GLOSSARY OF TERMS
ABG -- Anti-Blooming Gate. An electronic drain structure on a CCD chip which assures that electrons/voltage exceeding the full-well capacity of a pixel do not spill over to adjacent pixels.
ADU -- Analog-to-digital Unit. ADUs are employed as a measurement of pixel value or brightness. Pixel voltages (numbers of electrons) stored during CCD integrations are converted to ADU integers representing the measured voltage compared to maximum (full pixel) voltages in terms of the full Base 2 dynamic range of the CCD system (12 bit = 2^12th, 16 bit = 2^16th, etc.).
ASPECT RATIO -- The relationship of the X and Y scales of a 2-dimensional grid. Non-square CCD pixels are represented as square by video monitors and other output devices, yielding an aspect ratio not in accord with true sky coordinates unless the images are resampled to an aspect ratio that, in effect, squares the pixels.
AUTOGUIDING/GUIDING -- Telescope tracking controlled by feedback from real-time sensing of star movements within the field of view (FOV). Movement may be sensed by an electro-optical device, such as a CCD chip, or by the human eye comparing star movement to a eyepiece reticle intersection or a reticle grid. Autoguiding refers to automatic feedback to telescope drives provided by electronic devices, while manual guiding is accomplished by human feedback intervention using slow-motion controls on telescope drives.
BIAS SIGNAL -- The electrons and subsequent ADU generated by the voltage maintained over the CCD array during integration.
BINNING -- Combining the electrons from adjacent pixels to create, in effect, a single larger pixel.
BLOOMING -- The spilling over of electrons/voltage into adjacent pixels due to exceeding the full-well electronic capacity of a pixel.
CALIBRATION -- Generally, normalizing a system to a set of standards or constants. Specific to CCD imaging, to eliminate unwanted signal and reduce noise components by subtracting a dark frame and dividing by a flat-field frame.
CCD -- Charge-Coupled Device. A silicon wafer capable of generating electrons from impinging photons and compartmentalizing the generated electronic charge for subsequent transfer to data conversion and storage devices. Usually, CCDs are micromanufactured into two-dimensional grids of rows and columns, each intersection comprising a pixel several microns in both dimensions.
CHROMINANCE -- The color parameters of an image. Usually represented by hue and saturation.
CMY -- Cyan, Magenta, Yellow. Named the subtractive colors of the human visual spectrum, since cyan = white - red, magenta = white - green, and yellow = white - blue.
DARK CURRENT -- The electronic signal generated by the thermal characteristics of the CCD even in the absence of impinging light.
DARK FRAME -- An image of the dark current and camera readout and bias signal made by integrating an image while keeping the CCD array in total darkness.
DECONVOLUTION -- An iterative image processing filter that uses Fourier transform mathematics to restore a blurred image as nearly as possible to an unblurred state.
DN -- Data Number. Same as an ADU.
DYNAMIC RANGE -- The ratio of a CCD pixel's full-well capacity to the readout noise. Useful in determining the appropriate number of digitization levels that the analog-to-digital conversion system should use.
FIELD ROTATION -- Rotation of the FOV over time. With an alt-az system that does not control for field rotation, if tracking is otherwise perfect, the stars and other objects in the FOV will pivot around the center of the FOV during a CCD or film exposure or during a visual observing session. Equatorially mounted and driven systems will suffer from field rotation if polar alignment is imperfect.
FILTERING -- 1. Using color-dyed or interference-layered glass inserted into the optical path to restrict the passage of full-spectrum wavelengths. 2. Applying a mathematical function to the pixels in an image array that modifies each pixel's value according to the values of an assigned set of neighboring pixels. Primarily used to blur or sharpen localized aspects of an image.
FITS -- Flexible Image Transport System. The standard data file format for astronomical CCD images. FITS images use file extension names of FTS, FIT, or FITS.
FLAT-FIELD FRAME -- A CCD image of the irregularities in the optical system and the CCD chip. The image is integrated while the optical/CCD system is pointed at a wide-field, evenly illuminated source, such as that provided by a specially manufactured light box, the inside wall of an observatory dome, a large poster board positioned in front of the telescope, or a twilight sky.
FOCAL RATIO -- The effective focal length of an optical system divided by the diameter of the primary optical component.
FOV -- Field of View. The size of an imaged or visual scene in terms of sky dimensions, usually stated in degrees, arcminutes, or arcseconds. The approximate width of the FOV in arcseconds can be calculated by dividing the width of the CCD chip in microns by the focal length of the optical system in millimeters and multiplying the result by 206.
FWHM -- Full Width Half Maximum. A measurement of the size of a point source image, such as a star, in terms of the width of the 50% peak value circumference. Usually stated in units of arcseconds or pixels.
