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Old 02-09-2014, 08:12 AM
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Shiraz (Ray)
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Join Date: Apr 2010
Location: ardrossan south australia
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Quote:
Originally Posted by Peter.M View Post
Hi guys

I know this has been discussed by Ray quite a bit and I see his argument that when shot noise dominates an image exposing for longer has no benifit. Logically thinking about this at work at the moment and I think that it will still be better, here is the reason.

A long exposure that is already dominated by shot noise will be averaging the shot noise if it exposes longer. This would mean that stopping the exposure and starting another so you can stack more images to reduce shot noise would effectively already be happening in a longer exposure.

If I have this wrong I'm happy to be told so, just trying to logically understand what's going on.
Hi Pete. Sorry, I guess I have rabbitted on a bit

I think we agree, but....

If by "exposing for longer" you mean "exposing subs for longer", there is no value in longer subs. For example there is no SNR difference between a 20 minute sub and the average of 2x10 minute subs if both are "sky limited". The differences are that more stars will be saturated in the 20 minute sub, (so you have lost dynamic range) and the chance of a dud sub is higher (eg due wind gust or clouds). The issue is not that long subs are inferior from an SNR perspective, just that once you have subs that are long enough to be "sky limited", the SNR after stacking is the same from short or long subs - but you will lose in other areas by going longer.

If by "exposing for longer" you meant overall exposure, there is obvious benefit from going longer.

Last edited by Shiraz; 02-09-2014 at 04:52 PM.
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