for any errors in the telescope's drive system. Guiding can be done manually by watching a star through a crosshair eyepiece, or, more commonly, by using an autoguider to automatically guide. See also, Unguided Exposure, Autoguider, Self-Guiding, and Track & Accumulate.


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A histogram is a graph of number of pixels versus pixel value. Pixel values run from lowest (displayed as black) to highest (displayed as white). A bar is plotted for each pixel value showing the number of pixels in the image with that value. An astronomical image typically has more bars toward the lower (darker) end of the histogram since most astroimages contain are large amount of dark sky around a brighter (but small) object. For more information on histograms and how they are used for image processing and display, click here.

HyperStar is Starizona's improved version of Celestron's Fastar lens. Fastar optics allows the secondary mirror to be removed from an SCT and replaced with a lens assembly that allows fast f/1.8 imaging. The HyperStar lens has better optical correction than the original Fastar and is compatible with many more CCD cameras. It is available for Celestron's 8", 11" and 14" Schmidt-Cassegrain telescopes.


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Image Scale
Image scale is basically equivalent to magnification. Image scale is related to focal length in that the longer the focal length of a telescope, the greater the image scale will be. With film systems, image scale was typically measured in arcseconds per millimeter. But since CCDs have pixel sizes measured in microns, image scale is usually measured now in arcseconds per micron. This can be used to determine how much sky is covered by a given pixel in a CCD camera (which gives the resolution), or how large an object might appear in an image. The formula for image scale = 206 / Focal Length. This gives the result in arcsec/micron. A related equation gives resolution in arcsec/pixel. This formula = (Pixel Size * 206) / Focal Length. Focal length in these equations is in millimeters.

ISO was originally a measure of film speed. This has been adopted for digital camera use as well. In a film camera, changing the film was required if a change in film speed (sensitivity) was required. Typical ISO film speeds are 50, 100, 200, 400, and 800. Each correlates to a 2x improvement in speed over the previous ISO. Digital cameras have ISOs that are comparable to film speeds but can be changed electronically. Just like film, increasing the ISO of a digital camera increases noise (analogous to the grain in high-speed film). High ISOs are preferable for astronomical imaging, but many small digital cameras have high noise levels associated with high ISOs. Digital SLRs, on the other hand, tend to have low noise at high ISOs, making them suitable for astronomical imaging.


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Kernel Filters
Kernel filters are image processing algorithms applied to a CCD image, generally to smooth or sharpen an image. A kernel is a small grid which tells the computer how to change the value of a certain pixel based on the values of neighboring pixels. The most common types of kernel filters are low-pass ("smoothing" and "blurring" filters) and high-pass ("sharpening" filters). See also FFT filters.


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Linear Response
A CCD camera with a "linear response" has sensitivity such that doubling the exposure time of an object of a certain brightness will result in an image twice as bright. There is a linear relationship between exposure time and brightness. This is especially useful for making magnitude measurements of variable stars, comets, asteroids, or supernovae. A camera with a nonlinear response is not suitable for making magnitude estimates since a star which appears twice as bright in an image is not necessarily twice as bright in actuality. CCD cameras equipped with an anti-blooming feature are generally nonlinear. For taking pretty pictures linear response makes no difference.

Images taken with a black and white CCD camera through red, green, and blue filters are combined to make RGB images. An interesting effect of human vision is that we get most of the spatial information about an image from the brightness, orluminance, portion of the image and not from the color, or hue, portion. This means it is possible to take a low resolution color image and combine it with a high resolution black and white image to make an LRGB image (the L standing for luminance). This is a definite advantage for CCD imaging; since placing a filter in front of a camera decreases its sensitivity, color images can be binned to gain higher sensitivity at the expense of resolution. As long as the low-resolution color image is combined with a high-res luminance image (taken unfiltered and thus at the camera's maximum sensitivity) the full resolution is maintained and the total exposure time is decreased.


