.. _appendix1: 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 :ref:`appendix2` :ref:`appendix3`. :: 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: D2IMARR Header -------------------- The full, complete header of the ``D2IMARR`` extension as derived from the D2IMFILE discussed in :ref:`appendix3`. :: 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: 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 :ref:`appendix2`. 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.