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

[Patrick P Murphy]

FYI.

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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?

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?

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

Thanks very much!
-Ian

-- 
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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"Door meten tot weten" -- H.K.Onnes
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-- 
Patrick P. Murphy, Ph.D.               https://www.nrao.edu/~pmurphy/
Info Services Site Manager          NRAO Information Security Officer