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Maximum Entropy Deconvolution
This is an image processing routine found in the MaxIm DL software package. It is used to enhance detail in CCD images that are slightly blurred due to atmospheric effects, tracking errors, or imperfect optics. The routine works by attempting to match an ideal image (essentially a perfect star image) to the actual blurred image. The image is adjusted through a series of iterations to a closer approximation of an ideal image.

Median Combine
When combining images there are two methods employed. Most often images are summed to get as much information as possible. However, noisy images can be smoothed to some extent by taking the median value of all the pixels in the images. Median combining requires that there be at least 3 images. (2 images can be summed or averaged.)

Monochrome Imaging
A monochrome camera images in black and white. To create color images with this type of camera, red, green, and blue filters are used. An image through a given filter still appears in black and white, but when all three filtered images are combined the result is a full color image. One-shot color cameras differ in taking a single picture that is already full color.

CCD chips are usually small, and many deep-sky astronomical objects are big. Many CCDs, especially if used with long focal length telescopes, will not be able to fit a large object into the field of view. One solution is to make a mosaic using multiple images of smaller section of the object. Slightly overlapping images are taken of sections of the object and are combined in the computer. For more information on creating mosaics, click here.


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Narrowband Imaging
Narrowband imaging uses filters to isolate specific wavelengths of light. This allows very detailed images to be obtained, especially of nebulas. Using narrowband filters isolates certain gases, showing details not normally visible in regular RGB color images. Narrowband filters also allow imaging in light-polluted skies or during moonlit times of the month. Exposure times are increased with narrowband filters. Typical filters are Hydrogen-Alpha, Oxygen-III, and Sulfur-II.


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One-Shot Color Imaging
Normal black-and-white CCD cameras require a color filter wheel with red, green, and blue filters in order to obtain color images. One-shot color cameras, on the other hand, do not need separate color filters. They can capture a color image in a single exposure. Typically, one-shot color cameras are less sensitive than non-color cameras, but since they require 1/3 the number of exposures to capture a color image, the end result is often a shorter amount of time spent imaging.


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A pedestal is a value added by image processing software to the pixel values of an image to avoid zero values from appearing in pixel math calculations. Normally a value of 100 ADU is added to the standard pixel values. The prevents random noise from lowering a value to zero and affecting pixel math calculations. The pedestal means that if you measure a pixel value to be 2500 ADU, it is really 2400 ADU.

Periodic Error Correction (PEC)
All telescope mount drive gears suffer from periodic error. No gear can be machined perfectly, although some are certainly built with more precision than others (which is why some mounts cost $300 and some cost $10,000). Since the drive gear repeats its inherent errors at a given interval (however long the gear takes to revolve once, usually 5-10 minutes), the errors are repeated and, in theory, removable. PEC is built into certain mount electronics to record drive errors and repeat corrections to those errors. PEC allows longer unguided exposures. With guiding, PEC is usually not necessary since corrections are being made almost constantly.

Often small telescopes or camera lenses are piggybacked, or mounted on top of another telescope. Camera lenses and small refractors are popular for wide-field imaging and astronomers who own a larger scope such as an SCT will often mount the smaller instrument on top of the larger, using the big scope as a tracking platform. This eliminates the need for a separate mount for the smaller instrument and allows the larger scope to be used for guiding.

Pixel is short for "picture element", and describes the tiny squares which make up a CCD image. Pixels can refer either to the individual squares in an image or the actual light-sensitive squares on a CCD chip (also called photosites). The number of pixels in an image is often called the resolution, although resolution also refers to the physical size of the pixels (photosites) on a CCD chip.

Photometric measurements are used to determine the brightness of an object. This data is used to analyze variable stars, comets, asteroids, and more.

Photosites are the individual light-sensitive squares (or rectangles) of a CCD chip. A CCD chip is comprised of thousands or millions of photosites. In a CCD image, usually each photosites becomes an individual pixel in the final image displayed on screen, but sometimes the photosites are binned and 4 or 9 individual photosites work effectively as one larger photosite to create a single pixel. Photosites are almost always called pixels, except when a distinction needs to be made between photosites and image pixels.

