2.4 Headers and Keywords

ACS image headers for WFC, HRC, HRC-ACQ, and SBC observations have very similar formats. However, MAMA (SBC) headers require a slightly modified structure from the CCD format, and HRC-ACQ headers require several unique keywords.

ACS header keywords can be viewed in several ways. The example below shows how these tasks are used to get the values for group (or extension) keyword CRVAL1.
Python INPUT:

from astropy.io import fits
 
hdu = fits.open('j8cda1dsq_flt.fits')
hdr = hdu['sci',1].header
hdu.close()
 
print(hdr['CRVAL1'])

Python OUTPUT: 5.952775438936009
 
ACS image header keywords are described in Tables 2.6 and 2.7. When printing the header, note that the keywords are grouped together by type, for example, "Target Information," "Science Instrument Configuration," "Calibration Switches," and "Calibration Reference files." There are also convenience functions for querying header keywords. For more information, see the Astropy documentation.

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).

Table 2.6: ACS primary header keywords (FITS extension 0). Some keywords are no longer in use, but are included for users who may have older data.

KEYWORDDESCRIPTION
SIMPLEData conform to FITS standard (T/F)
BITPIXBits per data value
NAXISNumber of data axes, always set to zero for the primary header
EXTENDFITS data may contain extensions (T/F)
ORIGINFITS file originator
NEXTENDNumber of standard extensions
GROUPSImage is in group format (T/F)
DATEDate this file was written (YYYY-MM-DD)
FILENAMEName of file
FILETYPEType of data found in data file
TELESCOPTelescope used to acquire data
INSTRUMEIdentifier for instrument used to acquire data
EQUINOXEquinox of celestial coordinate system
DATA DESCRIPTION KEYWORDS (all detectors)
ROOTNAMERootname of the observation set
IMAGETYPType of exposure identifier
PRIMESIInstrument designated as prime
TARGET INFORMATION (all detectors)
TARGNAMEProposer's target name
RA_TARGRight ascension of the target (deg; J2000)
DEC_TARGDeclination of the target (deg; J2000)
PROPOSAL INFORMATION (all detectors)
PROPOSIDPEP proposal identifier
LINENUMProposal logsheet line number
PR_INV_LLast name of the principal investigator
PR_INV_FFirst name of the principal investigator
PR_INV_MMiddle name/initial of the principal investigator
EXPOSURE INFORMATION (all detectors)
SUNANGLEAngle between the Sun and V1 axis
MOONANGLAngle between the Moon and V1 axis
SUN_ALTAltitude of the Sun above Earth's limb
FGSLOCKCommanded FGS lock (FINE, COARSE, GYROS, UNKNOWN)
GYROMODENumber of gyros scheduled, T=3+OBAD (number of gyros available for use)
REFFRAMEGuide star catalog version
MTFLAGMoving target flag (T=moving target)
DATE-OBSUT date of observation start (YYYY-MM-DD)
TIME-OBSUT time of observation start (HH:MM:SS)
EXPSTARTExposure start time (modified Julian date)
EXPENDExposure end time (modified Julian date)
EXPTIMEExposure duration in seconds (calculated; exposure time as executed)
TEXPTIMETotal exposure time for drizzled product
DARKTIMETotal integration time for detector dark current in seconds (calculated)
EXPFLAGExposure interruption indicator

