2.4	Headers and Keywords

While the ACS image headers for WFC, HRC, HRC-ACQ, and SBC observations have very similar formats, two main differences are apparent. MAMA (SBC) headers require a slightly modified structure from the CCD (WFC, HRC) headers, and HRC-ACQ headers require several unique keywords.

ACS header keywords can be viewed in several ways. If you know the names of keywords you wish to look up, you can use the STSDAS tasks hedit or hselect tasks to query the images. The examples below show how these tasks are used to get the values for group (or extension) keywords CCDCHIP, CRVAL1, and CRVAL2. The contents of the entire primary or group header can be viewed on-screen, or by directing the output to a text file, using the task imheader.

--> hedit j9cm01jvq_flt.fits[sci,1] ccdchip,crval1,crval2 . j9cm01jvq_flt.fits[sci,1],CCDCHIP = 2 j9cm01jvq_flt.fits[sci,1],CRVAL1 = 125.9331885631965 j9cm01jvq_flt.fits[sci,1],CRVAL2 = 71.03615977254553   --> hselect j9cm01jvq_flt.fits[sci,2] ccdchip,crval1,crval2 yes 1 126.0134441294924 71.02376673000671   --> imheader j9cm01jvq_flt.fits[sci,1] l+ > j9cm01jvq_flt.fits_grp1.txt

ACS image header keywords are described in Tables 2.6 and 2.7. If you were to display the header using imheader, you’ll notice that the keywords are grouped together by their type, for example, “Target Information,” “Science Instrument Configuration,” “Calibration Switches,” and “Calibration Reference files.”

Keywords in the image’s primary header (FITS extension 0) contain parameter values that are common to the entire image dataset, and are given in Table 2.6.

Extension or group header keywords (FITS extensions > 0), as shown in Table 2.7, contain parameter values that are specific to the image group (however, many primary header keyword values are also accessible when querying an image group header).

Some keywords are no longer in use, but we include them for users who may have older data.

Most of the keywords described in Tables 2.6 and 2.7 are “user” keywords, i.e., keywords with values directly recorded in the image headers. A few keywords in the tables have the prefix “i_”; these are “standard” keywords. They cannot be accessed by tasks like eheader and imheader, but when queried by hedit and hselect, the values of standard keywords are determined on-the-fly by IRAF.

Table 2.6: ACS Primary Header Keywords (FITS extension 0)  

Text in normal font under the “Description” column are written as it appears in the image header. Entries in grey are keywords that do not appear in data from the current version of OTFR, but are retained here for historical reasons. Additional information is conveyed in italics.

