25.2 Header Keywords

http://archive.stsci.edu/keyword 

Some keywords can occasionally be confusing. Users should pay special attention to DATE, which refers to the date the file was written, as opposed to DATE-OBS, the date of the observation; FILTER1 and FILTER2, which are the numerical designations of the filter(s) used, while the filter names are FILTNAM1 and FILTNAM2; and PHOTZPT, which is not the photometric zeropoint (in the ST magnitude system) as normally understood, but rather the zeropoint in the ST magnitude system to be used after conversion to FLAM units (see "Photometric Zero Point" on page 28-1).

Astrometry keywords can also be confusing. The orientation of the image is specified by a global keyword, PA_V3, and a group keyword, ORIENTAT. The keyword PA_V3 gives the position angle (in degrees) of the telescope's V3 axis on the sky, measured from North through East. The V3 axis is roughly 225 degrees from the x axis of the PC. This axis differs by exactly 180 degrees from the ORIENT value specified in the Phase II submission, which refers to the U3 axis. The group keyword ORIENTAT gives, for each detector, the position angle of the y axis in the plane of the sky, measured from North through East. This angle differs from PA_V3 by roughly -135, -45, +45, and +135 degrees, for PC1, WF2, WF3, and WF4, respectively. See Figure 24.2 for a graphic representation of the geometric relationship between the four detectors. Accurate values of the positions and rotations between chips can be obtained from the Science Instrument Aperture File (SIAF). The SIAF is available via the WWW at:

http://www.stsci.edu/ftp/instrument_news/

Observatory/siaf.html