Quote:
Originally Posted by RickS
Hi Ray,
I definitely need flats with my KAF-16803. Interestingly, I take dusk and dawn flats every day when I have the scope set up in the back yard and I have found that having lots of flats does make a measurable improvement in SNR using PI noise analysis tools.
Richard Crisp did a flat field PTC analysis of one specific KAF-16803 and found that around half a million electrons was the sweet spot so that's what I usually target (around a dozen flats at 40K e- half well.) However, I recently discovered that if I use several times as many I can measure a significant SNR improvement. I must do some more PTCs myself when I get a chance. Maybe my sensor behaves differently to the one that Richard measured.
Cheers,
Rick.
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Hi Rick
Well that was interesting
. I went back to first principles and came up with what turned out to be a quite straightforward analysis and a simple rule of thumb for flat exposure:
if you want the flat induced noise to be below 10% of the total noise, then the total number of flat electrons must be 25x the total sky electrons in your lights. If you want it below 5%, you need 100x.
This is quite different to what I Understand of Richard's approach in that there is no single sweet spot for flats - if you expose longer, you need more flats - if your sky is brighter, you need more flats. Seems to make intuitive sense to me. Next step is to test it out in a model and then with a whole bunch of flats. When I have gone through the reasoning a few times, would be grateful if you would give it a once over before I post it (that is, if it holds up under testing - I still reserve the right to be wrong
).
If this is correct though, the more flats the better and you will always gain something (even if small) by adding flat data. It also means that those who are forced to image under bright skies should stock up on lots of flats and that the "mega" brigade should also go mega on flats. And maybe I can get some better results out of the many sets of lights that I currently have by just adding more flats to the calibration pot.
Must also have a look at darks and bias
regards ray
Edit: Just redid the analysis using full SNR rather than just noise and the requirement is not so stringent - you need flat electrons > 10x sky electrons to get out of the region where flat noise is a problem. Am now reasonably happy with the analysis - now to do some tests to validate.