[gnuastro-devel] [bug #51385] gal_wcs_pixel_scale output dimension order does not necessarily correspond to input

[Mohammad Akhlaghi]

URL:
  <http://savannah.gnu.org/bugs/?51385>

                 Summary: gal_wcs_pixel_scale output dimension order does not
necessarily correspond to input
                 Project: GNU Astronomy Utilities
            Submitted by: makhlaghi
            Submitted on: Wed 05 Jul 2017 01:07:48 AM CEST
                Category: Libraries
                Severity: 3 - Normal
              Item Group: Output not reasonable
                  Status: In Progress
                 Privacy: Public
             Assigned to: makhlaghi
             Open/Closed: Open
         Discussion Lock: Any

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Details:

The `gal_wcs_pixel_scale' function (new name for the `gal_wcs_pixel_scale_deg'
function of version 0.3 from commit 58ec3afd
<http://git.savannah.gnu.org/cgit/gnuastro.git/commit/?id=58ec3afdd2421b3>)
uses Singular value decomposition
<https://en.wikipedia.org/wiki/Singular_value_decomposition> (SVD) to find the
pixel scale from the full WCS matrix independent of its rotation.

However, GSL's SVD
<https://www.gnu.org/software/gsl/doc/html/linalg.html#singular-value-decomposition>
implementation orders the singluar values based on their values. It does not
preserve the original ordering of the input matrix.

Until now, `gal_wcs_pixel_scale' would just return the output of GSL's SVD
function. So it was correct when the pixel scales were identical in all
dimensions or when the scales decrease for higher dimensions.

It is possible to use the V matrix output to find the correct order and
permute the pixel scales correspondingly. I am now busy implementing it and
will push the fix when it is complete.






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