[fitswcs] WCS for co-moving maps

[Ian Avruch]

Hi,

Thanks for the replies, which I try summarize here.
However, I'm left unsure of the proper way to create an unambiguous
header for this case. Is it possible?
I hope I can make clear my question below.

The situation is that we are making maps of the sky around
moving solar system objects (SSOs), such as comets.  The telescope
slews in a pattern while tracking the apparent position of the SSO,
so that while on the sky the sample points are sparse, when reduced
to offsets relative to the SSO the sample points fill an area.

Neighboring points in the offset map differ by fixed increments
of angular distance parallel to RA or Dec (ICRS) from the
_instantaneous_ position of the SSO at the different sample times.
So they are not increments along RA or Dec; this should be clear.

A WCS with these pertinent keywords and values can be proposed:

CTYPE1  = 'OFLN-GLS'
CTYPE2  = 'OFLT-GLS'
CRVAL1  =                  0.0
CDELT1  =   -0.005555555555556 /
CRVAL2  =                  0.0
CDELT2  =    0.005555555555556 /
RADESYS = 'ICRS'
NAIFID  =                  499 /
ORBITFIL= 'H20130625_0002.LOE' /
HORIZSRC= 'DE405'
CROTA1  =                  0.0

With these keywords I try to describe a comoving map with 20-arcsec
pixel separation, where the pixels along axis 1 are parallel to
RA(ICRS) (though not, of course, in increments of RA), and likewise
axis 2 and Dec(ICRS).  If the axes are not parallel to RA & Dec
we'll use a nonzero CROTA1, though realizing it's depcrecated usage.

The ORBITFIL and HORIZSRC serve to define the observer and SSO
barycentric locations as functions of time.

**What concerns me is that RADESYS='ICRS' implies that CRVALn are
RA,Dec(ICRS), whereas they are not, of course. Am I wrong?

It was suggested to define a secondary, fixed, WCS to tie
the offset coordinates to the celestial sphere at a fiducial
time; the CRVALnA values could be the SSO coordinate values
at the fiducial time. I resist this because I think it's misleading;
the bare keywords imply that the two coordinate systems are so related
at all times.

Maybe it's not possible at the moment to unambiguously specify this
kind of comoving coordinate system in FITS? If so, we simply want to
implement something that won't be misleading, including to
software readers.

Any comments? I'm planning to implement a header like the one
above because it's not as wrong as what we have now!

Cheers
-Ian


On 09/14/15 18:46, Lucio Chiappetti wrote:
> On Sun, 13 Sep 2015, Dirk Petry wrote:
>
>> OFLN/OFLT sounds interesting also for our purposes at ALMA.
>> Is anyone working towards making this part of the FITS standard?
>
> I was not aware of the existence of this convention. It was never submitted to the registry of FITS conventions
> http://fits.gsfc.nasa.gov/fits_registry.html.
>
> But I presume it uses fully valid ("legal") FITS constructs. Just to make things clear:
>
> - if a convention uses valid constructs according to the FITS standard,
>    it is valid FITS.
>
>    Conventions inside a specific project or community are perfectly
>    legal.
>
> - if a convention has been submitted to the Registry, it simply means
>    it has a complete documentation and is advertised there. Inclusion
>    in the registry does not imply any endorsement. But if it was valid,
>    it remains valid. Anybody wishing to use it is encouraged to use it.
>
>    The registration is a suggestion : if you need to do this, you can
>    do it that way.
>
> - the IAU FITS WG (currently being dissolved and resurrected in the
>    framework of the IAU Commission reform) is currently working at
>    incorporating some registered conventions in the standard document.
>
>    WCS conventions are down the priority list, so most likely the
>    successor body will care about that.
>
>    Incorporation in the standard means : if you need to do this, then
>    you must do it that way.
>
>    But valid conventions remain valid and not forbidden even if they
>    are not "stamped" as part of the standard document.
>

-- 
Ian Avruch  Instrument Scientist / HIFI Calibration
             SRON (www.sron.nl)
             T (0031|0) 50 363 8759
             F (0031|0) 50 363 4033
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