QUALCOM1,

QUALCOM2,

QUALCOM3,

QUALITY

Data quality comments. These keywords are populated if there are problems with the exposure.
POINTING INFORMATION (all detectors)
PA_V3Position angle of the V3-axis of HST (deg)
TARGET OFFSETS (all detectors)
POSTARG1POSTARG in axis 1 direction
POSTARG2POSTARG in axis 2 direction
DIAGNOSTIC KEYWORDS (all detectors)
OPUS_VEROPUS software system version number
CSYS_VERCalibration software system version number
CAL_VERcalacs version number
PROCTIMEPipeline processing time (modified Julian date)
SCIENCE INSTRUMENT CONFIGURATION (all detectors except where noted)
OBSTYPEObservation type (imaging, spectroscopic, coronagraphic)
OBSMODEOperating mode
CTEIMAGEType of charge transfer image, if applicable
SCLAMPLamp status: NONE or name of lamp which is on
NRPTEXPNumber of repeat exposures in set (default=1)
SUBARRAYData from a subarray (T) or full frame (F)
DETECTORDetector in use: WFC, HRC, or SBC
FILTER1Element selected from filter wheel 1
FW1OFFSTComputed filter wheel offset
FW1ERRORFilter wheel position error flag
FILTER2Element selected from filter wheel 2
FW2OFFSTComputer filter wheel offset
FW2ERRORFilter wheel position error flag
FWSOFFSTComputed filter wheel offset 
FWSERRORFilter wheel position error flag
LRFWAVEProposed linear ramp filter wavelength
APERTUREAperture name
PROPAPERProposed aperture name
DIRIMAGEDirect image for grism of prism exposure
CTEDIRCTE measurement direction (serial or parallel; CCDs only)
CRSPLITNumber of cosmic ray split exposures (CCDs only)
MAMA OFFSETS (SBC only)
MOFFSET1Axis 1 MAMA offset (low-res pixels)
MOFFSET2Axis 2 MAMA offset (low-res pixels)
LOCAL RATE CHECK IMAGE (SBC only)
LRC_XSTSLocal rate check image exists (T/F)
LRC_FAILLocal rate check failed (T/F)
CALIBRATION SWITCHES: PERFORM, OMIT (all detectors except where noted)
STATFLAGCalculate statistics (not used; statistics are always calculated by calacs)
WRTERRWrite out error array extension (not used; always true for calacs)
DQICORRData quality initialization
ATODCORRCorrect for analog-to-digital conversion errors (CCDs only; not used by calacs)
BLEVCORRSubtract bias level computed from overscan image (CCDs only)
BIASCORRSubtract 2-D bias image (CCDs only)

