.. _appendix2: Appendix 2 - NPOLFILE Example ================================== The NPOLFILE reference file format includes a PRIMARY header describing what kind of image should be corrected by this file, along with extensions containing the corrections for each chip. FITS File Extensions -------------------- A sample NPOLFILE applicable to ACS/WFC F475W images has the FITS extensions:: Filename: /grp/hst/cdbs/jref/v971826aj_npl.fits No. Name Type Cards Dimensions Format 0 PRIMARY PrimaryHDU 35 () int16 1 DX ImageHDU 180 (65, 33) float32 2 DY ImageHDU 215 (65, 33) float32 3 DX ImageHDU 215 (65, 33) float32 4 DY ImageHDU 215 (65, 33) float32 The extensions with the name 'DX' provide the corrections in X for each of the ACS/WFC's 2 chips, while the 'DY' extensions provide the corrections in Y for each chip. Primary Header -------------------- The PRIMARY header of this file only includes the minimum information necessary to describe what exposures should be corrected by this reference file and how it was generated. A full listing of the PRIMARY header includes:: SIMPLE = T / Fits standard BITPIX = 16 / Bits per pixel NAXIS = 0 / Number of axes EXTEND = T / File may contain extensions ORIGIN = 'NOAO-IRAF FITS Image Kernel July 2003' / FITS file originator IRAF-TLM= '2011-09-09T13:24:40' NEXTEND = 4 / Number of standard extensions DATE = '2010-04-02T19:53:08' 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 Data Extension Header ---------------------- Each ACS/WFC chip has a shape of 4096 x 2048 pixels, yet the data arrays in this specific reference file only have 65x33 values. Each data extension ('DX' and 'DY') contains only those keywords necessary to properly interpolate the sub-sampled values from the arrays to apply to each individual pixel in the full ACS/WFC exposure. The full header for the ['DX',1] extension contains:: XTENSION= 'IMAGE ' / Image extension BITPIX = -32 / Bits per pixel NAXIS = 2 / Number of axes NAXIS1 = 65 / Axis length NAXIS2 = 33 / Axis length PCOUNT = 0 / No 'random' parameters GCOUNT = 1 / Only one group EXTNAME = 'DX ' / Extension name EXTVER = 1 / Extension version ORIGIN = 'NOAO-IRAF FITS Image Kernel July 2003' / FITS file originator INHERIT = F / Inherits global header DATE = '2004-04-28T16:44:21' IRAF-TLM= '16:42:00 (30/11/2006)' WCSDIM = 2 LTM1_1 = 1. LTM2_2 = 1. WAT0_001= 'system=physical' WAT1_001= 'wtype=linear' WAT2_001= 'wtype=linear' CCDCHIP = 2 / CCDCHIP from full size dgeo file LTV1 = 0 LTV2 = 0 ONAXIS1 = 4096 / NAXIS1 of full size dgeo file ONAXIS2 = 2048 / NAXIS2 of full size dgeo file CDELT1 = 64 / Coordinate increment along axis CDELT2 = 64 / Coordinate increment along axis