15.2 Header Keywords

The primary header carries global keywords which are shared by all extensions; the modification of a keyword in the primary header propagates that modification into all extension headers. In addition to the common keywords, the extension headers carry extension-specific keywords, which contain information relevant to the image in a particular extension. For example, observation parameters, calibration switches and reference file names are contained in the primary header. Exposure time and coordinate information, on the other hand, is contained in the header of each image extension because this information could vary from one group of extensions to another.

Table 15.4 below lists most of the keywords in the primary header of the science data files; the observer may find many of them useful. A complete description of the keywords can be found at the following WWW page:

Science Data File Primary Header Keywords

Keyword Name	Meaning
Image Keywords
NEXTEND	Number of extensions in the file (up to 130 for MULTIACCUM).
FILENAME	Name of the file
FITSDATE	Date on which the FITS file was generated
FILETYPE	Type of data: SCI - Science Data File SPT - Support File ASN_TABLE - Association Table
TELESCOP	HST
INSTRUME	Instrument used (NICMOS)
EQUINOX	Equinox of the celestial coordinate system (J2000.0 for HST observations)
Data Description Keywords
ROOTNAME	Rootname (IPPPSSOOT) of the dataset
IMAGETYP	Image type EXT = external image FLAT = flatfield image DARK = dark image
PARALLEL	Indicates if the observation was taken in parallel
PRIMESI	Primary Instrument used for the observation
Target Information
TARGNAME	Proposer's target name
RA_TARG	Right Ascension of the target (degrees, J2000)
DEC_TARG	Declination of the target (degrees, J2000)
Proposal Information
PROPOSID	Proposal's identification number
LINENUM	Exposure's logsheet linenumber, from the Phase 2 proposal
Exposure Information
ORIENTAT	Position angle of the image y axis (degrees East of North)
FGSLOCK	Commanded FGS lock (FINE, COARSE, GYROS, UNKNOWN)
DATE-OBS	UT date of start of the observation (dd/mm/yy)
TIME-OBS	UT time of start of the observation (hh:mm:ss)
EXPSTART	Exposure start time (Modified Julian Date)
EXPEND	Exposure end time (Modified Julian Date)
EXPTIME	Total integration time (sec)
Instrument Configuration Information
CAMERA	NICMOS camera used in the observation (1, 2, or 3)
PRIMECAM	NICMOS Prime Camera during the observation (for internal parallels)
APERTURE	Aperture used in the observation (NICi, NICi-FIX, NIC2-CORON)
OBSMODE	Observing mode (MULTIACCUM, ACCUM,...)
FILTER	Filter or grism used
NUMITER	Number of iterations in the exposure
NREAD	Number of ACCUM initial and final readouts
NSAMP	Number of MULTIACCUM or RAMP samples
SAMP_SEQ	MULTIACCUM exposure time sequence name
PATT_OFF	Pattern offset method (SAM, SAM-NO-REACQ, etc.)
PATTERN	Pattern type
PFRAME	Reference frame of pattern (DETECTOR, SKY)
PORIENT	Pattern orientation on the sky relative to North-to-East (degrees)
NUMPOS	Number of positions in the pattern
DITHSIZE	Size of the dither steps (arcseconds)
CHOPSIZE	Size of the chop steps (arcseconds)
PATT_POS	Position number of the exposure in the pattern sequence
FOMXPOS	X offset of the Camera FOV using NICMOS FOM (arcsec)
FOMYPOS	Y offset of the Camera FOV using NICMOS FOM (arcsec)
READOUT	Detector array readout rate (FAST, SLOW)
ADCGAIN	Analog-digital conversion gain (electrons/DN)
Photometry Keywords
PHOTMODE	Combination of <INSTRUME>+<CAMERA>+<FILTER>
PHOTFLAM	Inverse sensitivity (erg/cm2/angstrom/DN)
PHOTFNU	Inverse sensitivity (Jy*sec/DN)
PHOTZPT	ST magnitude system zero point
PHOTPLAM	Pivot wavelength of the photmode (Angstrom)
PHOTBW	Root Mean Square bandwidth of the photmode (Angstrom)
Calnica Calibration Reference Files (inputs to calnica)
MASKFILE	Static data quality file
NOISFILE	Detector read noise file
NLINFILE	Detector non-linearity file
DARKFILE	Dark current file
FLATFILE	Flat field file
PHOTTAB	Photometric calibration table
BACKTAB	Background model parameters table
Calnica Calibration Reference File Pedigree (outputs from calnica)
MASKPDGR	Static data quality file pedigree (values: GROUND dd/mm/yyyy - for reference files originated from Thermal Vacuum data; INFLIGHT dd/mm/yyyy - for reference files originated from on-orbit calibration observations)
NOISPDGR	Detector read noise file pedigree (values: GROUND dd/mm/yyyy; INFLIGHT dd/mm/yyyy)
NLINPDGR	Detector non-linearity file pedigree (values: GROUND dd/mm/yyyy; INFLIGHT dd/mm/yyyy)
DARKPDGR	Dark current file pedigree (values: GROUND dd/mm/yyyy; INFLIGHT dd/mm/yyyy; and MODEL dd/mm/yyyy for the synthetic darks, see next Chapter)
FLATPDGR	Flat field file pedigree (values: GROUND dd/mm/yyyy; INFLIGHT dd/mm/yyyy)
PHOTPDGR	Photometric calibration table pedigree (values: GROUND dd/mm/yyyy; INFLIGHT dd/mm/yyyy)
BACKPDGR	Background model parameters table pedigree (values: GROUND dd/mm/yyyy; INFLIGHT dd/mm/yyyy)
Calnica Calibration Switches (allowed values: PERFORM, OMIT)
BIASCORR	Correct wrapped pixel values
ZOFFCORR	Subtract MULTIACCUM zero read
MASKCORR	Data quality initialization (DQ array)
NOISCALC	Calculate statistical errors (ERR array)
NLINCORR	Correct for detectors non-linearities
DARKCORR	Dark correction
FLATCORR	Flat-field correction
UNITCORR	Convert to count rate
PHOTCALC	Populate photometry keywords
CRIDCALC	Identify cosmic ray hits (update of DQ arrays in *_ima.fits output of calnica for MULTIACCUM)
BACKCALC	Calculate background estimates
WARNCALC	Generate user warnings
Calnica Calibration Indicators (output from calnica; values: PERFORMED, SKIPPED, OMITTED)
BIASDONE	Correct wrapped pixel values
ZOFFDONE	Subtract MULTIACCUM zero read
MASKDONE	Data quality initialization (DQ array)
NOISDONE	Calculate statistical errors (ERR array)
NLINDONE	Correct for detectors non-linearities
DARKDONE	Dark correction
FLATDONE	Flat-field correction
UNITDONE	Convert to count rate
PHOTDONE	Populate photometry keywords
CRIDDONE	Identify cosmic ray hits (update of DQ arrays in *_ima.fits output of calnica for MULTIACCUM)
BACKDONE	Calculate background estimates
WARNDONE	Generate user warnings
Calibration Status
CALSTAGE	State of the calibration (values: CALNICA, CALNICB, UNCALIBRATED)
CAL_VER	Version number of the CALNIC code
Calnicb Calibration Information
ILLMCORR	Subtraction of background illumination pattern reference image (input values: PERFORM, OMIT)
ILLMDONE	Subtraction of background illumination pattern reference image (output values: PERFOMED, SKIPPED, OMITTED)
ILLMFILE	Background illumination pattern reference image filename
ILLMPDGR	Background illumination pattern file pedigree
MEAN_BKG	Mean background level (DN/sec), computed by calnicb
Association Keywords
ASN_ID	Association rootname
ASN_TAB	Name of the association table
ASN_MTYP	Role of the dataset in the association (e.g., targe, ; first background, second background, etc.; allowed values: EXP_TARG, EXP_BCKn, PROD_TARG, PROD_BCKn, where EXP=input exposure, PROD=output image, TARG=target, BCK=background, n=1-8)

