Appendix 1 - Sample ACS/WFC Image

The WCS of a single chip from an ACS/WFC exposure illustrates how the SIP keywords are populated based on the coefficients from the external IDCTAB reference file. In addition, this header includes the keywords referring to additional distortion corrections related to non-polynomial corrections from the NPOLFILE and to column-width corrections from the D2IMFILE. This sample illustrates how all three corrections can be specified at the same time in a FITS header using our rules for combining the SIP WCS convention and FITS Distortion Paper proposed syntax, while also using FITS WCS Paper I alternate WCS standards to maintain a record of the WCS information prior to being updated/recomputed to use the new reference information. The old WCS gets stored using WCS key ‘O’ and ‘WCSNAMEO’ = ‘OPUS’ to indicate it was originally computed by OPUS, the HST pipeline system.

FITS File extensions

The FITS file for this ACS/WFC image now contains extra extensions for the NPOLFILE and D2IMFILE corrections.

Filename: jbf401p8q_flc.fits
No.    Name         Type      Cards   Dimensions   Format
0    PRIMARY     PrimaryHDU     261   ()           int16
1    SCI         ImageHDU       184   (4096, 2048)   float32
2    ERR         ImageHDU        55   (4096, 2048)   float32
3    DQ          ImageHDU        47   (4096, 2048)   int16
4    SCI         ImageHDU       183   (4096, 2048)   float32
5    ERR         ImageHDU        55   (4096, 2048)   float32
6    DQ          ImageHDU        47   (4096, 2048)   int16
7    D2IMARR     ImageHDU        12   (4096,)      float32
8    WCSDVARR    ImageHDU        37   (65, 33)     float32
9    WCSDVARR    ImageHDU        37   (65, 33)     float32
10   WCSDVARR    ImageHDU        37   (65, 33)     float32
11   WCSDVARR    ImageHDU        37   (65, 33)     float32
12   WCSCORR     BinTableHDU     59   14R x 24C    [40A, I, 1A, 24A, 24A, 24A, 24A, D, D, D, D, D, D, D, D,
                                                    24A, 24A, D, D, D, D, J, 40A, 128A]

The last extension, named WCSCORR, contains a binary table providing a summary of all the WCS solutions that have been applied to this file and does not act as an active part of the WCS or its interpretation.

Primary Header

The PRIMARY header of HST data contains keywords specifying information general to the entire file, such as what calibration steps were applied and what reference files should be used. No active WCS keywords (keywords interpreted for coordinate transformations) are present in the PRIMARY header, but keywords specifying the applicable distortion reference files can be found in the PRIMARY header. Some keywords describing the distortion model and when the WCS was updated with the distortion information from the reference files can also be found in the PRIMARY header. These distortion and WCS related keywords from the PRIMARY header are:

             / CALIBRATION REFERENCE FILES

IDCTAB  = 'jref$v8q1444sj_idc.fits' / image distortion correction table
DGEOFILE= 'jref$qbu16420j_dxy.fits' / Distortion correction image
D2IMFILE= 'jref$v971826mj_d2i.fits' / Column Correction Reference File
NPOLFILE= 'jref$v971826aj_npl.fits' / Non-polynomial Offsets Reference File

UPWCSVER= '1.0.0   '           / Version of STWCS used to updated the WCS
PYWCSVER= '1.11-4.10'          / Version of PYWCS used to updated the WCS
DISTNAME= 'jbf401p8q_v8q1444sj-v971826aj-v971826mj'
SIPNAME = 'jbf401p8q_v8q1444sj'

The remainder of the PRIMARY header specifies the general characteristics of the image.

SCI Header Keywords

The following keywords only represent the WCS keywords from a sample ACS/WFC SCI header with 4-th order polynomial distortion correction from the IDCTAB reference file, along with NPOLFILE and D2IMFILE corrections from the specific reference files used as examples in Appendix 2 - NPOLFILE Example Appendix 3 - D2IMFILE Example.

XTENSION= 'IMAGE   '           / IMAGE extension
BITPIX  =                  -32
NAXIS   =                    2
NAXIS1  =                 4096
NAXIS2  =                 2048
PCOUNT  =                    0 / required keyword; must = 0
GCOUNT  =                    1 / required keyword; must = 1
ORIGIN  = 'HSTIO/CFITSIO March 2010'
DATE    = '2012-06-13' / date this file was written (yyyy-mm-dd)
INHERIT =                    T / inherit the primary header
EXTNAME = 'SCI     '           / extension name
EXTVER  =                    1 / extension version number
ROOTNAME= 'jbf401p8q                         ' / rootname of the observation set
EXPNAME = 'jbf401p8q                ' / exposure identifier
BUNIT   = 'ELECTRONS'          / brightness units

