[fitsbits] WCS keywords in tables

[William Pence]

So far there have been no objections to this simplifying WCS convention 
for images stored in FITS binary tables.  If that's really the case, 
then I will write up a short description (crediting Mark Calabretta as 
the originator) and submit it for inclusion in the Registry of FITS 
Conventions on the FITS support office web site.

Bill Pence

Pence, William D. (GSFC-6601) wrote:
> The following question about FITS WCS keyword usage occurred during some 
> recent discussions with a group that is in the process of designing the 
> format of their FITS data files:
> 
> This project intends to produce a large volume of FITS binary tables in 
> which several of the columns will each contain a small image (in a 
> 2-dimensional vector cell) in every row of the table.   Table 8.2 in the 
> FITS standard defines the set of WCS keywords that should be used in 
> this case (where the images are in column 4 of the table in this example):
> 
> WCAX4   =                    2 / number of World Coordinate System axes
> 1CRPX4  =             86.88896 / reference pixel
> 2CRPX4  =            261.93919 / reference pixel
> 1CRVL4  =    284.6423916705557 / right ascension  at reference pixel
> 2CRVL4  =    39.92821811003438 / declination (degrees) at reference 
> pixel
> 1CTYP4  = 'RA---TAN'           / the coordinate type for the first axis 
> 
> 2CTYP4  = 'DEC--TAN'           / the coordinate type for the second axis 
> 
> 1CDLT4  = -0.00110560439808150 / degrees per pixel, increasing eastward 
> 
> 2CDLT4  = 0.001105604398081504 / degrees per pixel, increasing northward 
> 
> (etc)
> 
> These keywords would need to be repeated for all the other columns that 
> contain images.
> 
> However, in their particular case, *all* the images contained in the 
> table have exactly the same WCS keyword values, so it is redundant to 
> repeat the same keywords for every column.  So, they would prefer to 
> just list all the WCS keywords once, using the standard keywords that 
> are used for simple FITS images:
> 
> WCSAXES   =                    2 / number of World Coordinate System axes
> CRPIX1  =             86.88896 / reference pixel
> CRPIX2  =            261.93919 / reference pixel
> CRVAL1  =    284.6423916705557 / right ascension at reference pixel
> CRVAL2  =    39.92821811003438 / declination (degrees) at reference 
> pixel
> CTYPE1  = 'RA---TAN'           / the coordinate type for the first axis 
> 
> CTYPE2  = 'DEC--TAN'           / the coordinate type for the second axis 
> 
> CDELT1  = -0.00110560439808150 / degrees per pixel, increasing eastward 
> 
> CDELT2  = 0.001105604398081504 / degrees per pixel, increasing northward 
> 
> (etc)
> 
> We were pleasantly surprised to find that Mark Calabretta's wcslib 
> library already supports this extension to the WCS usage.  The standard 
> image keywords can be used in binary tables to provide the default WCS 
> values for all vector columns in the table.  These WCS values may be 
> overridden in any particular column by providing the column-specific 
> form of the WCS keywords for that column.  This only works for images in 
> vector columns; it does not apply to 'pixel list' images in binary 
> tables because there is no well-defined association between coordinate 
> axis number and column number.
> 
> So, the question is, should we encourage projects to use this extension 
> to the WCS usage for vector images in binary tables?  Are there any good 
> reasons not to use this?  Do other WCS libraries besides wcslib support 
> this?
> 
> There is one other complexity in their case:  accurate astrometry is 
> essential, so they need to define the distortion corrections that should 
> be applied to the nominal WCS keywords.  For this purpose, they are 
> considering using the SIP convention (see 
> http://fits.gsfc.nasa.gov/registry/sip.html) which has keywords like:
> 
> AP_0_1
> BP_1_2
> etc.
> 
> Alternate forms of these SIP keywords have not been defined for use with 
> images stored as vector cells in binary tables, so they would either 
> need to define a new set of keywords for this purpose (which would be 
> difficult given the 8-character limit on keyword names), or just use the 
> existing keywords that were designed for simple FITS images.
> 
> Bill Pence

-- 
____________________________________________________________________
Dr. William Pence                       William.Pence at nasa.gov
NASA/GSFC Code 662       HEASARC        +1-301-286-4599 (voice)
Greenbelt MD 20771                      +1-301-286-1684 (fax)