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Hi, I'm trying to use a galaxy photometry routine created by my advisor that requires the wcs header keywords "crota" and "cdelt". Unfortunately, the wcs in my headers uses the cd rotation matrix. Is there a quick way to convert from the cd matrix to the crota/cdelt (I know it's fairly simple to go the other way)? Thank you!Heather

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Heather

Unless I'm not remembering things correctly:[code:1:7e560eab69]
cdelt1 = cd1_1
cdelt2 = cd2_2
crota1 = atan2 ( cd2_1, cd1_1)
crota2 = atan2 (-cd1_2, cd2_2)
[/code:1:7e560eab69]Near the poles or when the x-axis is the Dec/Lat these equations change and there are special conditions, but in most cases i think these work.For reference, the CROT/CDELT to CD-matrix transform is:[code:1:7e560eab69]
CD1_1 = CDELT1 * cos (CROTA2)
CD1_2 = -CDELT2 * sin (CROTA2)
CD2_1 = CDELT1 * sin (CROTA2)
CD2_2 = CDELT2 * cos (CROTA2)
[/code:1:7e560eab69]Hope this helps, hope others can offer needed corrections/elaboration.Cheers,
-Mike

That helps a lot. Thank you!Heather

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Heather