FLSHCORR

Post-flash correction (CCDs only)
CRCORRCombine observations to reject cosmic rays (CCDs only)
EXPSCORRProcess individual observations are cosmic ray rejection (CCDs only)
SHADCORRApply shutter shading correction (CCDs only; always set to OMIT for ACS)
GLINCORRCorrect for global detector non-linearities (SBC only)
LFLGCORRFlag pixels for local and global non-linearities (SBC only)
PCTECORRCTE correction (always set to OMIT for non-WFC exposures)
DARKCORRSubtract dark image
FLATCORRFlat-field data
PHOTCORRPopulate photometric header keywords
RPTCORRAdd individual repeat observations
DRIZCORRDrizzle processing
SINKCORRFlag sink pixels (affects WFC only since 2015)
CALIBRATION REFERENCE FILES (all detectors except where noted)
BPIXTABBad pixel table
MLINTABMAMA linearity correction table (SBC only)
CCDTABCCD calibration parameter table (CCDs only)
ATODTABAnalog-to-digital correction file (CCDs only; not used by calacs)
OSCNTABCCD overscan table (CCDs only)
BIASFILE2-D bias image file name (CCDs only)
FLSHFILEPost-flash correction file (CCDs only)
CRREJTABCosmic ray rejection parameters (CCDs only)
SHADFILEShutter shading correction file (CCDs only; not used by calacs)
PCTETABCTE correction table (always set to N/A for non-WFC exposures)
DRKCFILECTE-corrected dark image file name (always set to N/A for non-WFC exposures)
DARKFILEDark image file name
PFLTFILEPixel-to-pixel flat-field name
DFLTFILEDelta flat-field name
LFLTFILELow-order flat-field name
PHOTTABPhotometric throughput table (not used by calacs)
GRAPHTABHST graph table (not used by calacs)
COMPTABHST components table (not used by calacs)
IDCTABImage distortion correction table
DGEOFILEDistortion correction image (not used by AstroDrizzle for calacs)
MDRIZTABAstroDrizzle parameter table
CFLTFILECoronagraphic spot flat-field name
SPOTTABCoronagraphic spot offset table
IMPHTTABImage photometry table
D2IMFILEColumn correction reference files
NPOLFILENon-polynomial offsets reference files
SKNCFILEMap of sink pixels (affects WFC only since 2015)
COSMIC RAY REJECTION ALGORITHM PARAMETERS (WFC, HRC, HRC ACQ)
MEANEXPReference exposure time for parameters
SCALENSEMultiplicative scale factor applied to noise
INITGUESInitial guess method (MIN or MED)
SKYSUBSky value subtracted (MODE or NONE)
SKYSUMSky level from the sum of all constituent images
CRSIGMASStatistical rejection criteria
CRRADIUSRejection propagation radius (pixels)
CRTHRESHRejection propagation threshold
BADINPQDData quality flag bits to reject
REJ_RATERate at which pixels are affected by cosmic rays
CRMASKFlag cosmic ray-rejected pixels in input files (T/F)
OTFR Keywords (all detectors)
T_SGSTAROMS calculated guide star control
PATTERN KEYWORDS (all detectors)
PATTERN1Primary pattern type
P1_SHAPEPrimary pattern shape
P1_PURPSPrimary pattern purpose
P1_NPTSNumber of points in primary pattern
P1_PSPACPoint spacing for primary pattern (arcsec)
P1_LSPACLine spacing for primary pattern (arcsec)
P1_ANGLEAngle between sides of parallelogram pattern (deg)
P1_FRAMECoordinate frame of primary pattern
P1_ORINTOrientation of pattern to coordinate frame (deg)
P1_CENTRCenter pattern relative to pointing (yes/no)
PATTSTEPPosition number of this point in the pattern
POST-FLASH PARAMETERS (WFC, HRC, HRC ACQ)
FLASHDURExposure time in seconds (0.1 to 409.5)
FLASHCURPost-flash current (OFF, LOW, MED, HIGH)
FLASHSTAStatus (SUCCESSFUL, ABORTED, NOT PERFORMED)
SHUTRPOSShutter position (A or B)
ENGINEERING PARAMETERS (WFC, HRC, HRC ACQ)
CCDAMPCCD amplifier readout configuration
