Quote:
Originally Posted by Peter Ward
True, but I was trying to keep it simple. 
That said, you could make a further analogy but making our cup nice and clean while our bucket is a little grubby.... 
Larger pixel sensors, eg 14u-20u do however have massive dynamic ranges compared to 4um-5um offerings. ..90 db or more.
I think there there is a visible hallmark to tiny well sizes (small pixels): as many of the images taken by these devices show nearly all stars as saturated to white as it is tricky to keep those tiny wells from overflowing. |
Thanks Peter
Larger pixel sensors, eg 14u-20u do however have massive dynamic ranges compared to 4um-5um offerings. ..90 db or more.
yes, but we are not discussing that class of chip here - bit of a red herring.
I think there there is a visible hallmark to tiny well sizes (small pixels): as many of the images taken by these devices show nearly all stars as saturated to white as it is tricky to keep those tiny wells from overflowing.
Now I am confused. If you are saying that the icx694 will show nearly all saturated stars then you must also be implying that the ST8300 cameras that you sell* will show the same - after all, the KAF8300 CCDs (well depth 25,500) have only a little more well depth than the icx694 (20,000). There are enough fine images out there from 8300s to show that these CCDs do not necessarily have this problem, so there is no reason to expect that the icx694 will have it either.
There must be some other cameras out there that produce the saturated star images that you refer to? DSLRs, OSCs maybe?? I know the effect that you describe and suspect that image exposure strategies and processing techniques may play a significant role.
(*note - I have been told that you are an SBIG dealer, please correct if that is wrong)
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Quote:
Originally Posted by rally
My take is Well depth is still an important factor not to be ignored.
The choice of CCD for each person is very much dependent on so many things. : Budget, purpose (huge spectrum of requirements there - scientific vs artistic, planetary, Vs DSO, Vs photometric, blooming or antiblooming, spectral response and extended sensitivity at particular bands, binning capabilities, download speeds . . . ), the telescope being used and therefore image scale and all its considerations such as average local seeing conditions, degree of over sampling desired, well depth, dark noise, cooling, integrated guiding, filter wheels, adaptive optic capability, OAG options, light pollution etc etc etc
That is why there are books written on the subject and no one camera can ever suit all purposes or all people.
Thus its a personal thing because our needs are all a bit different.
Dynamic range is often a key factor in any decision (well depth e- / read noise e- or 20 * log(welldepth e- / readout noise e-) if you want it in db)
But Well Depth is one of the main things that allows us to image for a very long period of time to capture the very, very faint nebulosity without over saturating the rest of the image (ignoring stars).
Faint nebulosity has so few photons arriving that you either get a huge telescope or you expose for longer - since super high Qe is not generally affordable for mere mortals and true photon multiplying in silicon is likewise unavailable to us and really big telescopes are not quite so practical or affordable.
Exposing for longer is the cheapest, most affordable and available option !
So a long exposure with low well depth is harder to capture the full dynamic range of your target - (in one exposure) than a similar camera with greater well depth - despite having similar dynamic range.
After all, this is really the quest - to capture the enormous dynamic range of our chosen subject and compress it by so many orders of magnitude into an image that we look at (usually in an 8 bit format ! on screen or printed) and manipulate it to display the qualities we seek - dark dust clouds and shadows, enhanced colour, non linearly selectively stretched features etc to make it appealing and exaggerate these faint interesting and beautiful features
So to my mind there is some benefit to having a camera that has deeper wells than the Sony has (<>20,000e), irrespective of the chips dark noise or read noise and dynamic range calculation.
But they are making astro cameras more affordable and I think we are seeing the effects of this across the board.
Rally
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Hi Rally. thanks for the post.
I agree that well depth is very important - I just do not think it can be considered in isolation. To explain why, consider that you have two systems with the same aperture - one matched to an icx694 and the other to a Kodak 11002.
For discussion, lets say that the 11002 runs into star saturation (full wells of 60,000 electrons) at 20 minutes exposure. If you take four 5 minute images with the 694 system and add them together, you will run into star saturation at 20,000 electrons (full well) for each image or 80,000 electrons for the combined image. Thus you get more headroom than the 11002. The read noise adds in quadrature, so in the combined image it will be 2x single image read noise, or 10 electrons. This is less than the single image read noise from the 11002 of 13 electrons. The signal level will be the same from either 4x5 minutes or 20x1 minutes (assuming the QE is the same for simplicity). Thus, by stacking multiple frames, you get better headroom, lower noise and the same signal for the 694 when compared to the 11002. Note that this can only work if the read noise is low and that is a characteristic of the 694. I am not trying to say that the 11002 is no good, just that the 694 can be much better than it would seem to be from a consideration of well depth in isolation. It will be interesting to experiment with a variety of exposure strategies for the 694 - it really is a quite different beast.
I agree with your comments on the varied bases for people's decisions on cameras. I have been uneasy acting as a sort of defacto advocate for the Sony chip for this very reason. However, I considered that there had been some unjustifiably negative information posted about the icx694 chip and I thought that someone needed to point just how good its specs actually are - be a pity if anyone was dissuaded from buying one purely on the basis of dodgy technical comment - thereby missing out on a very capable device.
regards Ray