http://archive.stsci.edu/keyword

Table 15.5 lists some of the relevant keywords that are specific to image extensions; they appear in the extension headers, but not in the primary header:

Image Extension Header Keywords in Science Data Files

Keyword Name	Meaning
Data Description Keywords
EXTNAME	Name of the extension in an imset of the data file (SCI, ERR, DQ, SAMP, TIME)
EXTVER	Extension version; integer number to uniquely identify an IMSET in a science data file. A MULTIACCUM file can contain up to 26 IMSETs, i.e. up to EXTVER=26.
INHERIT	Switch to allow the image extension header to inherit the primary header keywords. Allowed values: T=TRUE, F=FALSE.
DATAMIN	Minimum pixel value
DATAMAX	Maximum pixel value
BUNIT	Brightness units; allowed values: COUNTS, COUNTS/S.
PIXVALUE	When ALL pixels in an image extension have the same value (e.g., the SAMP and TIME arrays in the *_ima.fits file from a MULTIACCUM exposure or the ERR, DQ, SAMP and TIME arrays in the *_raw.fits files from a MULTIACCUM, ACCUM or BRIGHTOBJ exposure) the pixel array of that extension is not generated, and the PIXVALUE keyword is instead populated with the common value of the pixels.
World Coordinate System of Image
CRPIX1	x-coordinate of image's reference pixel
CRPIX2	y-coordinate of image's reference pixel
CRVAL1	RA of reference pixel (degrees)
CRVAL2	DEC of reference pixel (degrees)
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
Readout Parameters
SAMPNUM	Sample number of the MULTIACCUM sequence
SAMPTIME	Total integration time (sec)
DELTATIM	Integration time of the sample (sec)
ROUTTIME	UT time of array readout (MJD)

In the SCI image extensions, additional keywords describing the data quality are present. They give the number of pixels which have a flag different from zero in the DQ extension, and a suite of statistical information (mean, standard deviation, minimum and maximum of good pixels in the entire detector and per quadrant) on the image.

Keywords giving the data on the ephemeris and engineering data on the status of the telescope and of the instrument during the observations are reported in the support file, *_spt.fits. Table 15.6 describes some of the relevant keywords from this file.

NICMOS Primary Header Keywords in the Support Files

Keyword Name	Meaning
PA_V3	Position angle of the V3 axis of HST (degrees)
RA_V1	RA of the V1 axis of HST (degrees in J2000)
DEC_V1	DEC of the V1 axis of HST (degrees in J2000)
RA_SUN	RA of the Sun (degrees in J2000)
DEC_SUN	DEC of the Sun (degrees in J2000)
RA_MOON	RA of the Moon (degrees in J2000)
DEC_MOON	DEC of the Moon (degrees in J2000)