KEYWORD	DESCRIPTION
SIMPLE	Data conform to FITS standard (T/F).
BITPIX	Bits per data value?
i_naxis NAXIS1, NAXIS2	Number of data axes, always set to zero for the primary header.
EXTEND	File may contain standard extensions (T/F).
ORIGIN	FITS file originator.
IRAF-TLM	Time of last modification.
NEXTEND	Number of standard extensions.
GROUPS	Image is in group format? (T/F).
DATE	Date this file was written (yyyy-mm-dd).
FILENAME	Name of file.
i_pixfile	Name of file.
FILETYPE	Type of data found in data file.
TELESCOP	Telescope used to acquire data.
INSTRUME	Identifier for instrument used to acquire data.
EQUINOX	Equinox of celestial coordinate system.
DATA DESCRIPTION KEYWORDS (All Detectors)
ROOTNAME	Rootname of the observation set.
IMAGETYP	Type of exposure identifier.
PRIMESI	Instrument designated as prime.
TARGET INFORMATION (All Detectors)
TARGNAME	Proposer’s target name.
RA_TARG	Right ascension of the target (deg) (J2000).
DEC_TARG	Declination of the target (deg) (J2000).
PROPOSAL INFORMATION (All Detectors)
PROPOSID	PEP proposal identifier.
LINENUM	Proposal logsheet line number.
PR_INV_L	Last name of principal investigator.
PR_INV_F	First name of principal investigator.
PR_INV_M	Middle name / initial of principal investigator.
EXPOSURE INFORMATION (All Detectors)
SUNANGLE	Angle between Sun and V1 axis.
MOONANGL	Angle between Moon and V1 axis.
SUN_ALT	Altitude of the Sun above Earth’s limb.
FGSLOCK	Commanded FGS lock (FINE, COARSE, GYROS, UNKNOWN).
TWOGYRO	Observation scheduled with only two gyros? (Y/N)
GYROMODE	Number of gyros scheduled, T=3+OBAD. (Number of gyros available for use.)
REFFRAME	Guide star catalog version.
MTFLAG	Moving target flag; T if it is a moving target.
DATE-OBS	UT date of start of observation (yyyy-mm-dd).
TIME-OBS	UT time of start of observation (hh:mm:ss).
EXPSTART	Exposure start time (modified Julian date).
EXPEND	Exposure end time (modified Julian date).
EXPTIME	Exposure duration (seconds)--calculated. (Exposure time as executed.)
EXPFLAG	Exposure interruption indicator.
QUALCOM1, QUALCOM2, QUALCOM3, QUALITY	Data quality comments. These keywords are only populated if there are problems with the exposure.
POINTING INFORMATION (All Detectors)
PA_V3	Position angle of V3-axis of HST (deg).
TARGET OFFSETS (POSTARGS) (All Detectors)
POSTARG1	POSTARG in axis 1 direction.
POSTARG2	POSTARG in axis 2 direction.
DIAGNOSTIC KEYWORDS (All Detectors)
OPUS_VER	OPUS software system version number.
CAL_VER	CALACS code version.
PROCTIME	Pipeline processing time (MJD).
SCIENCE INSTRUMENT CONFIGURATION (All Detectors, Except Where Noted)
OBSTYPE	Observation type (imaging, spectroscopic, coronagraphic).
OBSMODE	Operating mode.
CTEIMAGE	Type of charge transfer image, if applicable.
SCLAMP	Lamp status: NONE or name of lamp which is on.
NRPTEXP	Number of repeat exposures in set (default=1).
SUBARRAY	Data from a subarray (T) or full frame (F).
DETECTOR	Detector in use: WFC, HRC or SBC.
FILTER1	Element selected from filter wheel 1.
FW1OFFST	Computed filter wheel offset.
FILTER2	Element selected from filter wheel 2.
FW1ERROR	Filter wheel position error flag.
FW2OFFST	Computed filter wheel offset.
FW2ERROR	Filter wheel position error flag.
FWSOFFST	Computed filter wheel offset.
FWSERROR	Filter wheel position error flag.
FWOFFSET	Computed filter wheel offset (0, +1, -1).
FWERROR	Filter wheel position error flag (T, F).
LRFWAVE	Proposed linear ramp filter wavelength.
APERTURE	Aperture name.
PROPAPER	Proposed aperture name.
DIRIMAGE	Direct image for grism or prism exposure.
CTEDIR	CTE measurement direction: serial or parallel (CCDs only).
CRSPLIT	Number of cosmic ray split exposures (CCDs only).
MAMA OFFSETS (SBC Only)
MOFFSET1	Axis 1 MAMA offset.
MOFFSET2	Axis 2 MAMA offset.
LOCAL RATE CHECK IMAGE (SBC Only)
LRC_XSTS	Local rate check image exists. (T/F)
LRC_FAIL	Local rate check failed. (T/F)
CALIBRATION SWITCHES: PERFORM, OMIT (All Detectors, Except Where Noted)
STATFLAG	Calculate statistics?
WRTERR	Write out error array extension.
DQICORR	Data quality initialization.
ATODCORR	Correct for A to D conversion errors (CCDs only).
BLEVCORR	Subtract bias level computed from overscan image (CCDs only).
BIASCORR	Subtract bias image (CCDs only).
FLSHCORR	Post flash correction (CCDs only).
CRCORR	Combine observations to reject cosmic rays (CCDs only).
EXPSCORR	Process individual observations after cr-reject (CCDs only).
