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Originally Posted by codemonkey
Hmm... not sure why the prism would need to be any further away than usual? Either way, it's pie in the sky, I'm not going to make one so unless someone else does...
Interesting point about the guide cam. I don't think there's much out there that beats the QHY5L-IIM. Peak QE is about bang on compared to the Lodestar, the latter of which seems to get most of the benefit from having bigger pixels, but since you can bin the QHY, I'm not sure how much better it truly is. I did find a thread by the guy that makes Metaguide on Cloudy Nights where he normalised some values in comparing the two (image analysis, not just specs) and found the QHY to be, sensitivity-wise, better.
Having said that, I'm using a NIR filter at the moment and the Lodestar does have better sensitivity there. Either way I'm not sure if I'm going to get enough extra sensitivity to turn a complete lack of apparent stars into ones that I can guide on. I think I'd really just have to get a much bigger chip, which when combined with that kind of sensitivity... I'm probably looking at big bucks, and probably something more traditionally used for imaging than guiding.
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Hi Lee,
I was thinking that the prism needs to be far enough from optical axis so it does not block light falling on the CCD, in particular at 45degrees relative to the main CCD (cutting a corner); I feel that for larger CCDs that could be an issue. It is great that your OAG gives enough clearance at all angles - so obviously this can be overcome.
Perhaps it is true that QHY5L-IIM is more sensitive than Lodestar, but with 3.75micron pixels vs 8.2-8.4micorn pixels I honestly doubt it, in particular that to my knowledge binning 2x2 does not necessarily increase sensitivity by a factor of 4. Also, not sure whether 12bit with QHY would not induce some limitations as for guiding on brighter stars (saturated stars). I have been using the old version of Lodestar at 560mm fl (f5.6) and always have at least few good stars to choose from, and I image about 2km from Brisbane's CBD.
Another reason I can think of is that the size of a pick off prism can play its part too- a small prism would not fully illuminate the guide chip making some stars dimmer - as would placing the pick off prism far away from the optical axis - vignetting would limit the amount of light hitting the prism plus the stars could be spread over several pixels at larger distances from the optical axis.