[Shiraz (Ray)]

Was going through SNR theory for another purpose and suddenly realised that there is a very easy way to work out the best CCD sub exposure time while imaging.

Rather than go to online calculators and calibrate the system, all you need to do is make sure that the sub exposure length is long enough that the average background sky signal is above a target ADU value found from:

targetADU = Bias + 10*RN*RN/camera_gain

where the Bias is the normal bias level your camera produces, the camera_gain is the internal conversion factor (electrons/ADU) and RN is the camera read noise in electrons. This data should be fairly readily available for your camera.

When you have done the above calculation once for your camera, you thereafter use that targetADU value and (on the fly) choose exposures long enough to reach it, for whatever scope/filter/conditions etc that you have. It will provide you with an SNR that is within 5% of the best possible SNR, while preserving the dynamic range (in the same way as the online calculators). You don't need to know anything more than a single targetADU figure to ensure that your exposures are as effective as possible for whatever system you wish to put your camera on, or whatever the sky brightness is like.

As an example, my H694 has a bias of about 1100, a read noise of about 5.6 and a gain of about 0.3, so the targetADU is about 2150. On a clear night I will typically get this in 5+ minute subs on my current system, but hazy bright sky nights will only require about 2-3 minute subs. I can use the same targetADU figure for working out colour subs (which need to be longer than luminance subs), unless I choose shorter colour subs for extended dynamic range. When I get my new scope running, I will use the same targetADU value and choose exposures that yield the targetADU with that system.

You can use shorter subs (lower ADU) if you wish to get high dynamic range, but you will need to expose for a longer total time. Longer subs (higher background than the targetADU) will not change the overall SNR significantly, but will reduce the dynamic range. The calculation will work for narrowband as well, although you will need to include dark current in the average background signal estimate.

This process is so easy to use that I am embarrassed that I did not think of it before now. Hope it is useful and would be grateful for any feedback.

I don't think that this applies to dslr cameras, where the system noise varies with exposure time - it requires a camera with stable characteristics. Also, have no idea what complications the Bayer filters on an OSC introduce.

regards ray