2.2 Header Keywords

Both the primary header and the headers of each image extension in a science data file contain keywords. The keywords store a wide range of information about the observations themselves (e.g., observing mode, integration time, filters or grisms used), the processing of the data by the OPUS pipeline (e.g., calibration steps performed and reference files used), and the properties of the data themselves (e.g., number of image extensions, dimensions and data type of each image, coordinate system information, flux units, and image statistics).

The primary header carries global keywords which are applicable to all extensions. 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 World Coordinate System information, on the other hand, are contained in the header of each image extension because this information could vary from one set of extensions to another.

Table 2.4 below lists most of the keywords in the primary header of the science data files. The entries in this table are appropriate for data retrieved in 2009 from the HST Archive via on-the-fly reprocessing (OTFR - see Chapter 3). Some header keywords in older NICMOS data may be different. In particular, the observing pattern keywords have been changed for Cycle 11, and the dictionary below describes the new keyword format. The post-SAA dark keywords have been in use since Cycle 11. For data taken in Cycle 7 and 7N, these keywords are not populated. Since April 2007, the SAAclean task has been incorporated into the OTFR to clean SAA persistence from science observations. The related keywords can be found in the SAACLEAN and RUNCALSAA sections of Table 2.4. Keywords associated with the bright earth persistence and the bias-derived temperature were added to the OTFR in march 2008 in OPUS version 2008.1. The STScI Data Processing Team maintains a complete HST FITS header keyword dictionary with descriptions of the keywords used by NICMOS and other HST instruments.

 

