Creating mosaic with Andromeda

You will have to take equally long exposures.
Take one half of M31. Take the second half with equally long exposures. You can stitch the two halves together in photoshop. Here is an example. This is 5 twenty second exposures stitched together with photoshop
https://www.astrobin.com/401211/?nc=user
hope this helps
CS

The major problem with mosaics is the proper blending at the edges. You can try to process each panel individually and then stitch with Microsoft ICE. It's free and pretty great, I use it for lunar mosaics. I am not sure how well it handles the blending with DSO images.

What I use for mosaics is the AstroPixelProcessor. Granted, it's 60 EUR per year but the stacking features make it really nice and easy. I preprocess and stack each panel individually. I do not merge narrowband images into a color image. Instead I just the gradient removal tool to fix any light pollution and crop the outer edges a bit. Then I load my stacked and light-pollution-fixed panels as FITS file into a new session and select mosaic under registration and activate the blending under integration. This produces seamless mosaics.

There is a free trial. And a tutorial on mosaics on its forum.

Cheers,
Jesco

If you have enough overlap DSS Mosaic mode does work.

In DSS "Register Checked pictures…"   Takes you to the  "Register Settings" tab.  Hit the Stacking parameters button,  then select "Mosaic Mode".

  Very easy to use but you get what you get.  Be careful adjusting the number of stars under the "Register Settings"  advanced tab.  Too many  or too few and the stack might fail. I like numbers around 100 if I can get them.

 I've used this to make a mosaic registration "Master" that I then use as the reference image  in Pixinsight "Star Align".

Sorry for the late reply. I don’t have an answer to the filters you are using. I use a full spectrum DSLR. the only filter I use is a LPF.
I stitched my images together before I processed anything except stacking. I use DSS For all of my images

CS