35.2 Header Keywords

An HST observation is composed of packets of information sent down to the ground, and each observation packet has an embedded packet format code to identify the packet as SHP, UDL, or science. Each science packet has a packet format code (PKTFMT) that identifies the type of science. The default GHRS calibration switches are set by the PKTFMT keyword value, specified by the type of observation (Target ACQuisition, IMAGE, ACCUM, or RAPID mode). The default calibration switches have been selected to achieve the best possible calibration. The PKTFMT values are set when the observations are scheduled. They are used to determine the values of the science header keywords. This information is stated here for completeness.

The GHRS calibration steps are listed and described in "Recalibrating GHRS Data" on page 36-13.

Important GHRS Header Keywords

Keyword	Description and Values
GCOUNT	Number of data groups
FILETYPE	shp, udl, ext, sci, img, wav, flx
NBINS	Number of substep bins in this pattern
RPTOBS	Expected number of observation repeats
STEPPATT	Step pattern sequence
FP_SPLIT	FP-SPLIT (NO, TWO, FOUR, DSTWO, DSFOUR)
COMB_ADD	Comb-addition (NO, TWO, FOUR, DSTWO, DSFOUR)
FINCODE	Observation termination code
OBSMODE	Observation mode (ACCUM, RAPID, SPYBAL, DEFCAL, ACQ,)
DETECTOR	Detector in use (1 or 2)
GRATING	Grating, echelle, or mirror in use
APERTURE	Aperture name
DATE	Date this file was written (dd/mm/yy)
KZDEPLOY	COSTAR deployed for the HRS (T or F)
APER_FOV	Aperture field of view (arcsec), N/A for cal
FGSLOCK	Commanded FGS lock (FINE, COARSE, GYROS, UNKNO
DATE-OBS	UT date of start of observation (dd/mm/yy)
TIME-OBS	UT time of start of observation (hh:mm:ss)
EXPTIME	Exposure duration (seconds)-calculated