Re: Stacking via WCS results in blank images

Posted by cduston on Jul 07, 2017; 1:35pm
URL: http://astroimagej.170.s1.nabble.com/Stacking-via-WCS-results-in-blank-images-tp708p715.html

Hi Karen,

     This is great, thanks for your detailed answer.

I agree - I think a warning message is appropriate. If I was using IRAF I might have noticed the missing header keywords, but with something like AstroImageJ I had no idea what to look out for. Part of my decision making was driven by the fact that these images come from the itelescope network, so the presence of the WCS in the first place made me think they "knew what they were doing", and an additional plate solution would not be needed.

But, your solution worked perfectly - I just did another plate solution (via Astrometry.net) and the aligned images look fine. This was very easy, so I don't view this as "needing to be supported by AstroImageJ", but I do think a warning message when I tried to do the original alignment would have been appropriate, and could have directed me to a solution without having to bother you.

But, this solves my problem so thanks very much, I really appreciate the attention to this issue.

Chris

On Fri, Jul 7, 2017 at 6:52 AM, karenacollins [via AstroImageJ] <[hidden email]> wrote:

Hi Chris,

Thanks for the images. I do see the problem here and there is nothing you are doing wrong. However, the plate-solve headers that are added to the image headers are not in standard WCS format. There is enough info there to display RA and Dec, the plate scale, and the sky orientation arrows, but for WCS alignment to work, the images must have the WCS header keywords and values for CRPIX1, CRPIX2, CRVAL1 and CRVAL2, so that there is a specific pixel and associated RA and DEC that can be compared from image to image, and so the CRPIX1 and CRPIX2 values can be easily adjusted to account for the shift so that RA and Dec will display properly after the images have been aligned.

That's probably more than you wanted to know, but the bottom line is that the form of plate solution in your headers can not currently be used to align images. Your version of headers could probably be used for alignment when I can get time, but I won't be able to figure out how to implement that in the near future. The other issue is that there are several different formats of non-standard WCS headers, and it is probably not practical for me to support them all. I should at least consider showing an error message when the appropriate WCS headers are not available.

At least for now, you will have to plate solve using a different method (e.g. the AIJ link to astrometry.net, or astrometry.net directly, or TheSkyX since I know that it adds WCS compliant headers), or align using apertures, which I think would work fairly well for the sample images you sent. The downside of aligning using apertures is that the RA and Dec display may be wrong after alignment since the CRPIX1 and CRPIX2 are not in the headers for AIJ to adjust.

I apologize that I am not able to support the current format of plate solution in your headers at the present time,

Karen

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