I posted this in the Refractors forum, but I suspect there are more people on this forum who could comment about this.
This weekend I finished my test image of M31 using my AT130EDT + Riccardi 0.75x FF/FR + ASI6200MM. I wanted to make sure the scope could accommodate the full-frame sensor of the ASI6200MM. This combo worked great but out of curiosity, I plate-solved subs from each filter I used for the image to obtain the sub's image scale, and then I computed the corresponding focal length. Here is what I found using the following formulas:
pas = 206.265*(px size in microns) = 206.265*3.76 = 775.5564
FL = pas/(image scale)
Filter: image scale (FL)
Blu: 1.1290 (686.941)
Grn: 1.1290 (686.941)
Red: 1.1270 (688.160)
Lum: 1.1280 (687.550)
Ha: 1.1270 (688.160)
OIII: 1.1280 (687.550)
Obviously the registration process in PI takes care of the slightly different scales observed here. Since each color would be focused separately using a mono camera but not with a OSC camera, it seems that mono cameras have this additional advantage over OSC cameras for refractors. My conclusion is that unless you have a refractor with perfect color correction, a mono camera plus filters will have a two-fold advantage over a OSC camera with one multi-band filter: not only is there better spatial resolution without a Bayer matrix, you will get smaller star images with a mono camera since individual colors or bands will be individually focused.
BTW, my final image is here:
Edited by LPA, 21 September 2020 - 12:12 PM.