CCDGAINCommanded gain of CCD (electrons/DN)
CCDOFSTACommanded CCD bias offset for amplifier A
CCDOFSTBCommanded CCD bias offset for amplifier B
CCDOFSTCCommanded CCD bias offset for amplifier C
CCDOFSTDCommanded CCD bias offset for amplifier D
JWROTYPEASIC WFC readout type (DS_int for dual-slope integration or CLAMP for clamp-and-sample)
CALIBRATED ENGINEERING PARAMETERS (WFC, HRC, HRC ACQ)
ATODGNACalibrated gain for amplifier A (electrons/DN)
ATODGNBCalibrated gain for amplifier B (electrons/DN)
ATODGNCCalibrated gain for amplifier C (electrons/DN)
ATODGNDCalibrated gain for amplifier D (electrons/DN)
READNSEACalibrated read noise for amplifier A (electrons)
READNSEBCalibrated read noise for amplifier B (electrons)
READNSECCalibrated read noise for amplifier C (electrons)
READNSEDCalibrated read noise for amplifier D (electrons)
BIASLEVABias level for amplifier A (electrons)
BIASLEVBBias level for amplifier B (electrons)
BIASLEVCBias level for amplifier C (electrons)
BIASLEVDBias level for amplifier D (electrons)
TARGET ACQUISITION PARAMETERS (HRC ACQ only)
AUTHORHeaderlet created by this user
CATALOGAstrometric catalog used for headerlet solution
DESCRIPShort description of headerlet
DESTIMDestination observation rootname
HDRNAMEHeaderlet name
NMATCHNumber of sources used for headerlet solution
RMS_DECRMS in declination at reference pixel of headerlet solution
RMS_RARMS in right ascension at reference pixel of headerlet solution
ASSOCIATION KEYWORDS (all detectors)
ASN_IDUnique identifier assigned to association
ASN_TABName of the association table
ASN_MTYPRole of the member in the association
CRDS_CTXMaster calibration reference system file
CRDS_VERCalibration reference system version number
CTE CORRECTION KEYWORDS (WFC only)
CTE_NAMECTE algorithm name
CTE_VERCTE algorithm version
CTEDATE0Date of ACS installation
PCTETRSHCTE over subtraction threshold
CTEDATE1Date of CTE model pinning
PCTEFRACCTE scaling factor
PCTELENMaximum length of CTE trail
PCTERNOIRead noise amplifier clip limit
PCTENFORNumber of iterations in forward model
PCTENPARNumber of iterations in parallel transfer
FIXROCRFix readout cosmic rays
ASTRODRIZZLE KEYWORDS (all detectors)
UPWCSVERVersion of STWCS used to update the WCS
PYWCSVERVersion of PYWCS used to update the WCS
DISTNAMENames of all distortion model components used
SIPNAMESpecific polynomial distortion model
RULESVERVersion ID for header keyword rules file
BLENDVERVersion of blendheader software used
RULEFILEName of header keyword rules file
NDRIZNUMNumber of images drizzled onto output
D???OUUNUnits of output image (counts or counts/second)
D???MASKInput weighting image
D???FVALFill value for zero weight output pixels
D???KERNForm of weight distribution kernel
D???ISCLDefault IDCTAB pixel size (arcsec)
D???COEFSource of coefficients
D???SCALPixel size of output image (arcsec)
D???GEOMSource of geometric information
D???DATAInput image data
D???OUWEOutput weighting image
D???OUCOOutput context image
D???DECPInput image exposure time (seconds)
D???WKEYInput image WCS version used
D???OUDAOutput data image
D???VERDrizzle task version number
D???WTSCWeighting factor for input image
D???PIXFLinear size of drop
DEPRECATED KEYWORDS (all detectors, in alphabetical order)
ACQTYPEType of acquisition
BIASLEVCCD bias level used to process acquisition exposure
CCDBIASSCCD bias subtracted from target acquisition image (yes/no)
CENTMETHTarget acquisition centering method
CHECKBOXSize of checkbox for finding algorithm
CRELIMPerform cosmic ray rejection in acquisition

