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FITS Header Keywords provide information about the observation, type of data and post-observation processing. Full information for these can be found in the JWST Keyword Dictionary.


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Introduction 

All JWST data products are FITS files. The FITS header keywords of all the data files store important information characterizing the observations, telemetry received during the observations, and information that relates to the post-observation processing of the data. Following FITS conventions, each keyword is no longer than eight characters and these have values that can be integer, real (floating-point), or a character string. There are several keywords that are common to all JWST data while others are instrument specific. Knowledge of the keywords and where to find them is an important first step in understanding your data. By examining your file header, using fv, ds9, python's astropy.fits.getvalue function, you will find detailed information about your data including:

  • Coordinates of the target, program number, and other observation identifiers.
  • Date and time of observation including start, end and mid exposure times
  • Exposure parameters information such as the instrument configuration information (detector, filter, subarray names and subarray positions)
  • Readout definition parameters such as readpatt, nints, ngroups, and groupgap.
  • Exposure-specific information such as more detailed timing and world coordinate system information.
  • Calibration information such as the calibration switches and reference files used by the pipeline

Some of the keywords are only relevant to spectroscopic or imaging data, target acquisitions, and for specific instruments. Accordingly, users will find that the header on a particular file contains a unique combination of keywords appropriate for the type of observation. Long definitions for the keywords can also be accessed from the JWST Keyword Dictionary, which provides detailed explanations of the units, source, and many other attributes.

Keywords that deal with a particular topic, such as the program information or target information are grouped together logically throughout the headers. The sample of a data header below shows the keywords and groupings. These keywords can originate from the Proposal and Planning System (PPS), Observatory Status File (OSF), Science and Operations Center Project Reference Database (PRD),   Science Data Processing (SDP), or Calibration Software, the source along with the FITS header extension where these can be found and valid values are provided in the source attribute of the JWST Keyword Dictionary.



Sample Header

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SIMPLE = T / Data conform to FITS standard 
BITPIX = 8 / bits per data value 
NAXIS = 0 / number of data array dimensions 
EXTEND = T / file may contain standard extensions 
DATE = '2016-11-09T11:20:19' / [yyyy-mm-ddThh:mm:ss.ssssss] UTC date file creation
ORIGIN = 'STSCI ' / institution responsible for creating FITS file 
TIMESYS = 'UTC ' / principal time system for time-related keywords
FILENAME= 'jw98012001001_0210E_00001_mirimage_uncal.fits' / Name of the file 
FILETYPE= 'uncalibrated' / Type of data in the file 
SDP_VER = 'UNKNOWN ' / data processing software version number 
PRD_VER = 'PRDDEVSOC-D-012' / S&OC PRD version number used in data processing
TELESCOP= 'JWST ' / telescope used to acquire the data 
HGA_MOVE= F / High Gain Antenna moved during data collection 
RADESYS = 'ICRS ' / Name of the coordinate reference frame 

COMMENT / Program information 
TITLE = 'MIRI IMG-RAD-11B' / Proposal title 
PI_NAME = 'N/A ' / Principal investigator name 
CATEGORY= 'ENG ' / Program category 
SUBCAT = '' / Program sub-category 
SCICAT = '' / Science category assigned during TAC process 
CONT_ID = 0 / Continuation of previous program 

COMMENT / Observation identifiers 
DATE-OBS= '2016-01-24' / [yyyy-mm-dd] UTC date at start of exposure 
TIME-OBS= '07:47:22.005' / UTC time at the start of the exposure 
OBS_ID = 'V98012001001P000000000210E' / Programmatic observation identifier 
VISIT_ID= '98012001001' / Visit identifier 
PROGRAM = '98012 ' / Program number 
OBSERVTN= '001 ' / Observation number 
VISIT = '001 ' / Visit number 
VISITGRP= '02 ' / Visit group identifier 
SEQ_ID = '1 ' / Parallel sequence identifier 
ACT_ID = '0E ' / Activity identifier 
EXPOSURE= '1 ' / Exposure request number 

COMMENT / Visit information 
TEMPLATE= 'MIRI External Flat' / Observation template used 
OBSLABEL= 'IMAGER FLATS LAMP-ON ONLY' / Proposer label for the observation 
VISITYPE= 'PRIME ' / Visit type 
VSTSTART= '2016-01-24 06:59:44.0860000' / UTC visit start time 
WFSVISIT= '' / Wavefront sensing and control visit indicator 
VISITSTA= 'SUCCESSFUL' / Status of a visit 
NEXPOSUR= 16 / Total number of planned exposures in visit 
INTARGET= T / At least one exposure in visit is internal T
ARGOOPP= F / Visit scheduled as target of opportunity 

