[gnuastro-devel] [task #15128] TPV and SIP keywords when WCS distortions are necessary

[Mohammad Akhlaghi]

URL:
  <https://savannah.gnu.org/task/?15128>

                 Summary: TPV and SIP keywords when WCS distortions are
necessary
                 Project: GNU Astronomy Utilities
            Submitted by: makhlaghi
            Submitted on: Fri 21 Dec 2018 02:27:13 PM UTC
         Should Start On: Fri 21 Dec 2018 12:00:00 AM UTC
   Should be Finished on: Fri 21 Dec 2018 12:00:00 AM UTC
                Category: All Gnuastro
                Priority: 5 - Normal
              Item Group: None
                  Status: Postponed
                 Privacy: Public
        Percent Complete: 0%
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
                  Effort: 0.00

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Details:

Currently, Gnuastro just uses WCSLIB's raw output when it wants to write WCS
keywords that are distorted. In particular, it will use the SIP distortion
coefficients (which has become the commonly used standard recently).

But, as discussed in Proc. of SPIE Vol. 8451, 84511M
<http://web.ipac.caltech.edu/staff/shupe/reprints/SIP_to_PV_SPIE2012.pdf>, 
there are many software packages (Astromatic software
<https://www.astromatic.net/> in particular) that use TPV. 

In that SPIE paper, they suggest to include both conventions in the header
(with `CTYPE' referring to SIP, but TPV also present as `PV*' keywords. 

It would be good if we apply this standard in Gnuastro's outputs also (to
allow easy integration with Astromatic). 

WCSLIB does appear to have a flag for writing TPV keywords (`WCSHDO_PVn_ma',
when calling `wcshdo'), but I couldn't see any change when I changed the
current value (`WCSHDO_safe') in the call to `wcshdo' of `lib/fits.c' (using
WCSLIB 6.2).

So we might have to do this manually. That paper does give some of the basic
equations, but I couldn't find the `sip2pv' and `pv2sip' C programs it talks
about! However, they have apparently we-written it in Python and are
distributing that here <https://github.com/stargaser/sip_tpv>. 

So if there is no better solution, we can translate this Python code into C
and include it in Gnuastro's WCS library (for general use). 

Afterwards, it is trivial to also add a common option to all programs (maybe
called `--tpvinwcs'). When it is called, Gnuastro's programs can follow the
SPIE paper's suggestion and have both headers present.

For the time being, I am busy and won't have the chance to work on this, so if
you are interested please post a comment (for encouragement!), or just take up
the job (its just a relatively simple translation from Python).




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