Prime Focus
Prime focus imaging involves attaching a camera directly to a telescope without any intermediate eyepiece or camera lens. This is as opposed to afocal imaging which uses an eyepiece or eyepiece and camera lens to increase the magnification (such as for planetary imaging). Prime focus imaging is the normal setup used for deep-sky astrophotography as it allows wider fields of view and faster focal ratios.


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Quantum Efficiency
As photons of light hit a CCD chip they are converted into electrons which are stored and then read out at the end of the exposure. But not every photon that hits the chip is converted into an electron. How many photons are converted depends on the camera's quantum efficiency, or QE. QE is expressed as a percentage of the number of photons converted. If all the photons produced electrons, the QE would be 100%. Most amateur CCD cameras have QEs in the range of 25-50%. More advanced "back-illuminated" CCDs have QEs around 85%. Supercooled professional CCDs used at observatories have QEs closer to 98%. Compare this to film, which has a typical QE of around 2%. You can see why CCDs are so much faster!


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Readout Noise
This is electronic noise generated in the CCD camera when the data is read out from the camera to the computer.

There are two common definitions for resolution. One refers to the level of detail a CCD camera can capture, usually expressed in arcseconds per pixel. Smaller pixels produce a higher resolution, meaning smaller details can be seen. Resolution is also taken to mean the total number of pixels in a CCD chip or in a CCD image. Often this is given as width and height in pixels, similar to computer screen resolution. For example, a CCD chip might have a resolution of 1600x1200 pixels.

RGB stands for Red, Green, Blue. Most CCDs are inherently black and white and require a color filter wheel to take color images. When red, green, and blue filters are used to create a color image, it is termed RGB imaging and the resulting picture is an RGB image. This is the most common form of color imaging, but there are other techniques used as well, such as LRGB or CMYK imaging.


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Sampling refers to the proper matching of pixel size to focal length to achieve the best possible resolution. Sampling is the essentially number of pixels that a star image covers. If too few, the image is undersampled and does not achieve the best possible resolution. If too many, the image is oversampled which unnecessarily reduces efficiency.

Scaling is a way of adjusting an image to bring out more details. Scaling works by altering an image's histogram, usually to compress the range between bright and faint portions of the image so that both can be displayed by the limited range of a computer monitor.

Seeing conditions refer to the steadiness of the atmosphere. Seeing is critical for obtaining sharp images and is especially critical for planetary imaging. In fact, it is the single more important factor in capturing good planetary images. Seeing is often confused with transparency. Transparency is the clarity of the atmosphere, but has nothing to do with the steadiness of the air. In fact, the two are generally mutually exclusive. Nights of good transparency (dark, clear nights) often have poor seeing. And the steadiest skies often come on hazy nights of lousy transparency.

Self-Guiding is a feature of certain SBIG CCD cameras. In these cameras there are two CCD chips: one for imaging, and a smaller one for guiding the telescope. During long exposures, errors in a telescope's drive prevent perfect star images. Guiding eliminates this problem, but for most CCD cameras a second separate CCD must be used to guide. Self-Guiding CCDs simply use their built-in guiding chips to eliminate the need for a second camera. Also, a guidescope or off-axis guider does not need to be used, avoiding some of the other problems inherent in guiding a telescope using these methods.

Sharpening is a software routine used to enhance detail in a CCD image. There are numerous methods employed for sharpening images--there are even whole programs dedicated only to sharpening. Some are simple sharpening routines, while others involve more complex algorithms (such as the unsharp mask technique), and some employ iterative routines to produce successively sharper images while attempting to minimize the noise inherently generated in sharpening (the Lucy-Richardson and similar techniques).

Spherical Aberration
This aberration comes from the fact that a spherical lens or mirror cannot focus all the light rays from a star to a single point. A proper combination of spherical optics or an aspheric optic will minimize this aberration. The corrector lens on the front of a Schmidt-Cassegrain, for example, is an aspheric lens which corrects the aberration from the spherical primary mirror.

