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: http://www.stsci.edu/documents/dhb/web/c32_wfpc2dataanal.fm4.html
Дата изменения: Wed Jul 1 20:35:22 1998 Дата индексирования: Sat Dec 22 17:30:32 2007 Кодировка: Поисковые слова: solar system |
Because WFPC2 consists of four physically separate detectors, it is necessary to define a coordinate system that includes all four detectors. For convenience, sky coordinates (right ascension and declination) are often used; in this case, they must be computed and carried to a precision of a few mas, in order to maintain the precision with which the relative positions and scales of the WFPC2 detectors are known. It is important to remember that the coordinates are not known with this accuracy. The absolute accuracy of the positions obtained from WFPC2 images is typically 0."5 rms in each coordinate and is limited primarily by the accuracy of the guide star positions.
The recommended way to convert pixel coordinates into relative coordinates is to use the task metric, which can handle both WF/PC and WFPC2 images. For WFPC2 images, metric corrects for the geometric distortion introduced by the camera optics, primarily the field flattening lenses, and brings the four chips into the metachip reference system, defined so as to have the same orientation and plate scale as the WF2 chip at its center. These coordinates are then converted into right ascension and declination by using the position and orientation of the WF2 chip. A related task, invmetric, can be used to effect the opposite transformation, from right ascension and declination to chip and pixel position. The final relative positions are accurate to better than 0."005 for targets contained on one chip, and 0."1 for targets on different chips. Note that both metric and invmetric include specialized information about the geometry of WFPC2. They do not use the header parameters that describe the world coordinate system (CRVAL1 and 2, CRPIX1 and 2, and the CDMATRIX) to relate positions in different chips. Of these parameters, only the values for WF2 are used to convert the metachip positions to and from right ascension and declination. As a side effect, neither task can work on images that do not contain WF2, for which the xy2rd task can be used.
Early WFPC2 images contain header parameters with less accurate values of the plate scale and of the chip-to-chip rotations. The task uchcoord will modify the header parameters of these images to reflect more current information. The task uchcoord also corrects the header parameters for an error that occurred between April 11 and 19, 1994. The task will stamp the file after the transformation, to prevent users from unwittingly correcting the same image twice. It is recommended that uchcoord be run on all images taken before March 1995; the task will not apply any correction unless it is warranted, and thus it is safe to run on any WFPC2 image. There is also some evidence, both from internal exposures and from a study of many external WFPC2 exposures taken over the years (Ratnatunga et al., 1997), that the relative position of the four detectors may have shifted by up to 0."1 since launch, and especially across the cooldown of April 23, 1994. A regular monitoring program (CAL 7627) is in place to follow such variations. Should they occur again, the relevant tasks, metric and invmetric, will be modified to return the information appropriate to the date of observation.
Rough coordinates can also be obtained using the task xy2rd, which uses the world coordinate system parameters in each group to determine the coordinates associated with a given pixel position. However, xy2rd does not use the most recent information on the relative chip positions, and it does not apply the geometric correction. Each can result in an error of about 0."3, especially near the edges of the chip; typical errors are closer to 0."1.