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.. _appendix3:
Appendix 3 - D2IMFILE Example
==================================
The D2IMFILE reference file only contains a single 1-D array that should correct the
column (row) values based on the value of the 'AXISCORR' keyword in the SCI header.
FITS File Extensions
--------------------
This simple reference file, therefore, contains only 2 extensions; namely,
::
Filename: /grp/hst/cdbs/jref/v971826mj_d2i.fits
No. Name Type Cards Dimensions Format
0 PRIMARY PrimaryHDU 35 () int16
1 DX ImageHDU 18 (4096,) float32
PRIMARY Header
--------------------
The PRIMARY header only needs to contain information on what detector this file corrects,
along with any available information on how this file was generated. The ACS/WFC D2IMFILE
PRIMARY header only 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
DATE = '2010-02-01T20:19:11' / Date FITS file was generated
IRAF-TLM= '2011-09-02T13:04:07' / Time of last modification
NEXTEND = 1 / number of extensions in file
FILENAME= 'v971826mj_d2i.fits' / name of file
FILETYPE= 'WFC D2I FILE' / 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 '
USEAFTER= 'Mar 01 2002 00:00:00'
COMMENT = 'D2I calibration file created by Warren Hack 29 APR 2010'
DESCRIP = 'Column-width correction file for WFC images------------------------'
PEDIGREE= 'INFLIGHT 11/11/2002 11/11/2002'
HISTORY
HISTORY Fixed column (or row) width correction file. This is applied
HISTORY as a correction to the input pixel position and the output of
HISTORY this correction is to be used as input to the polynomial and
HISTORY non-polynomial distortion corrections.
HISTORY
HISTORY For ACS WFC data, the correction is stored as an image extension
HISTORY (D2IMARR) with one row. Each element in the row specifies the
HISTORY correction in pixels for every pixel in the column (or row) in
HISTORY the science extension; for ACS WFC, the correction is in the X
HISTORY direction.
HISTORY
HISTORY For a more in-depth explanation of this file, please see the
HISTORY draft writeup at:
HISTORY http://stsdas.stsci.edu/stsci_python_epydoc/stwcs/fits_conventions.html
HISTORY wfc_ref68col_d2i.fits renamed to v961506lj_d2i.fits on Sep 6 2011
HISTORY v961506lj_d2i.fits renamed to v971826mj_d2i.fits on Sep 7 2011
In this case, most of the keywords not required by FITS describe how this file
was computed while also describing how it should be applied.
Data Extension Header
----------------------
The header keywords for the actual DX array simply needs to provide the information
necessary to apply the values to the data; namely,
::
XTENSION= 'IMAGE ' / Image extension
BITPIX = -32 / Bits per pixel
NAXIS = 1 / Number of axes
NAXIS1 = 4096 / Axis length
PCOUNT = 0 / No 'random' parameters
GCOUNT = 1 / Only one group
EXTNAME = 'DX ' / Extension name
EXTVER = 11 / Extension version
ORIGIN = 'NOAO-IRAF FITS Image Kernel July 2003' / FITS file originator
INHERIT = F / Inherits global header
DATE = '2009-03-18T19:28:09' / Date FITS file was generated
IRAF-TLM= '16:05:02 (18/03/2009)' / Time of last modification
CRPIX1 = 0 / Distortion array reference pixel
CDELT1 = 0 / Grid step size in first coordinate
CRVAL1 = 0 / Image array pixel coordinate
CRPIX2 = 0 / Distortion array reference pixel
CDELT2 = 0 / Grid step size in second coordinate
CRVAL2 = 0 / Image array pixel coordinate
The fact that these values get applied without interpolation to each pixel in a row,
in this case, means that no translation terms are needed in the header, making for
a very simple header and very simple application to the data.
.. [DistortionPaper] Calabretta M. R., Valdes F. G., Greisen E. W., and Allen S. L., 2004,
"Representations of distortions in FITS world coordinate systems",[cited 2012 Sept 18],
Available from: http://www.atnf.csiro.au/people/mcalabre/WCS/dcs_20040422.pdf
.. [SIPConvention] Shupe D.L., Hook R.N., 2008, "The SIP Convention for Representing Distortion in FITS Image
Headers", [cited 2012 Sept 18], Available from: http://fits.gsfc.nasa.gov/registry/sip.html
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