[fitswcs] Status of WCS negotiations

[Don Wells]

Mark Calabretta writes:
 > ..  One question is whether keywords for the CD matrix elements would be
 > in fixed format such as Cmiiijjj (i.e. C\d\d{3}\d{3} as a perl regular
 > expression) or variable such as CDmi_j (i.e. CD\d\d{1,3}_\d{1,3} with the 8
 > character limitation).  

It appears to me that the semantics of the CDij notation that we are
discussing are identical to the semantics used in IRAF (Doug please
confirm). If that is so, then the agreement should specify the keyword
style that IRAF uses, because that decision would make several massive
archives compatible with the agreement and would enable a variety of
existing FITS reading software to read the agreed notation
immediately, without any changes.

I recommend appending the WCS version code, e.g. CD2_1B, CTYPE3D,
CRVAL1C, CRPIX2A, rather than embedding it. It appears to me that this
would simplify the pattern matching logic in readers and be easier to
document in the agreement. In principle we could adopt a variation on
the HST notation (CD1_1_2 gives the partial of the right ascension
w.r.t. x for the second CCD in WFPC2 headers), because we will almost
always have NAXISi<=9 and because we could use [A-Z0-9] to get up to
36 versions.  However I am nervous about a limit of 9 for NAXISi, and
would prefer a limit of 99. The worst cases CRPIX99Z, CRVAL99Z,
CD99_99Z, CTYPE99Z, PROJP99Z all fit into eight characters, so I
recommend we go with that.

Until today I was concerned that a limit of 26 alternative WCS
descriptions might be too small in certain applications. In
particular, I suspect that the worst case application might be
high-frequency radio spectroscopy with dozens (hundreds?)  of
molecular transitions in the bandwidths of spectrometers
simultaneously. Today I reviewed the situation with several
NRAO-Charlottesville people, and we concluded that 26 should be
sufficient for FITS interchange of radio spectroscopic data.

 > How would they be handled in binary tables headers
 > where the 8 character limitation really starts to bite?  ..

Bill Pence can comment on that.

 > Another question
 > concerns what needs to be duplicated in a multiple representation -
 > everything?  

We could decide that values of the main WCS keywords (CD2_1, CTYPE3,
CRVAL1, CRPIX2) would be used for each alternative WCS unless
overridden. This would produce smaller headers in many cases, but
would mean that if PROJP13 is set in the main WCS but isn't used in
alternative WCS C it would have to be explicitly reset with
PROJP13C=0. I would prefer to duplicate everything, because it would
simplify the text of the agreement and would be less likely to cause
implementation errors. The usual defaults should apply (1 on diagonal,
0 off diagonal for CDi_jm, 0 for PROJPim).

 > Hanish & Wells (1988) suggested writing CDELTn and CROTAn
 > together with the CD matrix for the benefit of old interpreters.  Is that
 > still a good idea?  

Certainly reasonable people can differ on this question. I have
changed my mind---I now think it will be better not to put CROTA2 in
the header along with CDi_j, but instead to provide standalone
conversion program(s) to translate headers for forward/backward
compatibility. My bet is that almost all of the old interpreters are
going to disappear quickly soon after we reach this agreement.

-Don
-- 
  Donald C. Wells         Associate Scientist         dwells at nrao.edu
		    http://www.cv.nrao.edu/~dwells
  National Radio Astronomy Observatory                +1-804-296-0277
  520 Edgemont Road,   Charlottesville, Virginia       22903-2475 USA