If this question has been covered elsewhere, I apologize, I was unable to find it.

I am trying to develop an understanding of the significance of the read noise of a CCD camera for the quality of an image.

I have read everything I can find, but there is am implicit logical leap in much of that material, that I need help making explicit.

Question1:

So far I understand that, simplistically, Full Well Capacity (Max signal) / Read Noise (Min signal) gives the effective dynamic range of a CCD. More dynamic range is better, because it allows a finer recording of subtle differences in light intensity. So, for example, with a FWC of 10,000 and a Read Noise of 10, there are 1,000 steps of dynamic range. With a FWC of 10,000 and a read noise of 6e, there are 1,666 steps of dynamic range.

This is where I am having my conceptual difficulty. I understand that the read noise establishes a floor for a signal (e.g. if read noise = 10e, then one needs 11e to create a signal, and the dynamic range expressed in electrons would be 10,000-10 = 9,990.

I don't understand how that translates into a dividing the full well capacity by the read noise to determine steps of dynamic range. There seems to be an assumption in this calculation that the size of each step of dynamic range of the sensor is equal to the read noise.

If read noise is 10e, is it not simply an adder to whatever number of electrons a photosite ends up capturing? A photosite that captures 7,000e will read off with 7010e (on average) for example, while one that captures 600e will read off with 610e (on average). In these terms, it seems an increase from 6e to 10e of read noise would be fairly de minimis. However, expressed in terms of steps of dynamic range, the increase in dynamic range is 2/3rds by going from 10e to 6e.

Question 2:

What is the relationship between the read noise and the necessary exposure time? I understand that generally, the advice is to expose for sub lengths sufficient to overwhelm read noise with sky noise, but how does that relate to the dynamic range of the camera?

Question 3:

If read noise is as important as it seems to be, then as between two similarly priced cameras with the same sensor and cooling capabilities (say the SBIG 16200 with 10e read noise and the FLI 16200 with 6e read noise) why wouldn't one always choose the camera wit h the lower read noise? Note - not trying to start a conversation on the relative merits of these two brands (which both appear to be amazing), just trying to understand the theory here based on what I have been reading.

I know I must have a basic conceptual break here, so apologize if any of the questions above are wrongheaded, and appreciate any enlightenment this amazing community can bring!