SHADCORR	Apply shutter shading correction (CCDs only).
GLINCORR	Correct for global detector non-linearities (SBC only).
LFLGCORR	Flag pixels for local and global nonlinearities (SBC only).
DARKCORR	Subtract dark image.
FLATCORR	Flat field data.
PHOTCORR	Populate photometric header keywords.
RPTCORR	Add individual repeat observations.
DRIZCORR	Drizzle processing.
CALIBRATION REFERENCE FILES (All Detectors, Except Where Noted)
BPIXTAB	Bad pixel table.
MLINTAB	MAMA linearity correction table (SBC only).
CCDTAB	CCD calibration parameter table (CCDs only).
ATODTAB	Analog-to-digital correction file (CCDs only).
OSCNTAB	CCD overscan table (CCDs only).
BIASFILE	Bias image file name (CCDs only).
FLSHFILE	Post flash correction file name (CCDs only).
CRREJTAB	Cosmic ray rejection parameter table (CCDs only).
SHADFILE	Shutter shading correction file (CCDs only).
DARKFILE	Dark image file name.
PFLTFILE	Pixel-to-pixel flat field file name.
DFLTFILE	Delta flat field file name.
LFLTFILE	Low order flat.
PHOTTAB	Photometric throughput table (not used).
GRAPHTAB	HST graph table.
COMPTAB	HST components table.
IDCTAB	Image distortion correction table.
DGEOFILE	Distortion correction image.
MDRIZTAB	MultiDrizzle parameter table.
CFLTFILE	Coronagraphic spot image (flat).
SPOTTAB	Coronagraphic spot offset table.
MDRIZSKY	Sky value computed by MultiDrizzle (in this section for SBC only).
COSMIC RAY REJECTION ALGORITHM PARAMETERS (WFC, HRC, HRC ACQ)
MEANEXP	Reference exposure time for parameters.
SCALENSE	Multiplicative scale factor applied to noise.
INITGUES	Initial guess method (MIN or MED).
SKYSUB	Sky value subtracted (MODE or NONE).
SKYSUM	Sky level from the sum of all constituent images.
CRSIGMAS	Statistical rejection criteria.
CRRADIUS	Rejection propagation radius (pixels).
CRTHRESH	Rejection propagation threshold.
BADINPDQ	Data quality flag bits to reject.
REJ_RATE	Rate at which pixels are affected by cosmic rays.
CRMASK	Flag CR-rejected pixels in input files. (T/F)
MDRIZSKY	Sky value computed by MultiDrizzle.
PATTERN KEYWORDS (All Detectors)
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 (arcsec).
P1_LSPAC	Line spacing for primary pattern (arcsec).
P1_ANGLE	Angle between sides of parallelogram pattern (deg).
P1_FRAME	Coordinate frame of primary pattern.
P1_ORINT	Orientation of pattern to coordinate frame (deg).
P1_CENTR	Center pattern relative to pointing (yes/no).
PATTSTEP	Position number of this point in the pattern.
POST FLASH PARAMETERS (WFC, HRC, HRC ACQ)
FLASHDUR	Exposure time in seconds: 0.1 to 409.5.
FLASHCUR	Post-flash current: OFF, LOW, MED, HIGH.
FLASHSTA	Status: SUCCESSFUL, ABORTED, NOT PERFORMED.
SHUTRPOS	Shutter position: A or B.
ENGINEERING PARAMETERS (WFC, HRC, HRC ACQ)
CCDAMP	CCD amplifier readout configuration.
CCDGAIN	Commanded gain of CCD (electrons/DN).
CCDOFSTA	Commanded CCD bias offset for amplifier A.
CCDOFSTB	Commanded CCD bias offset for amplifier B.
CCDOFSTC	Commanded CCD bias offset for amplifier C.
CCDOFSTD	Commanded CCD bias offset for amplifier D.
CALIBRATED ENGINEERING PARAMETERS (WFC, HRC, HRC ACQ)
ATODGNA	Calibrated gain for amplifier A (electrons/DN).
ATODGNB	Calibrated gain for amplifier B (electrons/DN).
ATODGNC	Calibrated gain for amplifier C (electrons/DN).
ATODGND	Calibrated gain for amplifier D (electrons/DN).
READNSEA	Calibrated read noise for amplifier A (electrons).
READNSEB	Calibrated read noise for amplifier B (electrons).
READNSEC	Calibrated read noise for amplifier C (electrons).
READNSED	Calibrated read noise for amplifier D (electrons).
BIASLEVA	Bias level for amplifier A (DN).
BIASLEVB	Bias level for amplifier B (DN).
BIASLEVC	Bias level for amplifier C (DN).
BIASLEVD	Bias level for amplifier D (DN).
TARGET ACQUISITION PARAMETERS (HRC only)
ACQTYPE	Type of acquisition.
ACQNAME	Rootname of acquisition exposure.
TARGET ACQUISITION PARAMETERS (HRC ACQ only)
ACQTYPE	Type of acquisition.
CENTMETH	Target acquisition centering method.
CRELIM	Perform cosmic ray rejection in acquisition.
CCDBIASS	CCD bias subtracted from target acq image? (y/n)
BIASLEV	CCD bias level used to process acquisition exp..
CHECKBOX	Size of checkbox for finding algorithms.
TARGAREA	Area of target (detector pixels).
ASSOCIATION KEYWORDS (All Detectors)
ASN_ID	Unique identifier assigned to association.
ASN_TAB	Name of the association table.
ASN_MTYP	Role of the Member in the Association.

