Best guide to use for a mosaic??

[mconsidine]

Apologies in advance if I have overlooked something obvious. This isn't really an issue, per se

I am trying to create a mosaic of two images, where the resulting image is the combination of the two original images, but without any cropping.

I can get the registered image from the examples I've seen, but I can't seem to get the resulting combined image.

Have I overlooked a parameter that turns off the cropping so that the result has dimensions covering the original as well as rotated/transformed image? I got the test_align.py script to execute without error. If there is something in there I should build upon, I'd appreciate a pointer.

TIA,
mconsidine

[martinberoiz]

Hi,

To create a mosaic, you can pad the target image with a pad region large enough to contain the transformed image entirely. Then before you call the register method you mask the original target region as explained here:

https://astroalign.quatrope.org/en/latest/mask.html#selecting-regions-with-masking

I think that should work. Let me know if you need more help.

[mconsidine]

Thanks for that. I'm working with two color fits images. So after selecting one channel out of each and adjust the histogram to knock down the number of stars, I get a transform that seems to work. Except that I really need to extract that transform, apply it to the original source image and then coadd the two I think.

The resulting 'registered' file, when written to first, is very large. Is there a bounding box function available that would help trim off what is unnecessary?

Thanks again for your help and for the package.
mconsidine

[martinberoiz]

Astroalign itself does not have any tool to downsample or trim images. But you can do an easy trim using numpy array slicing on the registered ndarray or astropy Cutout2D if you need to maintain the WCS info as well.

I'm not sure if this answer your question.

[mconsidine]

It does answer my question - thank you. And I seem to generally have things working out, except that I can't quite get the masking right to include both the target and the source. The source is correctly positioned inside a larger padded image, but the target image hasn't come through. If you have a further example you can point me to, I'll pull this apart. I'm very appreciative of the help you've given already.
Rgds,
mconsidine

[martinberoiz]

I converted this issue into a "discussion".

If you send me some working code along with a couple of images I can try helping you create the mosaic. Otherwise it's hard to give you a concrete example. There's a lot of finicking sometimes with images.

[mconsidine]

Thank you for switching this to "discussion" as I should have started out with that.I can send working code and links to 2 images tomorrow.I'm obliged.MattOn Feb 22, 2024 7:01 PM, Martin Beroiz ***@***.***> wrote: I converted this issue into a "discussion". If you send me some working code along with a couple of images I can try helping you create the mosaic. Otherwise it's hard to give you a concrete example. There's a lot of finicking sometimes with images. —Reply to this email directly, view it on GitHub, or unsubscribe.You are receiving this because you authored the thread.Message ID: ***@***.***>

[mconsidine]

Thank you again for your offer of help/tips. I've sent you links to 2 files and the code I'm using is attached here. If the two files A and B are in the same directory as the code, this should run "as-is." I've tried too to comment what I was doing/thinking at each point.
astroalign_test2.txt

[mconsidine]

Hello again,

I'm continuing to work on this and saw that the "mask" tutorial might be something I could use. The text doesn't show how the final result was obtained so I have been trying to replicate that on my own.

Would it be possible for you to present (or point me to, if I missed it) the workflow that place the source_half_degree.png file into the target image? I seem to be running into a matching triangles issue and am not sure what I need to tweak to reproduce those results.
Many thanks,
mconsidine

[martinberoiz]

Hi, apologies for not replying earlier. I just sent you an email with a jupyter notebook where you can see how you can make a mosaic with the images.

The final mosaic does not look well because of gain differences in the images (or something related to that, I didn't look) but the idea is there.

Let me know if it helps.