HST Data Handbook for WFPC2

2.2 Header Keywords

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Table 2.3 and table 2.4 list some of the more commonly used keywords found in a calibrated WFPC2 image (.c0h). A complete list of header keywords, with brief explanations, can be found in the WWW Keyword Dictionary.

The values of any of the header and group keywords can be determined using STSDAS tasks such as hedit, hselect, and imhead. The examples below illustrate the use of these tasks on a general image keyword (EXPSTART) and a group keyword (DETECTOR); the "[3]" string specifies the group desired.

Table 2.2: hedit, hselect and imhead examples

cl>hedit u5kl0101r.c0h[3] expstart,detector . u5kl0101r.c0h[3],EXPSTART = 5.159859322738E+04 u5kl0101r.c0h[3],DETECTOR = 3 cl> hsel u5kl0101r.c0h[3] expstart,detector yes 5.159859322738E+04 3 cl> imhead u5kl0101r.c0h[3] long+ | match EXPSTART EXPSTART= 5.159859322738E+04 / exposure start time (Modified Julian Date) cl> imhead u5kl0101r.c0h[3] long+ | match DETECTOR DETECTOR= 3

 

Most of the tasks provide only the keyword value; imhead shows both the value and the description for primary header keywords, but only the value for group keywords. Descriptions for group keywords can be viewed with a standard editor; each group keyword has three entries in the header file (e.g., c0h): one line defining the name and description of the group keyword, one specifying the space required for the keyword in the binary data file, and one providing the data type. As mentioned earlier, the group keyword value is stored in the binary data file and is only accessible via one of the STSDAS tasks. As an example, the entries for the group keyword DETECTOR are given below.

cl>page u5kl0101r.c0h ... PTYPE19 = 'DETECTOR' /CCD detector: PC 1, WFC 2-4 PDTYPE19= 'INTEGER*4' / PSIZE19 = 32 / ...

 

WFPC2 header keywords include items such as observing mode, integration time, filters and reference files used, calibration steps performed, and the properties of the data itself (e.g., number of groups, dimensions in pixels of each group); some additional calibration information is given at the end of the header file in the header HISTORY keywords. Group keywords include the reference pixels, coordinates, scale, flux units, and image statistics for each chip.

Observers new to WFPC2 data should peruse the header keyword comments in their images, as some keywords can occasionally be misleading. For example, DATE refers to the date the file was written while DATE-OBS refers to the date of the observation and PROCTIME refers to the date and time an image was processed through the On-The-Fly system, see section 3.1. The FILTER1 and FILTER2 keywords record the numerical designations of the filter(s) used while the filter names are stored in FILTNAM1 and FILTNAM2. Also, PHOTZPT 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 table 5.1).

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 (see figure 1.2). The V3 axis is roughly 225 degrees from the x axis of the PC. The V3 axis differs by exactly 180 degrees from the ORIENT¹ value, if any, specified in the Phase II submission. The group keyword, ORIENTAT, records, 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 1.2 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), which is available on the WWW. In addition, each group has eight coordinate-related keywords (listed under coordinate-related keywords below) that specify the transformation from pixels to world coordinates (right ascension and declination) in the tangent plane approximation at the center of the chip.

Table 2.3: Frequently-used WFPC2 Group Header Keywords

Keyword	Description
Image keywords
DETECTOR	CCD detector contained in group (PC1, WF2, WF3, WF4)
Coordinate-related keywords
CRVAL1	RA of reference pixel (deg)
CRVAL2	Dec of reference pixel (deg)
CRPIX1	X coordinate of reference pixel
CRPIX2	Y coordinate of reference pixel
CD1_1	Partial derivative of RA with respect to x
CD1_2	Partial derivative of RA with respect to y
CD2_1	Partial derivative of Dec with respect to x
CD2_2	Partial derivative of Dec with respect to y
Orientation
MIR_REVR	Is image mirror reversed?
ORIENTAT	Orientation of y axis (deg)-different for each group
Bias level information (based on pixels in columns 9-14, rows 10-790 in .x0h/.x0d file)
DEZERO	Bias level from engineering data
BIASEVEN	Bias level based on average of odd columns in.x0h/.x0d file (used for even columns in science image)
BIASODD	Bias level based on average of even columns in.x0h/.x0d file (used for odd columns in science image)
Pixel statistics
GOODMIN	Minimum value of "good" pixels (not flagged in DQF)
GOODMAX	Maximum value of "good" pixels
DATAMEAN	Mean value of "good" pixels
GPIXELS	Number of good pixels (out of 640,000 possible)
ATODSAT	Number of saturated pixels
Photometry keywords (populated if DOPHOTOM=yes)
PHOTMODE	Photometry mode
PHOTFLAM	Inverse sensitivity (units of erg/sec/cm2/Å for 1 DN/sec)
PHOTZPT	Zeropoint (currently -21.10)
PHOTPLAM	Pivot wavelength (in angstroms)
PHOTBW	rms bandwidth of filter (in angstroms)
Image statistics keywords
MEDIAN	Middle data value when good quality pixels sorted
HISTWIDE	Width of the histogram
SKEWNESS	Skewness of the histogram
MEANC10	Mean of a 10 x 10 region at the center of the chip
MEANC100	Mean of a 100 x 100 region at the center of the chip
MEANC300	Mean of a 300 x 300 region at the center of the chip
BACKGRND	Estimated background level

