[fitsbits] WCS keywords in tables

[Arnold Rots]

Bill,

I couldn't get enthusiastic about this convention, but if it's useful...

I do have some questions, though:

- How do we know which columns these keywords apply to?
  All that contain a 2D vector?

- How do we interpret a situation where there are 2D vector columns
  that do have explicit specifications (whether WCS or otherwise), in
  addition to there being a global specification of the kind you are
  proposing?

Cheers,

  - Arnold


William Pence wrote:
> 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)
> 
> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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