 

Table 2.7: ACS Extension Header Keywords (FITS Extensions > 0)  

Many primary header keywords also appear in the extension headers. However, only keywords that are unique to extension headers are listed here. Text in normal font under the “Description” column are written as it appears in the image header. Entries in grey are keywords that do not appear in data from the current version of OTFR, but are retained here for historical reasons. Additional information is conveyed in italics.

KEYWORD	DESCRIPTION
ASSOCIATION KEYWORDS (All Detectors)
XTENSION	Extension type.
BITPIX	Bits per data value.
NAXIS	Number of data axes.
i_naxis	Number of data axes. (For ACS images, value is set to 2.)
NAXIS1	Length of first data axis.
NAXIS2	Length of second data axis.
i_naxis1	Length of first data axis.
i_naxis2	Length of second data axis.
PCOUNT	No “random” parameters.
GCOUNT	Only one group (within the extension).
EXTNAME	Extension name.
EXTVER	Extension version .
INHERIT	Inherit the primary header.
EXPNAME	Exposure identifier.
BUNIT	Brightness units (counts, electrons, electrons/sec).
DATAMIN	Minimum data value.
DATAMAX	Maximum data value.
BSCALE1	Scale factor for array value to physical value.
BZERO1	Physical value for an array value of zero.
WFC CCD CHIP IDENTIFICATION (WFC only)
CCDCHIP	CCD chip (1 or 2).
WORLD COORDINATE SYSTEM AND RELATED PARAMETERS (All Detectors)
WCSAXES	Number of World Coordinate System axes.
CRPIX1	x-coordinate of reference pixel.
CRPIX2	y-coordinate of reference pixel.
CRVAL1	First axis value at reference pixel.
CRVAL2	Second axis value at reference pixel.
CTYPE1	The coordinate type for the first axis.
CTYPE2	The coordinate type for the second axis.
CD1_1	Partial of first axis coordinate w.r.t. x.
CD1_2	Partial of first axis coordinate w.r.t. y.
CD2_1	Partial of second axis coordinate w.r.t. x.
CD2_2	Partial of second axis coordinate w.r.t. y.
LTV1	Offset in X to subsection start. (x coordinate of the first science pixel in the SCI array.)
LTV2	Offset in Y to subsection start. (y coordinate of the first science pixel in the SCI array.)
LTM1_1	Reciprocal of sampling rate in X. (The reference pixel size in units of the current pixel size.)
LTM2_2	Reciprocal of sampling rate in Y. (The reference pixel size in units of the current pixel size)
ORIENTAT	Position angle of image y axis (degrees E of N).
RA_APER	RA of aperture reference position.
DEC_APER	Declination of aperture reference position.
PA_APER	Position Angle of reference aperture center(deg).
VAFACTOR	Velocity aberration plate scale factor.
READOUT DEFINITION PARAMETERS (All Detectors)
CENTERA1	Subarray axis1 center point in unbinned detector pixels.
CENTERA2	Subarray axis2 center point in unbinned detector pixels.
SIZAXIS1	Subarray axis1 size in unbinned detector pixels.
SIZAXIS2	Subarray axis2 size in unbinned detector pixels.
BINAXIS1	Axis1 data bin size in unbinned detector pixels.
BINAXIS2	Axis2 data bin size in unbinned detector pixels.
PHOTOMETRY KEYWORDS (All Detectors)
PHOTMODE	Observation configuration.
PHOTFLAM	Inverse sensitivity, ergs/cm2/Å/electron.
PHOTZPT	ST magnitude zero point.
PHOTPLAM	Pivot wavelength (Angstroms).
PHOTBW	rms bandwidth of filter plus detector.
DATA PACKET INFORMATION (All Detectors)
FILLCNT	Number of segments containing fill. (Number of segments for which data was lost in transmission and filled with a fill pattern prior to arriving at STScI.)
ERRCNT	Number of segments containing errors.
PODPSFF	PODPS fill present (T/F). (Indicates if there are missing packets in the data stream; in such cases, the OPUS pipeline inserts fill packets so that the image can be processed by analysis software.)
STDCFFF	Science telemetry fill data present? (T=1/F=0). (Indicates if errors were found in the science dataset at the Space Telescope Data Capture Facility; if there are missing packets or failed segments, fill data is inserted as placeholders.)
STDCFFP	Science telemetry fill pattern (hex).
ENGINEERING PARAMETERS (SBC Only)
GLOBRATE	Global count rate.
