[fitswcs] Status of WCS negotiations

[Patrick Wallace]

I wrote:

> >Now we turn to Section 5 of G+C, where we are asked to help old FITS
> >readers by writing a CROTA2 value...Equation 133 shows us how to do this:
> >
> >   CROTA2 = (arg(0,1)+arg(0,1))/2 = 90 deg

Mark Calabretta replied:

> ...you overlooked the factor/divisor of S = CDELT2/CDELT1 in the equations
> for rho_1 and rho_2 leading to Eq. 133.

Mark is quite right, but it's immaterial in this (special, but not unlikely)
case.  S was 0.0002/0.0001 in my example, so what I should have written was:

  CROTA2 = (arg(0,2)+arg(0,0.5))/2 = 90 deg

However, it doesn't change the answer.

> If you had included this you would have noticed rho_1 != rho_2 and
> realized something was amiss given that section 5 prescribes
> rho_1 = rho_2 for the computation of CROTA2...

For this particular case, you get 90 deg for both rho_1 and rho_2, so the
program does not notice anything is amiss.

OK, this can be patched up by yet more fine print in Section 5, and I
assume Mark will oblige.  But the objection remains that writers may have
a good reason to choose a style of PC matrix that happens not to comply
with Eq. 132, and will then have to accept that they can't also offer a
CROTA2-based description of the transformation.

Mark doesn't want to write CROTA2 at all, which is one way out, and even
sanctions opting out of using the new machinery altogether.  But why not
eliminate all the objections, at low cost, simply by choosing new names
for what the paper calls CDELTn?  Writers can then, if they wish, help old
readers by writing CDELTn/CROTA2 as well as PC/nuCDELTn, and can pick
whatever PC matrix formulation they like.

Incidentally, Section 5 of G+C ought to be relabelled "Compatibility with
previous formats" rather than "Converting previous formats" because it
addresses both cases: new readers interpreting old files and old readers
interpreting new files.


Patrick Wallace
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Starlink Project Manager                        Internet:  ptw at star.rl.ac.uk
Rutherford Appleton Laboratory                       Tel:    +44-1235-445372
Chilton, Didcot, Oxon OX11 0QX, UK                   Fax:    +44-1235-446667
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