bugGNU Astronomy Utilities - Bugs: bug #51385, Pixel scale measurement when...


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bug #51385: Pixel scale measurement when dimension scale isn't equal or doesn't decrease

Submitter:  Mohammad Akhlaghi <makhlaghi>
Submitted:  Tue 04 Jul 2017 11:07:48 PM UTC
Category:  Libraries Severity:  3 - Normal
Item Group:  Output not reasonable Status:  Fixed
Privacy:  Public Assigned to:  makhlaghi
Open/Closed:  Closed

Tue 04 Jul 2017 11:39:24 PM UTC, comment #1: 

This bug has been fixed and pushed to the main repo.

Mohammad Akhlaghi <makhlaghi>
Group administrator
Tue 04 Jul 2017 11:07:48 PM UTC, original submission:  

The `gal_wcs_pixel_scale' function (new name for the `gal_wcs_pixel_scale_deg' function of version 0.3 from commit 58ec3afd) uses Singular value decomposition (SVD) to find the pixel scale from the full WCS matrix independent of its rotation.

However, GSL's SVD 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.

Mohammad Akhlaghi <makhlaghi>
Group administrator


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    Follow 3 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2017-07-04 makhlaghi StatusIn Progress Fixed
        Open/ClosedOpen Closed
        Summarygal_wcs_pixel_scale output dimension order does not necessarily correspond to input Pixel scale measurement when dimension scale isn't equal or doesn't decrease

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