[fitswcs] WCS documents

[Clive Page]

On Tue, 16 Oct 2001, Eric Greisen wrote:

>       One could argue about whether anyone should be using sexagesimal
> in a FITS table - will people want it as a keyword value next?  I
> believe that the authors of the tables extension considered
> sexagesimal and rejected it.  Angles should be put in floating (double
> precision) degrees.  I can understand the use of sexagesimal to convey
> old tabular data, however.  But I do not like adding new keywords for
> a function that a current keyword really should perform.

Eric

I think perhaps you misunderstood the idea of the proposal (and since I
was just using it as an example perhaps I was too brief and should
explain it a little more).

The fact is that FITS binary tables are used extensively to hold things
like lists of detected sources.  Typically they have columns of RAs and
DECs which are held internally as floating-point values (types D or E);
the units are typically degrees.  TDISPn can be used to specify a
preference as to how many decimal digits a FITS viewer should show, but
that's about all.  If you have a column of integers you can even use
TDISPn to say that the results are best displayed in, say, octal or
hexadecimal.  But for floating-point values there's no way of saying you
want sexagesimal formatting rather than decimal formatting.

The Babylonian standard for angles has been around a lot longer than even
the FITS Standard, and I don't think we'll get astronomers to change.  I
think it would be useful, therefore, if one could make a FITS file
expressing a preference that a FITS viewer (like FV) should show such
angular coordinates in sexagesimal form, which is what the astronomer in
the street expects.  I just haven't worked out the best way of doing this
yet.  There's no problem with keywords, of course, you just use a
character string, e.g.
REF_DEC  = "-12:34:56.78"
You _could_ do the same thing with angular coordinates in a table, but it
wouldn't be at all efficient, and it's clear that in practice people
always use floating-point types if they can.

-- 
Clive Page,
Dept of Physics & Astronomy,
University of Leicester,    Tel +44 116 252 3551
Leicester, LE1 7RH,  U.K.   Fax +44 116 252 3311