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