I was hoping one of the CCDWare Jedi could finally put this issue to rest for me as I continue to be troubled by it. I totally understand your subexposure calculator- how it works and have calculated the exposure in my location to be around 3-4 minutes. Now, my understanding of this is that this calculation determines the MINIMUM exposure recommended to overcome camera read noise etc. for satisfactory outcomes. It says nothing regarding the maximum exposure, so for example if I decided to take 15 or 20 minute subs, the only issue there would be an increased chance of a satellite trail or airplane flying through or whatever, and possibly at some point you wouldn't necessarily be adding much additional data. Is this correct? I can't tell you how many times I am told in the various groups that my subs are too long, that they 'tweaked the ccdware subexposure calculator' and according to their calculations I should not be taking more than 8 min subs, or whatever! MUCH confusion surrounds this issue so I appreciate any help you can offer at resolving the dispute.
Thanks again, Dave

The CCDWare Sub Exposure Calculator does indeed tell you how long you need to expose to overwhelm read noise.

The fact is, once you reach this point, you start to get diminishing returns on that exposure as sky noise (light pollution) will start to interfere with the collection of photons from the desired object. At this point, it is best to stop exposing, and start a new one. This will give you the best possible amount of S/N for your precious imaging session.

As an example, here is an image comprised of 100 X 300 second exposures:
http://www.ewellobservatory.com/gallery/viewer.cfm?ID=58

Thats a total of 30000 seconds of exposure.

I could of taken 25 X 20 second exposures, but:
- the S/N would not have been as high
- I would of thrown away bad exposures
- I got better S/N because of dithering 100 exposurs vs. 25.

I hope that helps.

rb

Thanks Richard!