Combining, or stacking, images is a way to enhance the image by reducing noise. Multiple exposures of the same object are combined when stacking images. Since the subject in each image is the same, but noise in the image changes randomly from one picture to the next, stacking the frames together decreases the overall amount of noise. Mathematically, exposures can be stacked by summing or averaging the images, or by more complexalgorithms. Stacking is done automatically using image processing software.

Summing Images
Since astronomical objects tend to be faint, it is often desirable to add CCD images together to increase the detail visible in the image. This process is known as summing. Summing images will also, however, increase the noise in the image.

System Throughput
Also called Transfer Rate, or Download Speed, this describes how quickly the information in the CCD is transferred to the computer. There are several factors involved, including reading the data from the CCD chip, sending it to the computer, and then displaying it on the screen. The total download time is how long you wait from the end of the exposure until you see the image. For older large-format CCDs using parallel-port interfaces this might be as much as 3 minutes, while for newer cameras with high-speed USB 2.0 interfaces there might be less than 1 second of download time.


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Track & Accumulate
SBIG's patented system for taking the equivalent of a long exposure without having to guide the telescope. Instead of guiding during an exposure (which requires either a second CCD camera or Self-Guiding camera), corrections are made between each of a series of exposures which are then combined into a single longer exposure. By keeping the individual exposures short enough, tracking errors do not interfere. However, the individual exposures must be kept shorter than a certain limit imposed by the mount's inherent tracking ability. This is typically 30-60 seconds for most mounts.

Transparency refers to the clarity of the atmosphere. A night of high transparency is very dark and clear. This is ideal for deep sky imaging and observing. Transparency is often confused with seeing. Seeing conditions refer to the steadiness of the atmosphere, but has nothing to do with the clarity of the air. In fact, the two are generally mutually exclusive. Nights of good transparency (dark, clear nights) often have poor seeing. And the steadiest skies often come on hazy nights of lousy transparency.

Tri-Color Imaging
Most astronomical CCD cameras are inherently black-and-white. To take color images, red, green, and blue filters are placed in front of the camera to isolate each color. Each image is then combined later using image processing software to create a color image. This process is called tri-color imaging. See also, RGB, LRGB, and CMYK.


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Unguided Exposure
There are small tracking errors inherent in every telescope system. To avoid these errors ruining the exposure there are two methods that are used. One is to guide the telescope using either a second CCD camera or by using a Self-Guiding CCD. The other way to do it is to take an unguided exposure, short enough to keep the tracking errors from being seen. It is possible to combine multiple unguided exposures to create the effect of a single guided exposure. Often high-quality telescope mounts give tracking accuracy specifications in terms of how long an unguided exposure they are capable of taking.

Unsharp Mask
Unsharp mask is an image-processing technique used to sharpen an image and enhance detail. True unsharp masking is done by subtracting a blurred version of an image from the original. Unsharp mask routines in software simulate this effect and make it easier to have control over the final image.


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Vignetting is a darkening of the corners of an image, usually caused by light falling off toward the edge of the CCD chip due to the optical system. Typically vignetting is seen most prominently in fast-focal-ratio and wide-angle systems such as camera lenses. Vignetting is usually removed by using a flat field.


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An equatorial wedge is used for imaging with a fork-mounted telescope. Most computerized telescopes have alt-azimuth fork mounts where the fork point straight up and down. This configuration is easiest to use because the eyepiece is always in a convenient position and the telescope is less bulky. However, field rotation results if the telescope is not polar aligned. An equatorial wedge mounts between the telescope and tripod to allow the forks to be aimed toward Polaris so the telescope can track in just one axis, eliminating field rotation.

Well Depth
Well depth is a measure of how much charge an individual photosite on a CCD chip can contain. Well depth is generally measured in electrons. For example, a Kodak KAF-1300 CCD chip has a well-depth of 150,000 electrons. When more charge than this fills the photosite, blooming occurs unless the CCD chip has a built-in anti-blooming protection. In general, anti-blooming chips have much lower well depths, which is one of the reason non-anti-blooming CCDs are popular (they are much more sensitive).

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