[fitswcs] Long-slit spectroscopy WCS

[Malcolm J. Currie]

Thank you Frank and Mark.

Frank Valdes concluded in a step-by-step reply:
> So as you move in y (say along a night sky line) you get a variation in
> both RA and DEC in the coordinates.  So you use something like
>
> CD2_2 = scale * cos (PA)
> CD3_2 = scale * sin (PA)

So the answer's yes. (-:  Here the CDELTj are folded into the CD
elements, whereas I was thinking in terms of the PCji matrix with
CDELTj.  The question (in my mind but not clearly expressed) was more
one of whether the CDELTj (j=2,3) or the PCji be scaled by the
components of the position angle as above.  This general approach seemed
to be the obvious method; it's one that would be easy to follow for the
typical astronomer or programmer who doesn't know about projections and
the nitty-gritty of co-ordinate systems.  There was also a nagging doubt
that there was some better official alternative vindicated by Mark's
response implying the LONGPOLE keyword.  Is either acceptable?

Frank's notes and similar would be a useful addition to explain the
examples presented in Paper III or IV.  From designing the Starlink
NDF, I learnt that even programmers don't read the full documentation;
they want annotated examples.  Something like our SGP/38 and now Paper
II is somewhat intimidating.  Thus I urge the inclusion of examples
for common arrangements.  Those with trickier WCS problems will have
to read the small print or ask on fitsbits.

Mahalo,
 
Malcolm