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Old 03-12-2016, 04:39 PM
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Shiraz (Ray)
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Shiraz is offline
Join Date: Apr 2010
Location: ardrossan south australia
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Thanks for bringing this up Sam.

just did a test.

stacked 11 subs with no alignment and then again with star alignment.

- single sub noise ~48e. stacking 11 such subs should give ~14.5e

- stacked the aligned subs and the noise was ~14e, ie the noise reduced as expected by the sqrt of the number of subs

however, with no alignment, the noise after stacking was ~20e and it varied a lot with position (your factor of 1.5?).

My take on it that there is definitely a difference in SNR depending on alignment, but that the expected sqrt(n) relationship applies to the aligned stack. The unaligned stack is the odd one out and it has worse noise than expected due to the inherent persistence of FPN (it doesn't reduce with stacking unless it is decorrelated in some way)

You raise some very interesting issues - we use interpolation like this by default, without giving any thought to what it may mean to the images. For example, with heavy rejection of outliers, maybe it is possible to tighten up star profiles in the stacks. And what difference does the choice of interpolation algorithm make?

Last edited by Shiraz; 03-12-2016 at 05:57 PM.
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