When can you say "Enough subs!"

[rfdesigner]

English huh?

Internal CCD noise comes primarily from two sources, readout noise and dark current. Readout noise is constant and 'inflicted' on a per frame basis, less frames = less noise:

Experiment I've just run to prove this to myself.

I ran my lodestar at 1000th of a sec, 1 sec, 1 minute and 5 minutes getting dark frames for each.

I got a reading for the noise in each case of 20, 20, 30, 55 ADC counts (the camera's a bit warm hence these numbers are all a bit high)

So to take a 5 minute shot, you need 300,000x1000thsec frames, 300x1sec frames, 5x1min or 1x5min frame.

When you add noise you Square, Sum and Square Root so the noise works out as

20*sqrt(300,000) for 1000th sec subs or 11200 counts.

20*sqrt(300) for 1sec subs or 355 counts

30*sqrt(5) for 1min subs or 67 counts

55*sqrt(1) for 5min subs which of course is 55counts

Summary: longer subs are better, but once the dark current starts to be significant you don't loose much compared with a single long frame, i.e. 1m vs 5m.. if the camera were cooler then you'd want to lengthen the shots as the dark current halves for every ~8C where as the readout noise would only fall by a few percent over the same range.

Limiting case: tracking/passing aircraft/CCD saturation/skyglow/dawn:)

In your 2 minute subs case being able to go to 8 minute subs may well add as much benefit as doubling the total observing time.

i.e. you might find that 100x2min subs = 12x8min subs.

Derek

[namreg]

Thanks Derek. I'll read that again (and again) after I've researched dark noise and readout noise! Interesting stuff though

[FraserClarke]

Nice explanation and example Derek. Just to add that when you're taking data, the sky background usually dominates over the dark current for most cooled CCDs (i.e. 1 count/s of dark current vs 10 counts/s of sky, for example). So exactly the same principle applies, but for the sky background rather than dark current. The exception might be for narrow-band H-alpha imaging, where the sky is really dark.