Quote:
Originally Posted by marc4darkskies
I think your short on Lum then - only 3 x 10 min subs. Even with a 17" I'd guess you're sailing close to the wind for S/N in the fainter extremities.
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Yes a bit skinny in luminance although pretty bright.
With the CDK17 it is a compromise on a few things like any scope.
Central Obstruction size, baffling size, F ratio and vignetting. Vignetting is a little high at 20% with a 16803 chip but CO size is smallish at around 40%
so it gives good contrast and detail and good for visual and not just an astrograph.
But the vignetting means you have to have pretty good conditions or you'll have trouble with the processing. Of course flat fielding does most of the work but not all. So if there is any moon, light pollution, thin cloud the subs tend to be wasted as they will not calibrate easily and you'll have vignetting to deal with which can be difficult at times.
Ha images with the moon out are no good on this scope using a 16803 chip. During no moon they are fine. They may be fine with a smaller chip like an 8300 or 6303.
I did about 5 hours of Ha on the Helix and they showed virtually no contrast and you could hardly see the Helix and was a complete waste. Yet a 10 minute luminance during no moon was quite bright and detailed.
RCOS scopes are around 4% vignetting but they are F9 often and also ideally need a corrector. They are also twice the price, don't have a commercial reducer available except the AP 2.7 inch one which limits you to APS sized chips or less. Offaxis focus of stars is a bit better with the CDK design as well compared to most RCs (not all). RCOS RCs have a large central obstruction which means they lose a bit of contrast and are not good for visual or planetary. The CDK can do both. So its a design compromise toss up of pros and cons. No scope is perfect. These are the limitations of the CDK design.
If you were planning to get a CDK and use it for narrowband during the moon weeks and a widefield chip like 11002 or 16803 it may not be good.
If you were planning to use a smaller chip then it might be fine. I am not sure on that last point but will know in a month or so.
So once you know your scope's strengths and weaknesses you can play to its strengths and avoid its weaknesses. Overall this is a great scope, has high quality, easy to adjust (hasn't needed any collimation yet but it is easy to do as the primary is fixed), is very sharp and easy to focus and use.
It should also be good as a visual scope and planetary imager. Reflections from nearby bright stars (may be able to be improved with some careful flocking) and vignetting on large chips are its weaknesses. But at less than half the price of its RC competitors' 16 inch plus an extra inch aperture - its got a lot going for it. It also has a reducer available that makes it F4.5 which is tempting at some point. It does not have a rotator yet (in development) but you can swivel the camera but it involves loosening 3 bolts on the focuser so you'd really have to need to do that!
I know you were considering a 12.5 hence the writeup for your benefit.
Greg.