That is a pretty cool pic.
Actually, it is just something like that which prompted me to develop my first spreadsheet to try and figure it out. I had been imaging distant galaxy clusters - the splash screen is the very image that started it all
. As time went on I wrote an app, and then gave it to my software engineers to enhance it using the same algorithms. So, I can't take full credit, only perhaps 60%.
Couple of things to note:
The Seeing plays a major part in LM determination. If you measure it by looking at the FWFM on your image, it will be wrong because it will also contain the OTA diffraction induced blurring. A way to see what seeing really was, measure it in the image, and then look at the "Focused Star Size" on the System page in aCAD. Move the Seeing slider until it matches your measured FWHM. That was your seeing. In my case, using the C11, the measured FWHM of 3.8" corresponded to seeing of 2.8". We should add a small calculator to assist with that.
Distant galaxies look like stars, but if you measure their FWHM, it is larger than regular stars. So, if that is your goal, increase the Seeing to something like 4 - 6" to get a better idea.
~Chris