             / WFC CCD CHIP IDENTIFICATION

CCDCHIP =                    2 / CCD chip (1 or 2)

             / World Coordinate System and Related Parameters

WCSAXES =                    2 / number of World Coordinate System axes
CRPIX1  =                 2048 / x-coordinate of reference pixel
CRPIX2  =                 1024 / y-coordinate of reference pixel
CRVAL1  =        11.3139376926 / first axis value at reference pixel
CRVAL2  =        42.0159325283 / second axis value at reference pixel
CTYPE1  = 'RA---TAN-SIP'       / the coordinate type for the first axis
CTYPE2  = 'DEC--TAN-SIP'       / the coordinate type for the second axis
CD1_1   = -7.8194868997837E-06 / partial of first axis coordinate w.r.t. x
CD1_2   = 1.09620231564470E-05 / partial of first axis coordinate w.r.t. y
CD2_1   = 1.14279318521882E-05 / partial of second axis coordinate w.r.t. x
CD2_2   = 8.66885775536641E-06 / partial of second axis coordinate w.r.t. y
LTV1    =        0.0000000E+00 / offset in X to subsection start
LTV2    =        0.0000000E+00 / offset in Y to subsection start
LTM1_1  =                  1.0 / reciprocal of sampling rate in X
LTM2_2  =                  1.0 / reciprocal of sampling rate in Y
ORIENTAT=    51.66276166150634 / position angle of image y axis (deg. e of n)
RA_APER =   1.133205840898E+01 / RA of aperture reference position
DEC_APER=   4.202747924810E+01 / Declination of aperture reference position
PA_APER =              51.4653 / Position Angle of reference aperture center (de
VAFACTOR=   9.999374411935E-01 / velocity aberration plate scale factor

WCSCDATE= '18:41:12 (13/06/2012)' / Time WCS keywords were copied.
A_0_2   = 2.18045745103211E-06
B_0_2   = -7.2266555836441E-06
A_1_1   = -5.2225148886672E-06
B_1_1   = 6.20296011911662E-06
A_2_0   = 8.54842918202735E-06
B_2_0   = -1.7551668097547E-06
A_0_3   = 8.09354090167772E-12
B_0_3   = -4.2488740853874E-10
A_1_2   = -5.2903025382457E-10
B_1_2   = -7.6098727022982E-11
A_2_1   = -4.4821374838034E-11
B_2_1   = -5.1244088812978E-10
A_3_0   = -4.6755353102513E-10
B_3_0   = 8.48145748580355E-11
A_0_4   = -8.3665541956904E-17
B_0_4   = -2.1662072760964E-14
A_1_3   = -1.5108585176304E-14
B_1_3   = -1.5686763638364E-14
A_2_2   = 3.61252682019403E-14
B_2_2   = -2.6194214315839E-14
A_3_1   = 1.03502537140899E-14
B_3_1   = -2.6915637616404E-15
A_4_0   = 2.32643027828425E-14
B_4_0   = -1.5701287138447E-14
A_ORDER =                    4
B_ORDER =                    4
IDCSCALE=                 0.05
IDCV2REF=    256.6019897460938
IDCV3REF=    302.2520141601562
IDCTHETA=                  0.0
OCX10   = 0.001965125839177266
OCX11   =  0.04983026381230307
OCY10   =   0.0502766128737329
OCY11   = 0.001493971240339153
TDDALPHA=    0.246034678162242
TDDBETA = -0.07934489272074734
IDCXREF =               2048.0
IDCYREF =               1024.0
AXISCORR=                    1
D2IMEXT = '/grp/hst/cdbs/jref/v971826mj_d2i.fits'
D2IMERR = 0.002770500956103206
WCSNAMEO= 'OPUS    '
WCSAXESO=                    2
CRPIX1O =                 2048
CRPIX2O =                 1024
CDELT1O =                    1
CDELT2O =                    1
CUNIT1O = 'deg     '
CUNIT2O = 'deg     '
CTYPE1O = 'RA---TAN-SIP'
CTYPE2O = 'DEC--TAN-SIP'
CRVAL1O =        11.3139376926
CRVAL2O =        42.0159325283
LONPOLEO=                  180
LATPOLEO=        42.0159325283
RESTFRQO=                    0
RESTWAVO=                    0
CD1_1O  =   -7.81948731152E-06
CD1_2O  =    1.09620228331E-05
CD2_1O  =    1.14279315609E-05
CD2_2O  =    8.66885813904E-06
WCSNAME = 'IDC_v8q1444sj'
CPERR1  =                  0.0 / Maximum error of NPOL correction for axis 1
CPDIS1  = 'Lookup  '           / Prior distortion funcion type
DP1     = 'EXTVER: 1' / Version number of WCSDVARR extension containing lookup d
DP1     = 'NAXES: 2' / Number of independent variables in distortion function
DP1     = 'AXIS.1: 1' / Axis number of the jth independent variable in a distort
DP1     = 'AXIS.2: 2' / Axis number of the jth independent variable in a distort
CPERR2  =                  0.0 / Maximum error of NPOL correction for axis 2
CPDIS2  = 'Lookup  '           / Prior distortion funcion type
DP2     = 'EXTVER: 2' / Version number of WCSDVARR extension containing lookup d
DP2     = 'NAXES: 2' / Number of independent variables in distortion function
DP2     = 'AXIS.1: 1' / Axis number of the jth independent variable in a distort
DP2     = 'AXIS.2: 2' / Axis number of the jth independent variable in a distort
NPOLEXT = 'jref$v971826aj_npl.fits'