 
Table 2.4: Frequently-used WFPC2 General Header Keywords

Keyword	Description
Information about the groups
GROUPS	Multi-group image? Indicates whether data has groups.
GCOUNT	Number of groups per observation (1 to 4)
Image keywords
INSTRUME	Instrument used; always WFPC2
ROOTNAME	Rootname of the observation set
FILETYPE	SHP - standard header packet EXT - extracted engineering file EDQ - EED data quality file SDQ - science data quality file SCI - science data file
MODE	Mode: FULL (full resolution) or AREA (2 x 2 pixel summation)
SERIALS	Serial clocks: ON, OFF
Data type keywords
IMAGETYP	DARK/BIAS/IFLAT/UFLAT/VFLAT/KSPOT/EXT/ECAL
CDBSFILE	GENERIC/BIAS/DARK/FLAT/MASK/NO Is the image a reference file and if so, type is specified
Reference file selection keywords
DATE	File creation date
FILTNAM1	First filter name
FILTNAM2	Second filter name; blank if none
FILTER1	First filter number (0-48) (Used in SOGS - Science Operations Ground System)
FILTER2	Second filter number (0-48)
FILTROT	Partial filter rotation angle (degrees)
More reference file selection keywords
ATODGAIN	Analog to digital gain (electrons/DN)
Calibration switches
MASKCORR	Do mask correction: PERFORM, OMIT, COMPLETE
ATODCORR	Do A-to-D correction: PERFORM, OMIT, COMPLETE
BLEVCORR	Do bias level correction: PERFORM, OMIT, COMPLETE
BIASCORR	Do bias correction: PERFORM, OMIT, COMPLETE
DARKCORR	Do dark correction: PERFORM, OMIT, COMPLETE
FLATCORR	Do flatfield correction: PERFORM, OMIT, COMPLETE
SHADCORR	Do shaded shutter correction: PERFORM, OMIT, COMPLETE
DOSATMAP	Output saturated pixel map: PERFORM, OMIT, COMPLETE
DOPHOTOM	Fill photometry keywords: PERFORM, OMIT, COMPLETE
DOHISTOS	Make histograms: PERFORM, OMIT, COMPLETE
OUTDTYPE	Output image datatype: REAL, LONG, SHORT
Calibration reference files used1
MASKFILE	Name of the input static mask containing known bad pixels and charge traps
ATODFILE	Name of the A-to-D conversion file
BLEVFILE	Name of engineering file with extended register data
BLEVDFIL	Name of data quality file (DQF) for the engineering file
BIASFILE	Name of the superbias reference file
BIASDFIL	Name of the superbias reference DQF
DARKFILE	Name of the dark reference file
DARKDFIL	Name of the dark reference DQF
FLATFILE	Name of the flatfield reference file
FLATDFIL	Name of the flatfield reference DQF
SHADFILE	Name of the reference file for shutter shading
PHOTTAB	Name of the photometry calibration table
SATURATE	Data value at which saturation occurs (always 4095 for WFPC2, which includes the bias)
Ephemeris data
PA_V3	Position angle of V3 axis of HST
RA_SUN	Right ascension of the sun (deg)
DEC_SUN	Declination of the sun (deg)
EQNX_SUN	Equinox of the sun
Fill values
PODPSFF	0 (no fill), 1 (fill present)
RSDPFILL	Bad data fill value set in pipeline for calibrated image
Exposure Information
DARKTIME	Estimate of darktime (in sec)
EQUINOX	Equinox of the celestial coordinate system
SUNANGLE	Angle between sun and V1 axis (deg)
MOONANGL	Angle between moon and V1 axis (deg)
SUN_ALT	Altitude of the sun above Earth's limb (deg)
FGSLOCK	Commanded FGS lock (FINE, COARSE, GYROS, UNKNOWN)
Timing information
DATE-OBS	UT date of start of observation
TIME-OBS	UT time of start of observation (hh:mm:ss)
EXPSTART	Exposure start time (Modified Julian Date)
PROCTIME	Date and time of OTFR pipeline processing (Modified Julian Date)
EXPEND	Exposure end time (Modified Julian Date)
EXPTIME	Exposure duration (seconds)
EXPFLAG	How exposure time was calculated. (NORMAL, INTERRUPTED, INCOMPLETE, EXTENDED, UNCERTAIN, INDETERMINATE, or P REDICTED)
Proposal information
TARGNAME	Proposer's target name
RA_TARG	Right ascension of the target (deg) (J2000)
DEC_TARG	Declination of the target (deg) (J2000)
PROPOSID	RPS2 proposal identifier

1 Calibration reference file keywords are populated even if unused.

 
¹ ORIENT is an optional parameter used in HST proposals to request a particular orientation of the field of view on the sky and refers to the position angle of the U3 axis.