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CCD Cameras and Signal to Noise Ratio This is a BIG question. But first, let's move away from the "how bright..." idea completely because the answer you are looking for isn't just about brightness (which is the signal). The real answer lies in your signal to noise ratio (S/N). Here's the concept in short form:
Given all of this information, it follows that you can come up with some optimal subexposure time for your particular combination of sky brightness and camera readout noise such that even the background of your images is shotnoise limited. There is a calculator for just this on the CCDWare web site under resources, which you can use to determine the optimal subexposure time for your particular conditions. That's the Big Answer to the question implied by the idea of brightness in a CCD image. If you want more detail, I can suggest getting a copy of my Zone System book. Anacortes stocks it. One more consideration. Now that you have all those nice optimal subexposures, how do you combine them  sum or average? Simple answer: There is no difference! (This illustrates how nonintuitive noise really is...) What is an average? It's simply a scaled sum! The only potential problem you run into is if the precision in your sum is such that the scaling reduces the precision. Most CCD cameras only have 1415 bits of dynamic range, but you are using no smaller than 16bit numbers to contain the scaled result. Only if you have enough images to generate more than 16 bits of information do you need to worry. And even then the solution is extremely simple: just switch to floating point or 32bit integers to save your scaled result  then there is no difference whatsoever between a sum and an average. As it happens, as long as you retain the _real_ precision of your data (which is smaller than the 16bit container your software typically uses), an average is the gold standard for lowest noise. (except for outliers, which gets us into a WHOLE other area of discussion and now my fingers are tired.) 

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