All keywords related to the exposure itself, such as readout pattern, have been deleted from this SCI header listing for the sake of brevity.

D2IMARR Header

The full, complete header of the D2IMARR extension as derived from the D2IMFILE discussed in Appendix 3 - D2IMFILE Example.

XTENSION= 'IMAGE   '           / Image extension
BITPIX  =                  -32 / array data type
NAXIS   =                    1 / number of array dimensions
NAXIS1  =                 4096
PCOUNT  =                    0 / number of parameters
GCOUNT  =                    1 / number of groups
AXISCORR=                    1 / Direction in which the det2im correction is app
EXTVER  =                    1 / Distortion array version number
EXTNAME = 'D2IMARR '           / WCS distortion array
CDELT1  =                  1.0 / Coordinate increment along axis
CRPIX1  =               2048.0 / Coordinate system reference pixel
CRVAL1  =               2048.0 / Coordinate system value at reference pixel

WCSDVARR Header

Each of the WCSDVARR extensions has been derived based on the values for the NPOL correction found in the reference file described in Appendix 2 - NPOLFILE Example. The full header for the WCSDVARR extension with EXTVER=1 is:

XTENSION= 'IMAGE   '           / Image extension
BITPIX  =                  -32 / array data type
NAXIS   =                    2 / number of array dimensions
NAXIS1  =                   65
NAXIS2  =                   33
PCOUNT  =                    0 / number of parameters
GCOUNT  =                    1 / number of groups
EXTVER  =                    1 / Distortion array version number
EXTNAME = 'WCSDVARR'           / WCS distortion array
CRVAL2  =                  0.0 / Coordinate system value at reference pixel
CRPIX1  =                  0.0 / Coordinate system reference pixel
CRPIX2  =                  0.0 / Coordinate system reference pixel
CRVAL1  =                  0.0 / Coordinate system value at reference pixel
CDELT1  =                   64 / Coordinate increment along axis
CDELT2  =                   64 / Coordinate increment along axis
FILENAME= 'v971826aj_npl.fits' / name of file
FILETYPE= 'DXY GRID'           / type of data found in data file
OBSTYPE = 'IMAGING '           / type of observation
TELESCOP= 'HST'                / telescope used to acquire data
INSTRUME= 'ACS   '             / identifier for instrument used to acquire data
DETECTOR= 'WFC'                / detector in use: WFC, HRC, or SBC
FILTER1 = 'F475W   '           / element selected from filter wheel 1
FILTER2 = 'CLEAR2L '           / element selected from filter wheel 2
USEAFTER= 'Mar 01 2002 00:00:00'
COMMENT = 'NPOL calibration file created by Ray A. Lucas 29 APR 2010'
DESCRIP = 'Residual geometric distortion file for use with astrodrizzle-------'
PEDIGREE= 'INFLIGHT 11/11/2002 11/11/2002'
HISTORY   Non-polynomial offset file generated from qbu16420j_dxy.fits
HISTORY   Only added to the flt.fits file and used in coordinate
HISTORY   transformations if the npol reference filename is specified in
HISTORY   the header.  The offsets are copied from the reference file into
HISTORY   two arrays for each chip.  Each array is stored as a 65x33 pixel
HISTORY   image that gets interpolated up to the full chip size. Two new
HISTORY   extensions for each chip are also appended to the flt file
HISTORY   (WCSDVARR).
HISTORY qbu16420j_npl.fits renamed to v9615069j_npl.fits on Sep 6 2011
HISTORY v9615069j_npl.fits renamed to v971826aj_npl.fits on Sep 7 2011

Each of the WCSDVARR extension headers contains the same set of keywords, with only the values varying to reflect the axis and chip corrected by this extension.