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Old 06-03-2013, 07:14 AM
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Shiraz (Ray)
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Location: ardrossan south australia
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Originally Posted by rally View Post
I wonder if we are missing the fact that the Dynamic Range of the chip whilst important its only just one parameter of the many needed.

I'll give an example
For the argument lets take a hypothetical (extreme) example of a Skipper Chip that is 'available' to amateurs to illustrate the issue.
This hypothetical chip has the advantage of extreme low read noise per the published Skipper CCD design potential of 0.2e-
These chips have actually been built !

Its Dynamic Range is therefore 88db or 25,000 using the standard calculation

If we compare that to a few cameras we know and use . . .
STL11000m - 73db or 4545
Sony ICX294 - 77db or 4000
DSIPro ICX254 - 67db or 2261
KAF8300 (std) - 70db or 3188
ST10 KAF3200 - 79db or 8750

Then on first inspection 88db looks extremely impressive doesnt it.
Well ahead of the pack

But if I then reveal that the well depth in this hypothetical example is only 5000e- what would we say ?

The reality is that it probably wouldn't be much good for our purposes
Yet 5000 is actually 12.3 bits, so in theory it ought to be quite useful.

When Sony release a 15mp, 9um chip then we'll have something !

Well depth by itself does not tell you very much. Anyone who gets a chip like the one you describe into our hobby market will make all our current CCDs and high performance tracking mounts pretty much obsolete. Being able to use hundreds of short exposures without the noise going up much would open up the possibility of "lucky imaging" for DSO. It would also mean that there would be no need for mounts that could track perfectly for long periods of time. Provided there are no export restrictions, you may be able to currently buy EMCCD cameras with with less than 1 electron read noise (eg Andor, E2V) - if you have very deep pockets.

Originally Posted by PRejto View Post
Thanks Ray, and, yes, I always value an opinion!

May I ask another? What about well depth vs light pollution? Is it true that light pollution will have a greater effect on a chip with a smaller well depth? For example if I compared the KAI4022 to the ICX694 the KAI chip well is 2x the well size ICX694. If LP in a given subframe took up say 5000 that would be 25% of the ICX694 but only 12.5% of the KAI. Would that then give a better dynamic range to the KAI chip? If so, imaging in a light polluted environment, would it be better to trade higher QE for greater dynamic range?

I ask because I struggle with background noise imaging as I do from Sydney, and I'm wondering if what I really need is greater dynamic range rather than another camera with a small well. Or, have I got this wrong and the greater SNR of the ICX694 would make a big difference?


The QE advantage of the 694 is offset by the larger number of photons intercepted by the bigger 8300 pixels - at the expense of reduced resolution though - both chips have about the same broadband sensitivity with your scope, just different resolution.

I don't think well depth makes any difference in heavy light pollution. The sky signal comes with a whole bunch of Poisson noise and if the sky is bright, system SNR is completely determined as the sqrt of the number of photons detected. Thus, you could get almost the exact same result (in both SNR and headroom) from any camera by choosing an appropriate number of exposures and exposure durations - eg exposing 8 frames with an 8300 or 10 shorter frames with a 694 would give ~the same result (same target signal, same SNR, same headroom). The low noise of the 694 will not help at all in this situation, since read and thermal noise will be swamped by sky noise in almost any camera - unless you do a huge number of short exposures. If you are having star saturation problems due to the sky offset, you might try more numerous short exposures to reduce both sky and star signals - you should be able to take a lot of them before you will start to see any read noise.

If you decide to do narrow band imaging, there will be very little residual sky noise and the low read noise of the 694 would give it a significant advantage, even if you trade some of it off to give more headroom.

I clearly need to expand my model to incorporate various exposure strategies in different sky conditions - thanks for the inspiration to do this.

regards Ray
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