Quote:
Originally Posted by Shiraz
Very informative results Lee - thanks. The short exposures clearly work for your system and the extra subs all help with SNR.
I think that the SNR trend is the expected SNR=const*SQR(frames)..(ie doubling the number of frames gives 1.4x the SNR).. but the graph certainly does not have much obvious curvature in this region . I plotted your SNR against SQR(frames) and got a nearly perfect linear relationship, so that is an effective way to measure SNR.
Any idea how much resolution improvement is likely to be available by smarter stacking of shorter subs? I tried (again) to test this last night, but clouds appeared half way through.
regards Ray
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Thanks Ray, appreciate the confirmation! As I've said many times before, math is sadly not my strong suit, so it's good to get confirmation from someone such as yourself.
The (potential) resolution increase is going to be tricky to measure, I think, without a boatload of data. There's so many variables at play (seeing, cable drag, mount tracking etc). How are you planning to measure/compare it?
I can compare the FWHM of resulting images, but it's hard to measure the real impact when they've actually measured different things.
Quote:
Originally Posted by RickS
Interesting experiment, Lee.
Were the noise estimates done on drizzled integrations? That would "hide" some of the noise.
Did you get an improvement in FWHM with shorter subs?
Cheers,
Rick.
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Good point about drizzle, I hadn't thought much about its impacts on noise profile and what that might mean for the NoiseEvaluation's ability to measure it. What I'm interested in there is the relative noise though so as long as it remains consistent then I think we're ok.
I think I need more data to be able to say that I got better/worse/neutral FWHM. At the moment all I can say is that it looks about on par.
Quote:
Originally Posted by Atmos
Very interesting results Lee. How does the overall SNR compare between the same integration time of 36s vs the 480s? How close is it to the calculated 90%?
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Not very! But that's because I'm mildly retarded. Turns out I mathed wrong. 36s should give me ~82.3% of the SNR of the 480s sub,
not ~90%. Silly mistake converting ADU to e. Turned out that it gave me 82%, so basically exactly what the theory predicted.
Interestingly enough, this means that the 36s subs gave me (relative to integration time) ~80.7% of the SNR that would be achieved with a single 240min sub.
Based on the above, I'm going to up my (Luminance) sub length to 96s, which should give me about 91.3% of the SNR of a 240min sub. I'm happy with that trade off. Also means that my RGB can be significantly shorter and still be relatively well exposed, which is nice, since those sub lengths bothered me more than the luminance ones.
I've attached a spreadsheet (in snr.zip, the forum won't allow me to upload the xlsx spreadsheet directly) plotting SNR vs sub length for a 240min integration. The second tab contains my actual results. Please note the results in that second tab are based on a normal integration, not the drizzle integrations measured/shown in the original post.
The object/sky flux in the spreadsheet are measurements based off a single 36s exposure, with the object flux being that of PGC 962963, the small galaxy near M104.
This is based off the math from Steve Cannistra's article
Signal to Noise Ratio and the Subexposure Duration