COMMENT / Target information 
TARGPROP= '' / Proposer's name for the target 
TARGNAME= '' / Standard astronomical catalog name for target 
TARGTYPE= 'FIXED ' / Type of target (fixed, moving, generic) 
TARG_RA = 0.0 / Target RA at mid time of exposure 
TARG_DEC= 0.0 / Target Dec at mid time of exposure 
TARGURA = 0.0 / Target RA uncertainty 
TARGUDEC= 0.0 / Target Dec uncertainty 
PROP_RA = 0.0 / Proposer specified RA J2000 for the target 
PROP_DEC= 0.0 / Proposer specified Dec J2000 for the target 
PROPEPOC= 'Jan 1 2000 12:00AM' / Proposer specified epoch for proper motion 

COMMENT /Exposure parameters 
INSTRUME= 'MIRI ' / Instrument used to acquire the data 
DETECTOR= 'MIRIMAGE' / Name of detector used to acquire the data 
LAMP = '' / Internal lamp state 
FILTER = 'F1140C ' / Name of the filter element used 
CORONMSK= 'NULL ' / coronagraph mask used 
CCCSTATE= 'OPEN ' / Contamination control cover state 

COMMENT /Exposure information 
PNTG_SEQ= 1 / Pointing sequence number 
EXPCOUNT= 13 / Running count of exposures in visit 
EXP_TYPE= 'MIR_FLATIMAGE' / Type of data in the exposure 

