WCS alignment with a flip using v3.3.1

[TAI]

<tldr/> Align Stack using WCS doesn't seem to handle a flip

A sequence of FITS images were obtained with a GEM. About 75% through the sequence there was a flip. Before the images were imported in AIJ a third party software was run that plate solved each image and wrote WCS data to the FITS header.

Process -> Align Stack using WCS or apertures... with Use only WCS headers for alignment seemed to work though I'm not clear what the percentage refers to in the "Aligned_(30.8%)" window title.

Apertures are then placed in Multi-Aperture Measurements and after AIJ is told to do its thing lo, a Message window opens:

No signal for centroid in aperture T1 of image <the one taken right after a flip>. Multi-Aperture aborted.

Well, darn! Am I missing something in alignment process?

Thanks,
Jesse

[karenacollins]

Hi Jesse,

Stack aligner is only currently capable of aligning images by translating in x and y, so it can not correct for the rotation at meridian flip. However, since your images are plate solved, you do not need to align images to run multi-aperture, even with a meridian flip. All you need to do is enable the Multi-Aperture option "Use RA/Dec to locate aperture positions". Then AIJ will first locate aperture positions by RA and Dec (rather than X and Y) and will then centroid (if enabled).

Actually, with AIJ 3.3.1 and later, all you need to do is plate solve the first image and the first image after the meridian flip (or the first image after a large jump in pointing if its not a meridian flip). In this mode of operation, "Use RA/Dec..." mode places apertures by RA/Dec if an image is plate solved, and by X,Y coordinates of apertures in the previous image if not plate solved. To use this mixed mode of operation, the Multi-Aperture option "Halt processing on WCS or centroid error" must be DISabled.

Karen

[TAI]

Thanks, and please forgive me as I think you mentioned "Use RA/DEC" in an earlier post that I glazed over. -Jesse

[karenacollins]

No problem. I hope that works for you.

On 7/13/2019 12:37 PM, TAI [via AstroImageJ] wrote:

Thanks, and please forgive me as I think you mentioned "Use RA/DEC" in an earlier post that I glazed over. -Jesse

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