Table 2.4: Science Data File Primary Header Keywords

Keyword Name	Meaning
Image Keywords
NEXTEND	Number of extensions in the file (up to 130 for MULTIACCUM).
DATE	Date on which the file was generated
FILENAME	Name of the file
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
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)
ECL_LONG	Ecliptic longitude of the target (deg) (J2000)
ECL_LAT	Ecliptic latitude of the target (deg) (J2000)
GAL_LONG	Galactic longitude of the target (deg) (J2000)
GAL_LAT	Galactic latitude of the target (deg) (J2000)
	Other Coordinate System Information
APER_REF	Aperture used for reference position
ELON_REF	Ecliptic longitude at reference position (deg)
ELAT_REF	Ecliptic latitude at reference position (deg)
GLON_REF	Galactic longitude at reference position (deg)
GLAT_REF	Galactic latitude at reference position (deg)
Proposal Information
PROPOSID	Proposal’s identification number
LINENUM	Exposure’s logsheet line number, from the Phase 2 proposal
PR_INV_L	Principle investigator last name
PR_INV_F	Principle investigator first name
PR_INV_M	Principle investigator middle initial
Exposure Information
ORIENTAT	Position angle of the image y axis (degrees East of North)
SUNANGLE	Angle between sun and V1 axis
MOONANGL	Angle between moon and V1 axis
SUN_ALT	Altitude os the sun above earth’s limb
FGSLOCK	Commanded FGS lock (FINE, COARSE, GYROS, UNKNOWN)
GYROMODE	Number of gyros scheduled for observation
REFFRAME	guide star catalog version
DATE-OBS	UT date of start of the observation (dd/mm/yy or yyyy-mm-dd)
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)
EXPFLAG	Exposure interruption indicator (e.g., NORMAL)
Instrument Configuration Information
CAMERA	NICMOS camera used in the observation (1, 2, or 3)
PRIMECAM	NICMOS Prime Camera during the observation (for internal parallels)
FOCUS	In-focus camera for this observation
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 of the exposure
NREAD	Number of ACCUM initial and final readouts
NSAMP	Number of MULTIACCUM or RAMP samples
SAMP_SEQ	MULTIACCUM exposure time sequence name
FOMXPOS	X offset of the Camera FOV using NICMOS FOM (arcsec)
FOMYPOS	Y offset of the Camera FOV using NICMOS FOM (arcsec)
NFXTILTP	FOM X-tilt position (arcsec)
NFYTILTP	FOM Y-tilt position (arcsec)
NPXTILTP	PAM X-tilt position (arcsec)
NPYTILTP	PAM Y-tilt position (arcsec)
NPFOCUSP	PAM focus position (mm)
TIMEPATT	Timing pattern identifier for readout
READOUT	Detector array readout rate (FAST, SLOW)
SAMPZERO	Sample time of MULTIACCUM zeroth read (sec)
HCLKRATE	Horizontal clock rate (microseconds)
VIDEO_BW	Readout video bandwidth (kHz)
ADCGAIN	Analog-digital conversion gain (electrons/DN)
Photometry Keywords
PHOTMODE	Combination of <INSTRUMENT>+<CAMERA>+<FILTER>
PHOTFLAM	Inverse sensitivity (erg/cm2/angstrom/DN)
PHOTFNU	Inverse sensitivity (Jy*sec/DN)
PHOTZPT	ST magnitude system zero point
ZPSCALE	Temp dependent photometric zero-point scale factor
PHOTFERR	Relative error:temp dependent photometric zero-point
PHOTPLAM	Pivot wavelength of the photmode (Angstrom)
PHOTBW	Root Mean Square bandwidth of the photmode (Angstrom)
Bias-Derived Temperature Information
TFBDATE	Date that CalTempFromBias was run
TFBERR	Error (deg K) for temperature derived from bias
TFBMETH	CalTempFromBias algorithm type used
TFBTEMP	Temperature (deg K) derived from bias
TFBVER	Version of CalTempFromBias run
Bias-Derived Temperature Calibration Switches
TFBCALC	CalTempFromBias calc: PERFORM, OMIT, COMPLETE
Bias-Derived Temperature Calibration Indicators
TFBDONE	CalTempFromBias calc: PERFORM, OMIT, COMPLETE
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
TEMPFILE	Temperature dependent dark file
LINSCALE	Scaling factor for linear image
AMPSCALE	Scaling factor ampglow image
FLATFILE	Flat field file
TDFFILE	Temperature-dependent flat field
TDFGROUP	Imset from TDFFILE applied by calnica
PHOTTAB	Photometric calibration table
BACKTAB	Background model parameters table
IDCTAB	Image distortion correction 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 Chapter 3)
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
ZSIGCORR	MULTIACCUM zero read signal correction
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
BARSCORR	Bars 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
ZSIGDONE	MULTIACCUM zero read signal correction
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
BARSDONE	Bars 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
CALSTAGE	State of the calibration (values: CALNICA, CALNICB, UNCALIBRATED)
CAL_VER	Version number of the CALNICA (CALNICB) code for _cal.fits (_mos.fits) files
PROCTIME	Pipeline processing time (MJD)
OPUS_VER	OPUS software system version number
Post-SAA Dark Keywords (used with post-SAA darks in Cycle 11+)
SAA_EXIT	Time of last exit from SAA contour level 23
SAA_TIME	Seconds since last exit from SAA contour 23
SAA_DARK	Association name for post-SAA dark exposures
SAACRMAP1	SAA cosmic ray map file
Bright Earth Persistence Keywords
BEPSCALE	Level of persistence calculated
BEPVALLO	Minimum allowed value of the persistence to apply
BEPUSELO	Minimum allowed fraction of pixels used
BEPFRAC	Fraction of pixels used to calculate persistence
Runcalsaa Calibration Reference Files
SAADFILE	SAA dark reference image file
SAADPDGR	SAA dark reference file pedigree
SAACNTAB	Saaclean reference table (with task params)
SAACPDGR	Pedigree of saaclean reference table
PEDSBTAB	Pedsub reference table
PDSBPDGR	Pedigree of pedsub reference table
PMODFILE	Persistence model file
PMSKFILE	Persistence mask file
RUNCALSAA Calibration Switches
SAACORR	Correct for SAA signature
BEPCORR	Calculate and apply bright earth persistence
RUNCALSAA Calibration Indicators
SAADONE	Correct for SAA signature
BEPDONE	Calculate and apply bright earth persistence
Saa_clean Output keywords
SAAPERS	SAA Persistence image
SCNPSCL	Scale factor used to construct persistence image
SCNPMDN	Median used in flatfielding persistence image
SCNTHRSH	Threshold dividing high and low signal domains
SCNHNPIX	Number of pixels in high signal domain(HSD)
SCNLNPIX	Number of pixels in low signal domain(LSD)
SCNHCHI2	HSD chi squared for parabola fit
SCNHSCL	HSD scale factor for min noise
SCNHEFFN	HSD effective noise at SCNGAIN
SCNHNRED	HSD noise reduction(percent)
SCNLCHI2	LSD chi squared for parabola fit
SCNLSCL	LSD scale factor for min noise
SCNLEFFN	LSD effective noise at SCNGAIN
SCNLNRED	LSD noise reduction(percent)
SCNAPPLD	To which domains was SAA cleaning applied
RNLINCOR Calibration Reference Files
ZPRATTAB	Nonlincor zeropoint scaling table
RNLCORTB	Nonlincor nonlinearity power law table
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
Pattern Keywords
PATTERN1	Primary pattern type
P1_SHAPE	Primary pattern shape
P1_PURPS	Primary pattern purpose
P1_NPTS	Number of points in primary pattern
P1_PSPAC	Point spacing for primary pattern (arc-sec)
P1_LSPAC	Line spacing for primary pattern (arc-sec)
P1_ANGLE	Angle between sides of parallelogram pattern (deg)
P1_FRAME	Coordinate frame of primary pattern (POS-TARG or CELESTIAL)
P1_ORIENT	Orientation of pattern to coordinate frame (deg)
P1_CENTER	Center pattern relative to pointing (yes/no)
BKG_OFF	Pattern offset method (SAM or FOM)
PATTSTEP	Position number of this point in the pattern
PATT_POS	Position number in pattern sequence
Target Acquisition Keywords (useful for Coronagraphic Data)
NCHKBOXX	CHecKBOX location X
NCHKBOXY	CHecKBOX location Y
NTABOXSZ	TA checkBOX SiZe
NXCENT	X pos CENTroid (steps)
NYCENT	Y pos CENTroid (steps)
NXCENTP	X pos CENTroid (pixels)
NYCENTP	Y pos CENTroid (pixels)
NBOXSUM	checkBOX SUM
NOFFSETX	OFFSET maneuver X (steps)
NOFFSETY	OFFSET maneuver Y (steps)
NOFFSTXP	OFFSET maneuver X (pixels)
NOFFSTYP	OFFSET maneuver Y (pixels)
NSLEWCON	SLEW CONfiguration (Clear, Set)
Association Keywords
ASN_ID	Association rootname
ASN_TAB	Name of the association table
ASN_MTYP	Role of the dataset in the association (e.g., target, first background, second background, etc.; allowed values: EXP_TARG, EXP_BCKn, PROD_TARG, PROD_BCKn, where EXP=input exposure, PROD=output product, TARG=target, BCK=background, n=1-8)