Table 2.7: ACS extension header keywords (SCI, ERR, and DQ FITS extensions).

KEYWORDDESCRIPTION
ASSOCIATION KEYWORDS (all detectors)
XTENSIONExtension type
NAXIS1Length of first data axis
NAXIS2Length of second data axis
PCOUNTRequired keyword; must = 0
GCOUNTRequired keyword; must = 1
EXTNAMEExtension name
EXTVERExtension version number
INHERITInherit the primary header
EXPNAMEExposure identifier
DATAMINMinimum data value
DATAMAXMaximum data value
BUNITBrightness units (counts, electrons, electrons/second)
NPIX1Length of constant array axis 1
NPIX2Length of constant array axis 2
PIXVALUEValues of pixels in constant array
BSCALEScale factor for array value to physical value
BZEROPhysical value for an array value of zero
WFC CCD CHIP IDENTIFICATION (WFC only)
CCDCHIPCCD chip (1 or 2)
WORLD COORDINATE SYSTEM AND RELATED PARAMETERS (all detectors)
WCSAXESNumber of World Coordinate System axes
CRPIX1x-coordinate of reference pixel
CRPIX2y-coordinate of reference pixel
CRVAL1First axis value at reference pixel
CRVAL2Second axis value at reference pixel
CTYPE1The coordinate type for the first axis
CTYPE2The coordinate type for the second axis
CD1_1Partial of first axis coordinate with respect to x
CD1_2Partial of first axis coordinate with respect to y
CD2_1Partial of second axis coordinate with respect to x
CD2_2Partial of second axis coordinate with respect to y
LTV1Offset in x to subsection start (x-coordinate of the first science pixel in the SCI array)
LTV2Offset in y to subsection start (y-coordinate of the first science pixel in the SCI array)
RAW_LTV1Original offset in x to subsection start (x-coordinate of the first science pixel in the SCI array)
RAW_LTV2Original offset in y to subsection start (y-coordinate of the first science pixel in the SCI array)
LTM1_1Reciprocal of sampling rate in x (the reference pixel size in units of current pixel size)
LTM2_2Reciprocal of sampling rate in y (the reference pixel size in units of current pixel size)
ORIENTATPosition angle of image y axis (degrees E of N)
RA_APERRight ascension of aperture reference position
DEC_APERDeclination of aperture reference position
PA_APERPosition angle of reference aperture center (deg)
VAFACTORVelocity aberration plate scale factor
READOUT DEFINITION PARAMETERS (all detectors)
CENTERA1Subarray axis 1 centerpoint in unbinned detector pixels
CENTERA2Subarray axis 2 centerpoint in unbinned detector pixels
SIZAXIS1Subarray axis 1 size in unbinned detector pixels
SIZAXIS2Subarray axis 2 size in unbinned detector pixels
BINAXIS1Axis 1 data bin size in unbinned detector pixels
BINAXIS2Axis 2 data bin size in unbinned detector pixels
PHOTOMETRY KEYWORDS (all detectors)
PHOTMODEObservation configuration
PHOTFLAMInverse sensitivity (erg/cm2/Å/electron)
PHOTZPTST magnitude zero points
PHOTPLAMPivot wavelength (Angstroms)
PHOTBWRMS bandwidth of filter plus detector
DATA PACKET INFORMATION (all detectors)
FILLCNTNumber of segments containing fill value for which data were lost in transmission
ERRCNTNumber of segments containing errors
PODSPSFFPODPS fill present (T/F) indicating if there are missing packets in the data stream. Missing packets are replaced with fill packets by OPUS to facilitate pipeline processing.
STDCFFFScience telemetry fill present (T/F) indicating if there were errors found in the science dataset at the Space Telescope Data Capture Facility.
STDCFFPScience telemetry fill pattern (hex)
ENGINEERING PARAMETERS (SBC only)
GLOBRATEGlobal count rate
GLOBLIMWas global linearity level exceeded?
MDECODT1MAMA JMDECODT pwd temperature from eng snap 1 (deg C; temperature at the start of observation)
MDECODT2MAMA JMDECODT pwd temperature from eng snap 2 (deg C; temperature at the end of observation)
REPEATED EXPOSURES INFORMATION (WFC, HRC, HRC ACQ)
NCOMBINENumber of image sets combined during cosmic ray rejection
ONBOARD COMPRESSION INFORMATION (WFC only)
WFCMPRSDWas WFC data compressed? (T/F)
CBLKSIZSize of compression block in 2 byte words
LOSTPIXNumber of pixels lost due to buffer overflow
COMPTYPCompression type performed (Partial/Full/None)
IMAGE STATISTICS AND DATA QUALITY FLAGS (all detectors except where noted)
NGOODPIXNumber of good pixels
SDQFLAGSSerious data quality flags (bad pixels that are not included in calculations for header keyword image statistics, e.g., GOODMEAN, NGOODPIX, etc.)
GOODMINMinimum value of good pixels
GOODMAXMaximum value of good pixels
GOODMEANMean value of good pixels
