Quote:
Originally Posted by glend
Mike n Trish, my initial reaction was to just 'let this go through to the keeper', but hidden in your thesis is the attitude we so often hear. While your theory is of course sound, the 'caveats' and 'all things being equal' are real gotcha's in many peoples lives, and i would add another one - budget. In a perfect world of a very dark site in an observatory with a good pier, a high end mount perfectly aligned and corrected, no wind, perfect seeing, etc etc your long sub ccd nirvana camera will out perform on SNR. However, many of us lack the financial ability ( through lack of funds or other priorities), to build the nirvana system. We have to deal with the short comings of our particular situations, and thus compromises are required in order to produce and enjoy imaging. What the new, low entry cost, very low noise, reasonable QE (say 60%) cmos cameras provide to us humble masses, is the ability to achieve more than we possibly could in the past with our obvious constraints.  |
Excellently put, Glen. My very first astrocamera was a Canon EOS 20Da, which had a cmos chip. I was a
very good boy for mentioning the caveats and all-else-being-equals.
Quote:
Originally Posted by Shiraz
I doubt that anyone has seriously proposed 36,000* 1 second subs. However, let's look at something far more practical to see what is going on.
With a camera having a RN of 10e and using 1 hour subs, the total RN over a 16 hour period will be SQRT(16)*10 = 40. If you have a camera with a RN of 2e and take 1 hour subs you will get a total RN of SQRT(16)*2 = 8. the total number of photons detected will be the same in each case as will the shot noise, so the low RN camera will provide higher SNR.
Now try shorter subs on the low read noise camera. If you take 96* 10 minute subs, the read noise is SQRT(96)*2 = 19.6. ie with subs of 10 minutes, the total read noise is still less than that half that of the camera with 10e RN and 1 hour subs. The camera is still exposed for 16 hours so the number of photons detected and the shot noise will both be the same as before. Even with 10 minute subs, the low RN camera will provide better SNR than the 10e camera with 1 hour subs.
Clearly there will be a crossover point where total RN of the 2 cameras is the same. That will be when you have 25x as many subs with the 2e RN camera as with the 10e RN camera. If you have the same total exposure time, you will still have the same signal and shot noise and the 2e RN camera will produce identical SNR to the 10e RN camera - but with subs that are 1/25 as long.
Low read noise does not allow ridiculous results, but it does allow very much shorter subs to be used than previously. The situation for sky limited performance is even more striking and very short subs can be used in some circumstances.
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Cheers, Ray. We continue to be in total agreement. You've done a real-world comparison between a real CMOS and a real CCD, and shown that you can do shorter subs with the CMOS and get a better result. True !!! But it is still the case that if you can go a bit longer, your result is even better again !
I'm currently looking around for a 16 megapixel CMOS chip with a good software development kit. Until I can find one that I can fully control myself using C sharp, I'm stuck with my existing Apogee 16803, which comes with a really good DLL interface for back-yard developers such as myself.
Best,
Mike