GAMMA -- The logarithmic brightness value assigned to video monitors to allow replication of the logarithmic visual range of the eye.
GRAYSCALE -- The linear array of brightness values assigned to a monochrome image represented in black and white, where 0 = black and the maximum array value = white. For example, in an 8-bit dynamic range, 0 = black, 256 = white, and medium gray = 128.
HISTOGRAM -- A graph of the numbers of pixels at each brightness level in an image
HSL -- Hue, Saturation, Luminance. A particular conformation of color theory.
HUE -- Distinct color or shade.
INTEGRATION -- Acquisition of electronic data from a CCD array. Synonymous with image acquisition or exposure. Analogous to exposure in film terminology.
JPEG -- Joint Photographics Expert Group. A graphics data file format established as a standard for graphics file compression and storage. JPEG images are stored with a file extension name of JPG.
LIGHT BOX -- An internally illuminated container that uses indirectly placed light sources to evenly illuminate a translucent diffuser screen on one side of the container. The screen is sized to cover at least the entire aperture of a telescope so that accurate flat-field frames can be made.
LUMINANCE --The intensity of a source of light. Synonymous with brightness.
MICRON -- One millionth of a meter (0.000001 meter).
NOISE -- The degree of randomness or unpredictability in a signal. Noise components are a primary determinant of the quality of an image.
PIXEL -- Short for picture element. Each cell that constitutes an intersection of the grid of rows and columns on a CCD array is a pixel. A pixel is a single measurable entity for charge storage, release, and ADU conversion on a CCD.
PIXEL VALUE -- A measurement (usually in terms of ADUs or DNs) of the electronic charge obtained in a pixel during a CCD integration. After image processing, a measurement of the processed brightness of a pixel.
PSF -- Point Spread Function. The visible and measurable smear of photons from a point source. Optical diffraction and atmospheric distortion are primary factors causing light from point sources such as distant stars to be smeared into a PSF much larger than the geometrically calculable size of the point source.
QUANTUM EFFICIENCY -- The ratio of effectiveness with which a CCD converts received photons (quanta of light) to measurable electrons. Amateur CCDs operate from about 15% up to about 75%, depending on the wavelength of light. High-end CCDs built for professional use frequently run 80% or higher.
READOUT NOISE -- ADU randomness introduced by the collection, amplification, and analog-to-digital conversion of signal data.
RESAMPLE -- Increase or decrease of the number of pixels in one or both axes of an image using a method that correlates adjacent pixel values to create a smooth pixel-to-pixel brightness transition.
RGB -- Red, Green, Blue. Named the additive colors of the human visual spectrum, since red + green + blue = white.
SAMPLING -- The taking of readings from a single data source. In basic CCD imaging theory, for delivered optical system information not to be lost, the resolution of imaging CCD pixels must be at least twice as precise as the delivered resolution of the optical system (e.g., for appropriate sampling, a telescope delivering a PSF with an FWHM of 2 arcseconds calls for a CCD with pixels having a one-arcsecond FOV.) Less than this level of precision is termed undersampling. Significantly more is termed oversampling.
SATURATION -- Reaching or exceeding the full-well capacity of a pixel. Also, in color theory, the degree of purity of a hue in terms of mixture with white light.
SCALING -- see STRETCHING.
SCT -- Schmidt-Cassegrain Telescope.
SIGNAL --The measured value recorded by a pixel during a CCD integration. The signal in a CCD image usually comprises input from sky, thermal, and electronic sources. The natural degree of randomness in the pixel value is the noise component of the signal.
SNR -- Signal-to-noise ratio. Often shortened to S/N. The higher the SNR, the better the image.
STACKING -- Adding or averaging multiple CCD integrations of the same FOV to emulate the improved SNR of a single longer exposure.
STRETCHING -- Applying a mathematical transfer function to the pixel values in an array so that they are changed to better portray the image in terms of relative brightness levels. Often used synonymously with scaling.
SUBEXPOSURE -- Subintegration. One of a set of stacked CCD integrations.
SYSTEM GAIN -- The number of electrons represented by each ADU. Synonymous with conversion factor.
TELECOMPRESSION -- Reduction of the effective focal ratio of an optical system via the introduction of a positive lens system into the optical path between the basic optical elements and the focal plane.
TIFF -- Tag Image File Format. A graphics data file format established as a standard for uncompressed file storage. TIFF images are stored with a file extension name of TIF.
VIGNETTING -- Restriction of an FOV such that all parts of a desired imaging surface at the focal plane (such as a CCD chip, a piece of film, or the eye's pupil) are not illuminated by the entire primary optical element. Usually caused by an undersized or improperly placed secondary, or by narrow apertures in the focuser/camera assembly.