Quote:
Originally Posted by Atmos
That is some really nice detail Paul! It is an interesting comparison Re-Fainter halos.
Both scopes are 12". F/3.8 vs F/8. Pixel size is 9 vs 4.54. QE is ~ .6 vs maybe .45 at a guess.
Assuming both systems are read noise limited, you would need 2x the exposure time to get the same SNR on fainter areas. I imagine that with a 11002 sensor it would need far more than 20 minutes to reach that point at F/8 though. At a guess I would say that it is the sensor read noise that is the limiting factor.
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Quote:
Originally Posted by Shiraz
very high res image Paul - nice result.
just ran both systems through the spreadsheet. It agrees with your observation - with 20 minute subs on both, your system requires ~6x the time to get to the same depth in Ha.
As Colin pointed out, the f8/9micron and f3.8/4.5micron optics configurations are essentially equivalent, but your 11002 has about half the Ha QE of the 694 so you need 2x the exposure to compensate. The biggest killer though is that the 11002 has about double the read noise of the 694 and, since neither system is sky limited at 20 minutes under dark skies, the read noise makes a real difference - you need an extra ~3x exposure to compensate. You don't have many options to do anything about this, short of using much longer subs so that you introduce less read noise. However, you would need maybe 10 hour subs to really help a lot, so that has practical and dynamic range limitations. A camera with lower read noise would help, but it must have ~9 micron pixels to match the rest of your system. The only solutions I can think of would be one of the Sony 694/814 chipped cameras used in 2x2 bin to get large pixels (but only 1.5/2mp) - or maybe even a ZWO/QHY 16mp CMOS camera with software binning for 4mp. Neither would be much good for wider fields, but good for smaller objects such as PNs. If you want to try this approach, let me know - you can experiment with my 694 if that would help. With the 694 in bin2, you would get back the missing 6x factor using you existing scope.
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Quote:
Originally Posted by RickS
[Ray beat me to it with a more comprehensive answer, but we said similar things  ]
Hi Paul,
Your system is approx one third the speed of Mike's based on the small difference in image scale, a little more obstruction and the larger difference in QE (for Ha the QE of the KAI-11002m is down around 30%.)
I'd agree with Colin that with 20 min NB subs you're going to be taking a big hit from read noise, which is up around 12e- for that sensor.
Longer subs would help, but you're at a big disadvantage based on QE and read noise. It is definitely the sensor that is the weak link in the chain.
Cheers,
Rick. |
Thanks guys for the responses. I agree the read noise is the biggest problem here, with QE coming in a close second, then the scope itself.
I have been evaluating whether I will continue down this path for much longer and have considered other systems for quite some time. Imaging faint obscure objects is currently a criteria for this analysis. I am involved in some research imaging and really need a faster system for that purpose.
Agree that using a 20 minute sub was probably a mistake in the equation this time, though I thought the integration would have made up the difference and it has slowly over time but not enough to really be worthwhile, and there are diminishing returns here. Perhaps I should have used say 30- 60 minute subs (would welcome alternative comments here).
Thanks again.