Thanks for your reply, riccio. Yes, these are ballpark estimates just intended to get a wisp of nebulousity to imprint on the subframe. I decided that I desperately needed a tool like this when I was flummoxed as to why I couldn't get a print of the Fireworks Galaxy (NGC 6946) with a 30 sec exposure after I just got a print of the Ring Nebula at 30 sec -- after all, the catalog says that they are both magnitude 8.8. After that is when I discovered Surface Brightness.

The weather here has been terrible. I am lucky to get one night per week. Since then I did verify that a 52 sec exposure of the Dumbbell Nebula is roughly equivalent to a 30 sec exposure of the Ring Nebula, that according to my program. I have more testing to do. Right now I am hacking my 50-year old telescope and mount to enable auto-guiding. I have been pretty much limited to 1 minute exposures due to periodic error. I have now replaced the original synchronous motor with a stepper motor of my design, and putting the final touches on periodic error correction. I would be happy if I can do 5 minutes unguided with only minimal star elongation. The next step will be to write an INDI driver and do auto-guiding. First, though, I will try to go unguided.

Thanks for your input!

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