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

[William Thompson]

On 09/11/15 12:57, David Berry wrote:
> 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.
>>

...

>> 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.

I see this as being very similar to what we do for solar images, where we've 
defined a solar-specific coordinate system signified by the axis labels HPLN and 
HPLT, with the orientation aligned to the solar north polar axis.  This is a 
convention, but one which has been adopted by several recent solar missions.

I found documentation for OFLN/OFLT here:

http://www.starlink.rl.ac.uk/docs/sun211.htx/sun211ss281.html

I'm not sure I completely understand it.  Are OFLN/OFLT always defined to be 
offsets in the RA/Dec system, or could they also be offsets in some other system 
such as ecliptic?  The AST documentation talks about additional AST-specific 
keywords being written to the header to allow the original "SkyFrame" to be 
reconstructed.  This leads me to wonder whether using OFLN/OFLT is useful 
without the full AST library implementation.

>
>> 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.

I would argue for including them for maximum clarity, similar to the way that 
LONPOLE is recommended to be included even when it has the default value.

> David
>
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>

-- 
William Thompson
NASA Goddard Space Flight Center
Code 671
Greenbelt, MD  20771
USA

301-286-2040
William.T.Thompson at nasa.gov