Siril Mosaic stitching not aligned

Software Siril
1

I’m trying to put together a mosaic of two images shot using a 350mm scope. The images are flat and the same size. When I combine them using the Deep Space Astro’s process things come together but the area where there is overlap has a doubling of stars. Across the entire seam the pairs of doubled stars are the same distance apart, and at the same relative angle, which tells me there’s no real issue with edge distortion. If I could only nudge one image a bit it would line up, but I’m not aware of Siril having that manual capability. What settings can I adjust to get these two images to align better?

2

Hello,
there’s manual registration available in the registration tab but i feel the window to check the alignment in your case would be pretty much useless (it’s designed for mostly overlapping images, not mosaic tiles).
Can you share your 2 tiles so that i can have a look as to why astrometey would not align properly?

Cheers

C.

3

I’m happy to do that, are you asking for the two actual fits files I’m working with? If so they are large, what method of file sharing do you prefer, is a google drive link ok?

4

Yes, the 2 stacks you are trying to align for the mosaic. Google drive link is fine.
Thanks!

5

https://drive.google.com/drive/folders/1J4FYQp7C3ZFNB2ygid31Cr5mUfF3YB1j?usp=sharing

Let me know if there’s an issue, but the two files should be visible. Thank you for the assistance!

6

Thanks,

got your images all right. So the problem is there an astrometric solution in your images which is inaccurate. If I use a conesearch command and inspect catalog stars positions vs their actual position, I can see they don’t match, in particular on the edges:

image
image548×481 88.5 KB

It’s not so much there is distorsion, it’s just the center is slightly off.
So if we use this info to register the 2 tiles together, it won’t work.

The steps to get a correct mosaic without using a script:

  • create a sequence with the 2 images
  • open the platesolve dialog, tick the Sequence/Astrometric registration and untick Skip already solved images (we really want to overwrite the currently stored solution)
    image
    image546×107 10 KB
  • In the Registration tab, select Apply existing registration and framing= maximum, click Register.
  • And then in Stacking, just press stack. You will need to feather a bit and normalize on overlap.
    All those steps are detailed in this tutorial:
    Siril - Mosaics

Here-below the overlap zone, no feathering, no normalisation, just to check the alignement:

image
image642×476 75.6 KB

Cheers,

C

7

Thank you, that makes sense, and it worked. Really appreciate the help!

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