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Charge capacity (QE)
Electronics (noise)
Cooling (noise) - accurate temperature control.
Simplicity (ease of use)
Easy to adapt to camera lenses
Software compatibility
Customer service
Price
Bonus items are;
In-built guiding chip, preferably using the main optics with no requirement for a separate guide scope.
Hi Rowland,
You've got a few concepts not right there. QE stands for Quantum Efficiency which means how sensitive the camera is. In other words what % of photons hitting the chip actually get converted to an electron by a pixel that can be measured by the electronics. QE of 60% is good. One shot colour are usually around 23 to 40% depending on chip and what colour (different sensitivities for different colours, green is usually more sensitive). Most mono CCD cameras are 50-60% QE. Anything over 60% is exceptional. 60% is plenty though. 50% is fine as well. 23% is limiting you to brighter objects with faster scopes with larger aperture.
Charge capacity is called well depth. It is not really a measure of sensitivity. It is how many electrons can a pixel hold. It is measured in numbers of electrons. 20,000 electron well capacity is about as low as it goes, 110,000 is about as high as usual chips go. It is not 4022 is medium/low but workable. Small well depth means you may have to shoot shorter subexposures on faster telescopes than long well depth. It means bright stars will end up overexposed and white - no colour- fairly easily and is something you have to keep an eye on.
40,000 well depth or more is what you want. 100,000 is great. You'll get colourful bright stars even with 15-20 minute exposures.
Cooling (noise) is not about how accurate you can regulate it (I think almost all regulated cooled cameras now are accurately regulated) its about how powerful the cooling is. Highly regulated weak cooling still leaves lots of noise and artifacts in the chip. Most CCD chips have noise reduced by half with every extra 6 degrees C cooling. So running a typical chip at -20C versus -35C you are talking about 1/5th less noise in the -35C no matter how well regulated the temp is. Regulated temperature is important and all the major brands do that as far as I am aware. I think Starlight Express did not always do it but Apogee FLI and as far as I know QSI and others do this quite well. It is true accurate controlled temperature is valuable so your darks match your light exposures and do a clean dark subtract.
Weak cooling means you now HAVE to have super well done darks. I often use only 3-6 darks for a master with powerful cooling. Weak cooling would take 16-32. Again not that big of a deal perhap but almost all defects in a chip are reduced by powerful cooling. Some vertical lines in chips do not dark subtract out easily no matter how accurate your darks are. That then gives you a problem processing the images with these little background vertical lines showing up.
Also with better electronics there is FAR less noise in the first place to subract making them more reliable images.
My 8300 chip is as clean as anything at -35C yet I noticed when it was warm it actually has a slight vertical line in it. I don't know what temp it is when that disappears but I think its past -20C. I usually run it at -35 or -40C. So if your camera maxed at -20C that line would still be visible and you would be relying on your darks to remove it. That may or may not work.
Adapters to lenses are available for any camera. Precise Parts will make an adapter for anything so that is not really a difference between brands. I have had lens adapters for my SBIG, Apogee and FLI cameras.
Compatibility with software. As far as I know they are all compatible with the major software. CCDsoft does need a camera plug in to work with non-SBIG cameras. But Maxim DL I don't think does.
I thought I would clarify those points.
SBIG self guiding is handy for sure but it is useless for Ha or Narrowband so you end up needing an off axis guider or guide scope anyway. Self guiding is a brilliant idea but its weakness is the guide camera looks through the filters. Guide exposures are usually a few seconds or less. But to get a guide star through a Ha filter may require 30seconds to a minute to see one. I don't know of a mount that will handle 30 to 60 secod guide exposures and get round stars at long focal length. Not even a Paramount ME would do that. If you only do colour and no Ha (and there goes all your nebula shots) then its not a problem.
So you see there is no simple work around these laws of Physics we are up against when imaging.
Greg.
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