After doing this for awhile now, I'm wondering about what your thoughts are on the following: many including myself are making perhaps a single calculation for a project which takes many nights to complete, and taking all the subs for the whole project (multiple nights)at that specific duration. But your ADUs are going to vary between filters and also between nights (with moonlight and perhaps high clouds, etc). So wouldn't a better way to reduce total signal/noise in the final result be to do a subexposure calculation every night before starting your imaging? Maybe this is obvious, but just checking.
Thanks,
Dave

Dave,

I will not answer your question with qualitative data but with my hands-on experience on this subject.

1. With your clear (or Lum) filter, calculate your background on a moonless night (or when the moon is not up). Use this as the basis for sub-exposures. If the moon is up, then you will get minimum advantage to changing the exposure to a smaller number. For example, no moon = 300 sec exposure, half moon = 220 seconds. I normally just leave the exposure at 300 seconds for simplicity sake. If the math was done, I would suspect (means: I don't have supporting data) that the difference in s/n for that exposure would be minimal.

2. In regards to color. I take the filter that has the lowest transmission and use that for my test. So if the Red filter lets through the least light, I use that as my test, and use that number for all the other color filters. Again, the difference from 330 seconds for one filter and 270 for another may just make life more difficult with no appreciable s/n gain.

I tend to try to keep things simple and although your idea has a very logical intent (and would have an effect on s/n) I suspect (means: I don't have supporting data) that it might not be worth all the extra effort.

The best way to test this would be to take a 3 min exposure with all three colored filters and use the recommended value for sub-exposure and measure s/n on all three subs. Then use the same number for all three, and test the s/n and see what the difference is. If it is less than 10%, I would not go through the hassle of measuring every evening. You could use that time to take an extra exposure.

rb

ok. thanks richard!
dave