bugGNU Astronomy Utilities - Bugs: bug #55217, --align not working as expected in...


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bug #55217: --align not working as expected in left-hand oriented WCS

Submitter:  Mohammad Akhlaghi <makhlaghi>
Submitted:  Thu 13 Dec 2018 01:52:58 AM UTC
Category:  Warp Severity:  3 - Normal
Item Group:  Output not reasonable Status:  Postponed
Privacy:  Public Assigned to:  None
Open/Closed:  Open

Thu 28 May 2020 04:09:57 AM UTC, comment #1: 

Currently, this can be fixed by flipping the image along the problematic axis ('--flip=1,0' or '--flip=0,1'). In Commit 6dc1fd447fd a warning has been added to explain the problem.

But unfortunately '--flip' has to be called in a separate/prior call to Warp, it can't be in the same call, because we only implement the warping matrix in the WCS at the end of all rotations. So one useful step in this bug may be to apply the WCS correction after each separate warp.

Mohammad Akhlaghi <makhlaghi>
Group administrator
Thu 13 Dec 2018 01:52:58 AM UTC, original submission:  

When the input WCS dataset is left-hand oriented (East is to the left of North, what ever direction North has), then the `--align' operator doesn't work as expected.

This problem can be reproduced with a simple mock image:

$ echo "0 25 25 1 2 1 45 0.5 1 5"                    \
       | astmkprof --oversample=1 --mergedsize=49,49 \
$ astwarp in.fits --flip=0,1 --rotate=20
$ astwarp in_warped.fits --align

The output `in_warped_aligned.fits' will not be aligned with the standard RA/Dec directions (North to the top and East to the left).

However, apparently no information is lost, because after opening `in.fits' and `in_warped_aligned.fits' in ds9 and matching them by their WCS, things seem reasonable. So --align is apparently just failing in finding the wrong rotation, it doesn't look like a lower-level problem (in mixing pixels for example).

Mohammad Akhlaghi <makhlaghi>
Group administrator


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