[fitswcs] Distortions

[Frank Valdes]

There has not been any movement towards putting forward a standard for celestial or higher dimensional spectral/spatial coordinates.  There are a few distorted celestial systems under the conventions approach that have fairly wide use.  See the WCS registered conventions here <https://fits.gsfc.nasa.gov/fits_registry.html>.   At NOAO TPV has become the main exported version though internally I like ZPX as better suited to optical telescope data.  Some downstream users convert to SIP.  Software libraries pretty widely do implement TPV.  In my pipeline work I’ve adopted exactly the same approach for data products as you mention; two flavors of unrebinned and rebinned to TAN in the N-up/E-left convention.

So you should study SIP and TPV and, if it is suitable for you, use one of them before inventing something else.  As I said, TPV is handled in, at least, IRAF, DS9, Terapix (SExtractor/Scamp,etc), the SAO WCS tools, and, I believe, the usual libraries such as WCSLIB.

But I think you are talking about non-celestial or mixed celestial and dispersion systems.  There is, of course, paper 3 for 1D non-linear spectra.  The question of distortions in general and in spectra in particular started with a proposal I made.  It did not go further because other authors claimed to be entitled take over and be lead authors on such a paper and I did not want to get into the politics.  Anyway, digging into my past documentation the early proposal is:

http://iraf.noao.edu/projects/fitswcs/spec3d.html <http://iraf.noao.edu/projects/fitswcs/spec3d.html>

Some of the thoughts given there may be of use.  In principle this would be combined with the spectral WCS of paper 3 for the dispersion axis.

Cheers,
Frank



> On May 8, 2019, at 7:50 AM, Thompson, William T. (GSFC-671.0)[ADNET SYSTEMS INC] <william.t.thompson at nasa.gov> wrote:
> 
> Folks:
>  
> It’s been a while since I’ve heard any chatter about WCS.  What’s the status of Paper IV on distortions?  Are distortions handled in any of the software packages, such as WCSLIB?
>  
> It appears that we may need to use distortions for an upcoming mission.  Ray tracing for a spectrometer for the Solar Orbiter mission shows that the relationship between the pixel positions I,J and the real-world coordinates have significant second-order terms.  Our plans are to provide data in two formats, one in the original detector pixel space, and one where the data have been interpolated to linearize and rectify the data.  The latter is no problem, but I would like to accurately describe the coordinates in the former.
>  
> Second order effects show up in two places.  The most significant is that the plate scale in arc seconds increases linearly with wavelength.  In other words
>  
> S = a0 + a1 * J * (1 + a2*I)
>  
> where S is the spatial dimension along the slit, J is the pixel dimension associated with the spatial dimension, and I is the pixel dimension associated with the spectral dimension.  Here, the second order effect comes from the J*I term.
>  
> The other second order effect comes from the fact that the slit images are slightly bowed, so that the wavelength has a dependence on J^2.
>  
> Thanks,
>  
> Bill Thompson
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