SOFTERRSNumber of soft error pixels (DQF=1)
SNRMINMinimum signal to noise of good pixels
SNRMAXMaximum signal to noise of good pixels
SNRMEANMean value of signal to noise of good pixels
MEANDARKAverage of the dark values subtracted (electrons)
MEANBLEVAverage of all bias levels subtracted (electrons; CCDs only)
MEANFLSHMean number of counts in post-flash exposure (electrons; CCDs only)
MDRIZSKYSky value computed by AstroDrizzle
DISTORTION CORRECTION PARAMETERS (all detectors)
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, A_3_2, B_3_2, B_1_4, B_2_3, B_0_5, A_5_0, A_4_1, B_4_1, B_5_0, A_1_4, A_0_5, A_2_3Non-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 for use by the pipeline software and AstroDrizzle.
A_ORDER, B_ORDEROrder of the polynomial used to describe geometric distortion corrections
IDCSCALEPixel scale in the IDCTAB table
IDCXREFReference pixel location in x as specified by the IDCTAB
IDCYREFReference pixel location in y as specified by the IDCTAB
IDCV2REFReference pixel V2 position as derived from the IDCTAB reference table
IDCV3REFReference pixel V3 position as derived from the IDCTAB reference table
IDCTHETAOrientation of the detector's Y-axis relative to V3 axis, as derived from the IDCTAB reference table
OCX10Linear distortion term without image plate scale, directly from distortion model (IDCTAB)
OCX11Linear distortion term without image plate scale, directly from distortion model (IDCTAB)
OCY10Linear distortion term without image plate scale, directly from distortion model (IDCTAB)
OCY11Linear distortion term without image plate scale, directly from distortion model (IDCTAB)
D2IMEXTReference file used by updatewcs to create a D2IMARR extension, which holds the column or row corrections
D2IMERR1Maximum error of NPOL correction for axis 1
D2IMDIS1Detector-to-image correction type for axis 1
D2IMERR2Maximum error of NPOL correction for axis 2
D2IMDIS2Detector-to-image correction type for axis 2
D2IM1.AXIS.1, D2IM1.AXIS.2, D2IM1.EXTVER, D2IM1.NAXES, D2IM2.AXIS.1, D2IM2.AXIS.1, D2IM2.EXTVER, D2IM2.NAXESWCS NPOL correction parameters
WCSNAMEOWCS solution label
WCSAXESONumber of WCS axes
CRPIX1O, CRPIX2O, CDELT1O, CDELT2O, CUNIT1, CUNIT1O, CUNIT2, CUNIT2O, CTYPE1O, CTYPE2O, CRVAL1O, CRVAL2O, LONPOLE, LONPOLEO, LATPOLE, LATPOLEO, CD1_1O, CD1_2O, CD2_1O, CD2_2O, CDELT1, CDELT2, RADESYS, RADESYSOWCS solution parameters
CDERR1, CPDIS1, CDERR2, CPDIS2, CPERR1, CPERR2 DP1.AXIS.1, DP1.AXIS.2, DP1.EXTVER, DP1.NAXES, DP2.AXIS.1, DP2.AXIS.2, DP2.EXTVER, DP2.NAXESDistortion solution parameters, if performed
NPOLEXTNPOLFILE used, if applicable
TDD_CTBWFC time-dependent distortion parameter (skew angle in Y-axis)
TDD_CXBWFC time-dependent distortion parameter (scale in X-axis)
TARGET ACQUISITION KEYWORDS (HRC ACQ only)
TG_ENAMETarget science data extension name
TG_EVERTarget science data extension version number
DEPRECATED KEYWORDS (all detectors, sorted alphabetically)
APERA1FSW located subarray axis 1 coordinate of aperture
APERA2FSW located subarray axis 2 coordinate of aperture
APERLKA1Axis 1 detector pixel of acquisition aperture center
APERLKA2Axis 2 detector pixel of acquisition aperture center
AXISCORRAxis to which the DET2IM correction is applied
BOPOFFA1Axis 1 offset object moved off aperture (arcsec)
BOPOFFA2Axis 2 offset object moved off aperture (arcsec)
MAXCHCNTCounts in the brightest checkbox
OCD1_1Partial of first axis coordinate with respect to x
OCD1_2Partial of first axis coordinate with respect to y
OCD2_1Partial of second axis coordinate with respect to x
OCD2_2Partial of second axis coordinate with respect to y
OCRPIX1x-coordinate of reference pixel
OCRPIX2y-coordinate of reference pixel
OCRVAL1First axis value at reference pixel
OCRVAL2Second axis value at reference pixel
OCRTYPE1The coordinate type for the first axis
OCRTYPE2The coordinate type for the second axis
ONAXIS1x-axis length
ONAXIS2y-axis length
OORIENTAPosition angle of image y-axis (deg E of N)
TARGA1FSW located subarray axis 1 coordinate of target
TARGA2FSW located subarray axis 2 coordinate of target
TDDALPHAWFC time-dependent distortion coefficient alpha

TDDBETA

WFC time-dependent distortion coefficient beta
TDD_CTAWFC time-dependent distortion coefficient alpha
TDD_CYAWFC time-dependent distortion coefficient alpha
TDD_CYBWFC time-dependent distortion coefficient alpha
TDD_CXAWFC time-dependent distortion coefficient alpha
WCSDATETime WCS keywords were copied