COMMENT / Exposure times 
EXPSTART= 57411.3245602429 / UTC exposure start time 
EXPMID = 57411.32536320594 / UTC exposure mid time 
EXPEND = 57411.32616616898 / UTC exposure end time 
READPATT= 'FAST ' / Readout pattern 
NINTS = 5 / Number of integrations in exposure 
NGROUPS = 10 / Number of groups in integration 
NFRAMES = 1 / Number of frames per group 
GROUPGAP= 0 / Number of frames dropped between groups 
NSAMPLES= 1 / Number of A/D samples per pixel 
TSAMPLE = 10.0 / Time between samples (microsec) 
TFRAME = 2.77504 / Time between frames (sec) 
TGROUP = 2.77504 / Time between groups (sec) 
EFFINTTM= 27.7504 / Effective integration time (sec) 
EFFEXPTM= 139.25 / Effective exposure time (sec) 
CHRGTIME= 0.0 / Charge accumulation time per integration (sec) 
DURATION= 138.752 / Total duration of exposure (sec) 
NRSTSTRT= 0 / Number of resets at start of exposure 
NRESETS = 0 / Number of resets between integrations 
ZEROFRAM= F / Zero frame was downlinked separately 
DATAPROB= F / Science telemetry indicated a problem 
SCA_NUM = 0 / Sensor Chip Assembly number 
DATAMODE= 1 / post-processing method used in FPAP 
COMPRSSD= F / data compressed on-board (T/F) 
FRMDIVSR= 1 / Divisor applied to frame-averaged groups 
SUBARRAY= 'FULL ' / Subarray used 
SUBSTRT1= 1 / Starting pixel in axis 1 direction 
SUBSTRT2= 1 / Starting pixel in axis 2 direction 
SUBSIZE1= 1032 / Number of pixels in axis 1 direction 
SUBSIZE2= 1024 / Number of pixels in axis 2 direction 
FASTAXIS= 1 / Fast readout axis direction 
SLOWAXIS= 2 / Slow readout axis direction 
PATTTYPE= 'NONE ' / Primary dither pattern type 
PATT_NUM= 0 / Position number in primary pattern 
PATTSTRT= 0 / Starting point in pattern 
NUMDTHPT= 0 / Total number of point in pattern 
SUBPXNUM= 0 / Subpixel sampling pattern number 
PATTSIZE= '' / Primary dither pattern size 
SUBPIXEL= T / Subpixel sampling performed 
XOFFSET = 0.0 / x offset from pattern starting position 
YOFFSET = 0.0 / y offset from pattern starting position 
COORDSYS= '' / Ephemeris coordinate system 
EPH_TIME= 57143 / UTC time from ephemeris start time (sec) 
JWST_X = -1436696.08564455 / X spatial coordinate of JWST (km) 
JWST_Y = -713108.747788469 / Y spatial coordinate of JWST (km) 
JWST_Z = -606536.187076095 / Z spatial coordinate of JWST (km) 
JWST_DX = 0.0288294864316793 / X component of JWST velocity (km/sec) 
JWST_DY = -0.0465945808751849 / Y component of JWST velocity (km/sec) 
JWST_DZ = 0.0174011518182956 / Z component of JWST velocity (km/sec) 
APERNAME= '' / PRD science aperture used 
PA_APER = 0.0 / Position angle of aperture used (deg) 
DVA_RA = -1.7189778168302E-07 / Velocity aberration correction RA offset (rad) 
DVA_DEC = 1.23020859538226E-07 / Velocity aberration correction Dec offset (rad)
VA_SCALE= 1.000000060550096 / Velocity aberration scale factor 
BARTDELT= 0.0 / Barycentric time correction 
BSTRTIME= 0.0 / Barycentric exposure start time 
BENDTIME= 0.0 / Barycentric exposure end time 
BMIDTIME= 0.0 / Barycentric exposure mid time 
HELIDELT= 0.0 / Heliocentric time correction 
HSTRTIME= 0.0 / Heliocentric exposure start time 
HENDTIME= 0.0 / Heliocentric exposure end time 
HMIDTIME= 0.0 / Heliocentric exposure mid time 
V2_REF = 0.0 / V2 coordinate of the reference pixel (SIAF:V2Re
V3_REF = 0.0 / V3 coordinate of the reference pixel (SIAF:V3Re
DEC_V1 = -38.74923877352641 / [deg] Dec of telescope V1 axis 
NEXTEND = 3 / Number of file extensions 
WCSAXES = 2 / number of World Coordinate System axes 
CRPIX1 = 1024.0 / Axis 1 coordinate of the reference pixel in the
CRPIX2 = 1024.0 / Axis 2 coordinate of the reference pixel in the
CRVAL1 = 348.9278676961044 / First axis value at the reference pixel (RA in 
CRVAL2 = -38.74923877352641 / Second axis value at the reference pixel (RA in
CTYPE1 = 'RA---TAN' / First axis coordinate type 
CTYPE2 = 'DEC--TAN' / Second axis coordinate type 
CUNIT1 = 'deg ' / units for first axis 
CUNIT2 = 'deg ' / units for second axis 
CDELT1 = 0.11 / first axis increment per pixel, increasing east
CDELT2 = 0.11 / Second axis increment per pixel, increasing nor
PC1_1 = 0.6404219811118805 / linear transformation matrix element cos(theta)
PC1_2 = -0.7680232327922992 / linear transformation matrix element -sin(theta
PC2_1 = 0.7680232327922992 / linear transformation matrix element sin(theta)
PC2_2 = 0.6404219811118805 / linear transformation matrix element cos(theta)
S_REGION= '' / spatial extent of the observation, footprint 
RA_REF = 348.9278676961044 / Right Ascension of the reference point (deg) 
DEC_REF = -38.74923877352641 / Declination of the reference point (deg) 
ROLL_REF= 354.9135542146994 / Telescope roll angle of V3 North over East at t
VPARITY = 1 / Parity (sense) of aperture settings (1, -1) 
V3I_YANG= 0.0 / Direction angle in V3 (Y) 
GS_ORDER= 0 / index of guide star within list of selected gui
GSSTRTTM= '1999-01-01 00:00:00' / UTC time when guide star activity started 
GSENDTIM= '1999-01-01 00:00:00' / UTC time when guide star activity completed 
GDSTARID= '' / guide star identifier 
GS_RA = 0.0 / guide star right ascension 
GS_DEC = 0.0 / guide star declination 
GS_URA = 0.0 / guide star right ascension uncertainty 
GS_UDEC = 0.0 / guide star declination uncertainty 
GS_MAG = 0.0 / guide star magnitude 
GS_UMAG = 0.0 / guide star magnitude uncertainty 
PCS_MODE= 'NONE ' / Pointing Control System mode 
GSCENTX = 0.0 / guide star centroid x postion in the FGS ideal 
GSCENTY = 0.0 / guide star centroid y postion in the FGS ideal 
JITTERMS= 0.0 / RMS jitter over the exposure (arcsec). 
VISITEND= '2016-01-24 07:58:05.1420000' / Observatory UTC time when the visit en
RA_V1 = 348.9278676961044 
PA_V3 = 50.17670723125901 
END

 


 



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Last updated

Published July 31, 2017


 

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Published March 2, 2017