---

1

The SAACRMAP keyword is no longer being used by the OTFR pipeline.

 

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. Table 2.5 lists some of the relevant keywords that are specific to image extensions; they appear in the extension headers, but not in the primary header.

Table 2.5: 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 to save space.
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)

Keywords in the support file, *_spt.fits, report information from the ephemeris and engineering data on the status of the telescope and of the instrument during the observations. Information common to all readouts of a MULTIACCUM sequence are stored in the primary header of the support file, while information which may change from readout to readout (e.g., detector temperature) is stored in the headers for each corresponding extension. Table 2.6 describes some of the relevant primary header keywords from the *_spt.fits file, and Table 2.7 gives some useful keywords from the extension headers.

Table 2.6: 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)

Table 2.7: Some Useful NICMOS Extension Header Keywords in the Support Files

Keyword Name	Meaning
NDWTMP11	Camera 1 and 2 Mounting Cup temp (deg K)
NDWTMP13	Camera 3 Mounting Cup temp (deg K)

Table 2.8: NICMOS and NCS Telemetry Mnemonics

Mnemonic	MSID	Sample time	Units	Operational limitsa	Description
MNCIRSPD	M25L563A	60s	rps	0-1250	NCS CIrc Rot SPeeD
MNCORSPD	M25L565A	15s	rps	0-7360	NCS COmp Rot SPeeD
MNCONTRL	M32J991A	60s	K	?-103.344	NCS CONTRoL point temp
MNPDSTPT	M32J993A	60s	K	77.1	NCS PiD SeT Point Temp
MNCRADAT	M26T780A	60s	C	-70-+70	NCS Cpl RAD A Temp
MNHTREJT	M25T788A	60s	C	-45-+45	NCS HeaT REJect Temp
MNPNCOLT	M25T551A	60s	K	?-197.102	NCS Pri NiC OutLet Temp
MNRNCILT	M25T548A	60s	K	?-103.310	NCS Red NiC InLet Temp
MNRNCOLT	M25T547A	60s	K	?-103.420	NCS Red NiC OutLet Temp
NDWTMP11	N12T656A	30s	K	?-77.43	NIC1 Mounting Cup Temp
NDWTMP13	N12T658A	30s	K	?-76.28	NIC3 Mounting Cup Temp
NDWTMP14	N12T659A	30s	K	?-76.81	Cold Well Temp
NDWTMP21	N12T662A	30s	K	48.15-263.15	NIC2 Cold Mask Temp
NDWTMP22	N12T663A	30s	K	48.15-263.15	NIC3 Colds Mast Temp

a. Operational limits are a range of values over which the sensor will operate normally.