[wfc] Result of WFC Vote on New FITS Standard

[Steve Allen]

On Wed 2008-01-23T14:57:42 -0500, Arnold Rots hath writ:
> 5) 4.4.2.2 Equinox - Why should an EQUINOX value of 2000 imply ICRS?
> How does one indicate FK4 or FK5 coordinates that were precessed to year
> 2000?
> And what  does "exactly 2000" mean for a floating point value? Maybe I
> misunderstand, but this sounds like a major change that will cause an
> incorrect interpretation of at least some archived FITS data (even
> though the numerical difference is probably negligible). It would make
> more sense to define a RADECSYS keyword (as done in the WCS papers and
> the VO SSAP data model paper) but even then, defining a default
> reference frame could effect the interpretation of archived FITS files.

4.4.2.2 is effectively updating the meaning of the "legacy" keyword
EQUINOX from the initial Wells et al.  FITS paper in situations when
the rest of the keywords from the WCS papers are not present.

> This seems to imply the assumed reference frame should vary with the
> value of EQUINOX, and one should assume FK5 for EQUINOX=2000 and ICRS
> if EQUINOX is not specified.

This is exactly as in the CCS paper, which specified defaults to be
presumed when not all of the WCS keywords are present.  They were
basically heuristics for interpretation of various transitional forms
of WCS keywords in FITS headers.

> Does this contradict the earlier definition in 4.4.2.2 or am I missing
> something?

So by including 4.4.2.2 I see that this standard adds one nuance of
default which is not present in the CCS paper.

In the absence of the rigor of the full set of WCS keywords it seems
likely to me that the milliarcsecond distinction between FK5 J2000.0
and ICRS is not only mostly harmless, but totally harmless.  It is not
at all clear what EQUINOX means.

I see the effect as just another reminder for FITS data providers to
consider updating their software to support the now-official IAU
coordinate system.

--
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