[fitswcs] [forwarded] WCS for co-moving maps

[David Berry]

On 11 September 2015 at 17:45, Patrick P Murphy <pmurphy at nrao.edu> wrote:
> FYI.
>
> ------- start of forwarded message -------
> From: Ian Avruch <i.avruch at sron.nl>
> To: fitswcs at listmgr.nrao.edu
> Subject: WCS for co-moving maps
> Date: Fri, 11 Sep 2015 18:42:43 +0200
>
> Hello all,
>
> I'm seeking help in specifying the WCS for maps that track moving
> objects.
>
> Our data are spectra taken with a single-dish radio telescope.
> The (space) telescope observes in a pattern with respect to the
> changing apparent position of the solar system object (SSO).
>
> Our software produces spectral cubes in which the frequencies are in the rest
> frame of the SSO, and the spatial axes are a GLS projection of the
> angular offsets from the SSO.
>
> Here is an example header with my best guess of a proper WCS, based on
> the meager examples I could find.  This is for a cube in which there is
> one pixel directly on the SSO. Axis1 is offsets from the SSO parallel
> to RA (ICRS), in increments of 20 arcsec, axis2 is parallel for Dec
> (ICRS), and the frequency axis is 1GHz wide centered at 1800.0 GHz.
>
> -----------------------------------------------------------
> 123456789 123456789 123456789 123456789 123456789 123456789
>
> WCSNAME = 'Offset from SSO'
> NAXIS   =                    3
>
> CTYPE3  = 'FREQ    '
> CUNIT3  = 'GHz     '           / WCS: Unit axis 3
> SPECSYS = 'SOURCE  '
> SSYSOBS = 'TOPOCENT'
>
> NAXIS3  =                 2001
> CRPIX3  =               1000.0 / [] WCS: Reference layer index, unit=Scalar
> CRVAL3  =            1800.0000 / [] WCS: Wavelength, time, ... of reference layer; unit=length,time,...
> CDELT3  =               5.0E-4 / [] WCS: Scale in 3rd dimension, unit=Length, Duration, ...
>
> NAXIS1  =                   11
> NAXIS2  =                   11
> CTYPE1  = 'OFLN-GLS'
> CTYPE2  = 'OFLT-GLS'
> CUNIT1  = 'deg     '           / WCS: Unit axis 1, default=""
> CUNIT2  = 'deg     '           / WCS: Unit axis 2, default=""
> CRVAL1 =                   0.0
> CRPIX1  =                 10.0 / [] WCS: Reference pixel position axis 1, unit=Scalar
> CDELT1  =   -0.005555555555556 / [] WCS: Pixel scale axis 1, unit=Angle
> CRVAL2 =                   0.0
> CRPIX2  =                 10.0 / [] WCS: Reference pixel position axis 2, unit=Scalar
> CDELT2  =    0.005555555555556 / [] WCS: Pixel scale axis 2, unit=Angle
>
> CROTA1  =                  0.0 / [] The Rotation angle
> CROTA2  =                  0.0 / [] The Rotation angle
>
> NAIFID  =                  499 / [] Solar system object NAIF identifier
> ORBITFIL= 'H20130625_0002.LOE' / [] Spacecraft geocentric orbit ephemeris
> -----------------------------------------------------------
>
> My questions are:
>
> 1) Is CTYPE(1,2) = ('OFLN-xxx', 'OFLT-xxx') a correct/standard usage?

OFLN/OFLT is the convention used by the AST library for representing
offsets from moving objects. I don't think it's an accepted standard
in any sense. JCMT produces data using this convention.

> 2) How do I convey that the pixles along axis1 are offsets from the SSO
>     parallel to the ICRS RA axis, and not some other RA,Dec system?

I'd say use RADESYS.


> 3) Do you see any other problem with the above header?

CUNIT1/2 are not really needed as the FIST-WCS standard mandates degrees.

David