Quote:
Originally Posted by clive milne
Ray,
As you shorten the focal ratio, AO actually becomes more doable.
It becomes less necessary as you shorten the focal length.
~c
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although a short scope makes it easier to move the correcting element, there were two implementation problems at f4.
First off, at f4 you need a coma corrector - the Paracorrr and MPCC only provide about 55 mm of back focus, so the only coma corrector that could be used with the AO I looked at is the RCC1. As an example, the Orion AO requires about 98mm, which just fits in the 100mm or so back focus provided by the RCC1 - provided there are no adapters etc needed. Then when you look at where the light goes in, the f4 light bundle 98 mm out from the focal plane is large enough to be severely vignetted by the input aperture and the pickoff prism. Opto-mechanically it does not work well at f4, but would be OK at about f8 or more.
the other issue is that, at short fl, the pickoff prism is a fair angle away from the centre of the image - the typical isoplanatic region is not large enough to allow fast corrections, since some of the guide solution will be derived in turbulence that is not correlated with that of the image - the AO adds in new errors and even though it may still produce round stars they can be enlarged. Even at slow update rate, the AO can potentially put in slow uncorrelated turbulence corrections that add to the tracking error. This problem is also pretty well fixed if the scope is slow and the pixels big.
After thinking about it, I decided that it all seemed far too hard at f4, at least with the gear I looked at.
As an aside, the issue of deriving guidance signals from outside of the isoplanatic patch will apply to an OAG as well - not sure yet what the best guiding solution will be and may need to try both OAG and a separate guidescope aligned with the image to see which does best.
regards ray