5.2 Header Keywords

• Configuration or optical relay (OPTCRLY): f/96 or f/48.
• Filters employed (FILTNAM*).
• Spectrographic mirror position (SMMMODE).
• Image size (SAMPPLN and LINEPFM).
• Position of starting pixel on photocathode (SAMPOFF and LINEOFF).
• Pixel size (PXFORMT): normal or zoomed.
• Reference pixel in the chosen format (CRPIX1 and CRPIX2). For easy reference, Table 5.2 lists these header keywords, their definitions, and the possible keyword values for the commonly used FOC observing modes.

  The quickest way to learn how each observation was actually performed is to use the iminfo task in the STSDAS toolbox.headers package. This task provides a user-friendly synopsis of the most relevant header information, extracted from the ASCII header and the group parameters in the binary data file. Figure 5.1 shows sample results of running iminfo on the final calibrated data file for an FOC image. Included in the listing are the target name, target RA and Dec, observation date, exposure time, basic image statistics, basic instrument configuration, basic observing mode, the calibration steps performed, and the number of groups in the image (only one for the FOC).

cl> imheader x2x10108t.c1h long+ | page

The values of the target keywords are extracted from the proposal prior to execution, with the orientation keyword ORIENTAT providing the angle between North and the image's y axis. The exposure keywords, on the other hand, describe the actual execution of the observation. For example, if a problem interrupted the exposure, the EXPFLAG keyword would report this condition. After the observation has been taken, standard processing supplies information on the filters, format, and optical relay used for the image. The PHOTMODE keyword concisely summarizes the image configuration, and the inverse sensitivity keyword PHOTFLAM gives the factor which converts count rates to flux units (see Chapter 3 for more on HST photometry keywords).

FOC images taken after mid-November 1993 contain the KX_DEPLOY keyword in their headers. This keyword has the value "T" if COSTAR is deployed and "F" otherwise. Before November 1993, the KX_DEPLOY keyword did not exist, but in most cases the name of the data file itself will tell you whether the image is aberrated. Rootnames of observations taken prior to the December 1993 servicing mission begin with x0 or x1, while FOC images taken after the servicing mission begin with x2, x3, or x4 and benefit from COSTAR correction. A small number of images were taken after the first servicing mission but before COSTAR deployment; however, these were generally uninteresting calibration images.

An HST Keyword Dictionary is available via the world wide web at:

http://archive.stsci.edu/keyword/