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Old 29-10-2013, 10:26 AM
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Shiraz (Ray)
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Join Date: Apr 2010
Location: ardrossan south australia
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thanks for the comprehensive response Dave - pleasure to have a detailed discussion on this topic.

I will try to summarise my response to the points you raise, but that will require a bit of a critique of the Anstey paper - so sorry to go off track a bit. Also, if it sounds a bit pompous in places - sorry..

first off, you questioned whether sub length is proportional to read noise squared. I think that it is - the read noise and signal both increase linearly with multiple subs, but the shot noise only goes up with the square root. If you double the read noise, you need to increase the signal by 4x to get a doubling in shot noise, ie you need to expose for 4x as long if you want the shot noise to cover a doubling of read noise.

The other main points come from the underlying assumptions of the Anstey article, which are:
- you need to look at the noise in the target when you set sub length
- short subs will be messed up by quantisation noise.

I disagree on both points:
1. In any imaging I have done, the bright bits of the target are never an issue - they take care of themselves. The big problem is the noise that blights the dim parts of the scene when the target is barely visible or not present at all. For broadband imaging the primary source of noise in this region is shot noise from the sky background and target signal/ noise is quite unimportant. Hence, I consider that the Smith/Starizona approach is valid and that subs should be chosen to keep read noise well below shot noise from the sky background. Noise is noise and it doesn't matter where it came from - there is nothing special about the target noise.

2. I think that the quantisation issue is a non-starter. To take your example, consider a target with an average of 1 photon every five minutes. Some of the 2.5 minute subs (on average slightly less than half of them) will have no target photons, some will have one and maybe the odd one will have two or more. Add the signals over 12 such subs and you will typically get around 6 target photons - the same as you would get from one long 60 minute exposure. This of course depends on the stacking method - it won't work if the stacking is done by simply averaging the camera signals at the same bit resolution as the original (then you might get either 1 or 0 for the target). However, any stacking system worth its salt will at least add up the total signal and then divide by the number of subs to give an average. If the internal signal representation is floating point, you may get something like "averagesignal = 0.5" and if you want to know how many photons you collected then just multiply by the number of subs - no signal has been lost by having sub signals below 1 photon on average. The final average signal will need to be stretched more than it would for the longer exposure, but the SNR will be exactly the same, since the noise will also have been divided by the number of subs the get an average. ie, there is no quantisation issue at all. Even if the stacking system produces a fixed point representation, all is not lost - see http://www.stark-labs.com/craig/reso...thStacking.pdf. Maybe I have it wrong on stacking, but with my current understanding, I do not accept any of the Anstey arguments based on quantisation error.

A couple of other comments:
- cameras with 1 electron read noise are available - they just cost an arm and a leg.
- your conclusion that shooting something like the Orion nebula is similar to NB is valid, but only for the bright bits. I find that, to bring out the fine detail around it, you need to take into account the sky noise - the Smith/Starizona approach is still valid. Maybe it isn't on something like the moon or a planet, but then the issue is not one of long exposures anyway.
- the simplest way to resolve some of these questions is by experiment, will try to do so if I can ever again find a nice clear night with good transparency and then force myself to devote some imaging time to something that may be of limited interest.

Cheers and regards Ray

Last edited by Shiraz; 29-10-2013 at 11:33 AM.
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