Calibration: The process to extract the final (calibrated) image from the raw CCD frame. For astrometric purposes, dark frame and flat field calibration is recommended. Further image processing (e.g., histogram scaling, application of filters or deconvolution) is not recommended for astrometric or photometric applications, because it will distort the centroids and flux of the objects in the image.
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Centroid: The "center of gravity" of the image of an object, defining the position of the object with subpixel precision.
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Contact Details: The postal and email address given in the header of the MPC Report File. The contact details must include the name of a person connected with the observing program (i.e., the principal observer), a postal address (not necessarily the postal address of the observatory), and an e-mail address for that person. For a description of the format, see http://cfa- www.harvard.edu/iau/info/ObsDetails.html.
Dark Frame: A calibration image, taken with the same integration time and at the same temperature as the CCD image, but with the shutter closed. A dark frame therefore samples the dark current of the CCD chip that accumulates during a given integration time with the CCD chip cooled to a certain temperature. By subtracting the dark frame from the CCD image, the dark current that accumulated during the exposure is removed.
Flexible Image Transport System (FITS):
The standard image file format for astronomical images. Basic FITS files contain the image data (numeric value for each pixel in the image) and a header with mandatory (e.g., size if the image) and additional (e.g., date and time when the image was taken) information. Astrometrica can read FITS images that are stored using 8 or 16 bit integer values.
The definition of the FITS format is available online at http://archive.stsci.edu/fits/fits_standard/.
Flat Field Frame: A calibration image, taken with the telescope pointing towards a uniformly illuminated area (e.g., the observatory dome or the twilight sky). The flat field frame records the variations in the sensitivity of the individual pixels of the CCD chip, the vignetting of the optical system, and the shadows cast by dust particles on surfaces in the optical path. By applying a flat field correction, these effects are removed.
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Flux: The stream of photons coming from a light source. The number of photons collected by a pixel, and therefore the pixel value, is a measure of the flux, and thus of the brightness of the light source.
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Full Width Half Maximum (FWHM): The width of a curve (for example, the PSF) at half of it's peak value. In astronomy, the "diameter" of a stellar image as it appears on a CCD frame is often described by the FWHM of the PSF of the source. Although a number of factors control the FWHM of a stellar image (like focusing, telescope optics, and vibrations), it is usually dominated by the seeing. The FWHM is the same for all point-sources in the image (if optical aberrations can be neglected). Most notably, it is independent of the brightness of the object. Bright stars appear larger on the image only because the faint outer extensions of the PSF are visible. For faint stars, these parts drown in the noise and are therefore not visible.
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Histogram: A graph or table showing the number of pixels with certain pixel values.
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Log-File: A file that logs the results obtained in the process of measuring an image. It contains detailed information on the reference stars, the plate constants, and the objects detected by the software.
Minor Planet Center (MPC): The international clearing house for discoveries and astrometric observations of minor planets and other minor bodies of the solar system, located at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. On behalf of the International Astronomical Union (IAU), the MPC collects all astrometric observations of minor planets and comets worldwide, calculates orbits for these objects, and publishes these data in the Minor Planet Circulars and Minor Planet Electronic Circulars. The homepage of the MPC, which provides many useful items of information and services for observers, is located at http://cfa-www.harvard.edu/iau/mpc.html.
MPCOrb Database: A daily updated database containing current orbital elements for all minor planets, supplied by the MPC.
MPC Report File: A file that lists the results of the astrometric data reduction for submission to the MPC in a standardized format. This format is described at http://cfa-www.harvard.edu/iau/info/OpticalObs.html.
Observatory Code: A numeric or alphanumeric code assigned to observatories working in astrometry of minor planets or comets, assigned by the MPC. A list of observatory codes is available at http://cfa-www.harvard.edu/iau/lists/ObsCodes.html.
Packed Designation: A packed from of the designation of a minor planet or comet, as described at http://cfa-www.harvard.edu/iau/info/PackedDes.html.
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Plate Constants: The coefficients of a polynomial function that is used to transform the rectangular coordinates measured on the CCD image to normalized standard coordinates, which then can be transferred to spherical coordinates (Right Ascension and Declination).
Point Spread Function (PSF): A function that describes the distribution of light from a point source in the focal plane of a telescope, after it has passed the atmosphere and the optics of the telescope. A Gaussian (bell shaped) curve is generally a good model for the PSF.
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Residuals: Generally, the difference between a measured value and the value predicted from a model. In Astrometry, the term is applied to the difference between the observed position (or magnitude) of a reference star and the position (or magnitude) listed in the star catalog. It is also used for the difference between the observed position of an object (minor planet or comet) and the position calculated from an orbit.
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Reference Stars: Star with known position and/or magnitude, that are used to find the plate constants and photometric calibration of an image.
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Root Mean Square (RMS): The square root of the average value of the square of residuals. This value generally describes how good a model fits to a set of data.
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Roving Observer Format: While the MPC assigns an Observatory Code to observatories or fixed observing places, it has created a special format for roving observers and non-permanent observing sites. The format is described at http://cfa-www.harvard.edu/iau/info/RovingObs.html.
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Signal to Noise Ratio (SNR): The ratio of the signal to the random noise in a set of data. In general, a higher SNR means that more accurate and more reliable information can be extracted from the data.
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Star Catalog: The software must have access to a star catalog, which lists the known position and magnitude of stars. By comparing the data from the star catalog with the position of the stars measured from the image, the software is able to calculate the Plate Constants, and thus, also the celestial coordinates of all objects on the image. Currently, the software supports access to the UCAC 2, the USNO-A2.0, the USNO-SA2.0, the the USNO-B1.0 star catalogs.
UCAC 2: The US Naval Observatory's CCD Astrograph Catalogue (UCAC) is a high-precision, high-density star catalogue. At this time, however, the UCAC 2 covers the complete southern hemisphere and the northern hemisphere up to about +45° declination only. The final version, that will cover the whole sky, is expected to be released in 2004. For more information on the UCAC, visit UCAC web site at http://ad.usno.navy.mil/ucac/.
Universal Time (UT): Same as Greenwich Mean Time (GMT).
USNO-A2.0: This star catalog, compiled by the US Naval Observatory, includes astrometric and photometric data on nearly half a billion stars, from the brightest naked eye stars down to magnitude 23. It is distributed by the USNO on a set of 11 CD-ROMs.
USNO-B1.0: The USNO-B catalogue is the successor of the USNO-A2.0 and presents positions, proper motions and magnitudes for more than a billion objects. Due to the enormous size (80 Gigabytes), this catalogue is not distributed on some media by the USNO, but it is available online trough the Internet. If you select this catalogue in the Program Settings, Astrometrica will automatically connect trough the Internet to download reference star data.
USNO-SA2.0: The USNO-SA2.0 is a subset of the USNO-A2.0 star catalog. It contains data on about 55 million objects, extracted from the USNO-A catalog to produce a spatially uniform net of faint (mainly magnitude 16 to 19) reference stars. Due to the particular selection method for the stars in that catalog, visual reference star identification can be difficult.
World Coordinate System (WCS): World Coordinate System keywords in the FITS-Header define the relationship between pixel coordinates in the image (x/y) and sky coordinates (Right Ascension and Declination). Software that can read WCS keywords can display sky coordinates for any pixel in the image without finding the complete astrometric solution for the image.
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