Re: Subtracting a Galaxy template

Posted by cduston on Jan 25, 2024; 7:18pm
URL: http://astroimagej.170.s1.nabble.com/Subtracting-a-Galaxy-template-tp2302p2344.html

I thought I would add some here, as I've been trying to tackle this problem with no success.

The technical aspects of the image subtraction does not seem to be the problem - I found that aligning via image stabilization was better than WCS, and the virtual stack was not necessary - just saving as a stack worked fine. The creation of the template as we outlined above worked fine as well - meaning to blank out everything outside of a particular region around the galaxy.

However, the subtraction was never satisfactory because of the normalization. I tried removing the background across the entire image (typically with options "sliding paraboloid" and "disable smoothing", otherwise the background level went below zero), and then normalizing the stack, but the normalization never made sense - the brightness of the galaxy was different across the images, so the subtraction didn't work. The same appeared to be true of a few random stars I checked, and the closest I got was by individually scaling each image so the galaxy brightness was consistent - not feasible for more than a few images in the stack, although I was eventually able to extract a light curve from one particular target.

So perhaps a way to fix this would be to have options in the normalization window - scale to brightest stars, or within a set of apertures, so then you could pick what objects you wanted to have consistent brightness in the image.

Also missing in this particular task would be a way of dealing with the different seeing - I think the usual thing to do is smooth with a Gaussian to match the worst seeing in the stack, but it doesn't seem like that is possible in AIJ. I have to say, I don't know how critical this smoothing step is to this kind of task. Possibly just including the normalization option would at least open up this avenue.

So I think this remains a task for, i.e., AstroPy. Thanks for all the advice here!