Quote:
Originally Posted by Shiraz
I am primarily interested in imaging galaxies and smaller objects at high resolution, so anything much over 0.5 degrees and about 6mpix would be a waste of time and money in my seeing. I don't want images of a galaxy surrounded by an acre of stars, but for someone who wants to image large nebulas, more etendue would be a good thing - but it isn't a goal for all astronomy.
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Hi Ray,
Well funny enough my main interest has also always been small faint galaxies and hence why I was content with an ST7 then an ST8 for a long time. However when it came time to upgrade for various reasons, such as better cooling that I needed in Australia vs Canada, remote operation hence sealed CCD chamber (no desiccant swaps), better image transfer system etc, I also started looking at various CCDs. The good old KAF3200ME had a lot going for it except for it's tendency to bloom easily (high QE, 6.8um pixels, no ABG = PITA for processing), and it was still fairly costly. KAF6303M looks good except again no ABG and lower QE (due to no microlenses) than my ST8XME. So the obvious choice was the KAF16803 which has all the good stuff and decent QE, plus allowed me to use my scope for more than just galaxies, I could actually image nebulas larger than the Horsehead! I don't think the latest Sony chips were available then, they would have been contenders except for the pixel size mismatch.
Quote:
Originally Posted by Shiraz
but you wouldn't put a 694 on a 2+m telescope - it would be a heavily oversampled system. If you put it on a 1m scope (ie at the same sampling, as was specified - thanks S), it will cover the same sky as an 11002 on a 2m scope using just 2 panels. eg an 11002 on a 10inch f8 will have the same resolution, sampling, etendue etc as a 694 on a 10 inch f4. However, the 10 inch f4 with a 694 will have twice the Ha sensitivity on targets within it's field of view, due to the higher QE.
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Changing the focal length to get the same sampling muddies the waters since for constant aperture (hence sky flux) the fratio must decrease for the smaller pixel CCD, and the problems associated with fast optics rear their head - no free lunch.
At constant aperture a bigger CCD is 'usually' easier to use than a small one. Yes it implies longer focal length, but that does not mean longer tube (enter catadioptrics), nor different requirements on tracking (where pixel scale is the only factor), it means slower optics which are generally more tolerant in the making and holding in place, plus longer focus zone.
Quote:
Originally Posted by Shiraz
Don't get me wrong, I am not advocating that anyone with a large scope change over to the Sony chips
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And I am not certainly advocating everybody go out and buy a KAF16803. I think we actually see things in much the same way, though with slightly different experiences colouring our perception. Frank discussion is always interesting.
Quote:
Originally Posted by Shiraz
and why Adaptive Optics cannot possibly give 15-20% more detected flux on extended targets.
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This is an aside that could be it's own thread
IMHO after using an AO8 for 5+ years, amateur AO doesn't provide much boost for big CCDs if the mount is good enough to start with (most PMEs that are not overloaded, almost all Astrophysics mounts if the legend serves to be correct, and any Renishaw encoder equipped mount qualifies). Maybe one day I will use AO again and change my mind, but for the cost+complexity to add to my system it will be awhile
However, that being said, from empirical observation the signal from extended sources such as faint jets or spiral arms can be greatly affected by the seeing conditions. Perhaps it is the faint structures that get washed out diminishing the contrast, or simply the feature being smeared out against the background, not sure, but a single low FWHM subs is always worth many crappy high FWHM subs in a stack.
Quote:
Originally Posted by Shiraz
why can't ONsemi achieve similar QE/noise specs on their big pixels?
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Perhaps the answer is simple, they don't have the tech, can't afford to develop the tech, and Sony just doesn't give a hoot about the large CCD market segment. Amateur, and to some extent pro astronomy, have always been at the mercy of other commercial endeavours (e.g. medical imaging, surveillance) for CCD development, I can't see how this will change in the forseable future.
Best regards,
EB