GLOBLIM	Was global linearity level exceeded?
MDECODT1	MAMA jMDECODT pwb temp from eng snap 1 (deg. C) (Temperature at the start of observation.)
MDECODT2	MAMA JMDECODT pwb temp from eng snap 2 (deg. C) (Temperature at the end of observation.)
REPEATED EXPOSURES INFO (WFC, HRC, HRC ACQ)
NCOMBINE	Number of image sets combined during CR rejection.
ON-BOARD COMPRESSION INFORMATION (WFC Only)
WFCMPRSD	Was WFC data compressed? (T/F).
CBLKSIZ	Size of compression block in 2-byte words.
LOSTPIX	Number of pixels lost due to buffer overflow.
COMPTYP	Compression type performed (Partial/Full/None).
IMAGE STATISTICS AND DATA QUALITY FLAGS (All Detectors)
NGOODPIX	Number of good pixels.
SDQFLAGS	Serious data quality flags. (Bad pixels that are not included in calculations for header keyword image statistics, e.g., GOODMEAN, NGOODPIX.)
GOODMIN	Minimum value of good pixels.
GOODMAX	Maximum value of good pixels.
GOODMEAN	Mean value of good pixels.
SOFTERRS	Number of soft error pixels (DQF=1).
SNRMIN	Minimum signal to noise of good pixels.
SNRMAX	Maximum signal to noise of good pixels.
SNRMEAN	Mean value of signal to noise of good pixels.
MEANDARK	Average of the dark values subtracted (electrons).
MEANBLEV	Average of all bias levels subtracted (DN).
MEANFLSH	Mean number of counts in post flash exposure (electrons).
OCRVAL11	First axis value at reference pixel.
OCRVAL21	Second axis value at reference pixel.
OCRPIX21	y-coordinate of reference pixel.
OCRPIX11	x-coordinate of reference pixel.
ONAXIS21	y-axis length.
ONAXIS11	x-axis length.
OCD2_21	Partial of second axis coordinate w.r.t. y.
OCD2_11	Partial of second axis coordinate w.r.t. x.
OORIENTA1	Position angle of image y axis (degrees E of N).
OCTYPE11	The coordinate type for the first axis.
OCD1_11	Partial of first axis coordinate w.r.t. x.
OCD1_21	Partial of first axis coordinate w.r.t. y.
OCTYPE21	The coordinate type for the second axis.
WCSCDATE	Time WCS keywords were copied.
A_0_2, B_0_2, A_1_1, B_1_1, A_2_0, B_2_0, A_0_3, B_0_3, A_1_2, B_1_2, A_2_1, B_2_1, A_3_0, B_3_0, A_0_4, B_0_4, A_1_3, B_1_3, A_2_2, B_2_2, A_3_1, B_3_1, A_4_0, B_4_0	Non-linear or high-order polynomial coefficients, in the SIP convention, describe the geometric distortion model for each image group. They are present for use by SIP-enabled code such as DS9 and may be used by future versions of the pipeline software, multidrizzle, and other STSDAS tasks.
A_ORDER, B_ORDER	Order of the polynomial used to describe geometric distortion corrections.
IDCSCALE	Pixel scale in the IDCTAB table.
IDCV2REF	Reference pixel V2 position as derived from the IDCTAB reference table.
IDCV3REF	Reference pixel V3 position as derived from the IDCTAB reference table.
IDCTHETA	Orientation of the detector’s Y-axis relative to V3 axis, as derived from the IDCTAB reference table.
OCX10	Linear distortion term without image plate scale, directly from distortion model (IDCTAB).
OCX11	Linear distortion term without image plate scale, directly from distortion model (IDCTAB).
OCY10	Linear distortion term without image plate scale, directly from distortion model (IDCTAB)
OCY11	Linear distortion term without image plate scale, directly from distortion model (IDCTAB)
TDDALPHA	Time-dependent distortion coefficient alpha (ACS ISR 2007-08) (WFC only)
TDDBETA	Time-dependent distortion coefficient beta (ACS ISR 2007-08) (WFC only)
TARGET ACQUISITION PARAMETERS (HRC ACQ Only)
TARGA1	FSW located subarray axis1 coord. of target.
TARGA2	FSW located subarray axis2 coord. of target.
APERA1	FSW located subarray axis 1 coord. of aperture.
APERA2	FSW located subarray axis 2 coord. of aperture.
APERLKA1	Axis 1 detector pixel of acq aperture center.
APERLKA2	Axis 2 detector pixel of acq aperture center.
MAXCHCNT	Counts in the brightest checkbox.
BOPOFFA1	Axis 1 offset (arcs) object moved off aperture.
BOPOFFA2	Axis 2 offset (arcs) object moved off aperture.

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1

Values before geometric distortion corrections.