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They could have obtained the results shown if they used a very big scope and arranged the guide star and target to be close together - they don't say what they did, but it sure wasn't using an 8 inch f6 with an OAG. Furthermore, most of the SBIG graphical data seems to be the results of a simulation - it's not specified what was simulated, but it doesn't look like it included de-correlation of the guide and target errors. Undoubtedly AO can help a bit, but we are not going to get results like those shown by bolting one on to our existing systems - SBIG used a specialised system to get those results. EDIT: according to Stuart, they used a 20 inch f8.3 scope....no details on distance between guide and test stars.
Interestingly, SBIG have previously noted that tip/tilt AO has a limited field of view - to quote from the SBIG AO7 manual: The isoplanatic patch commonly referred to in the literature is the angular extent over which the
higher order aberrations are correlated, and is typically only a few arc seconds. The odds of
having a suitably bright star so close to an object of interest is small, which is what has
motivated the development of laser guide stars, which can be put where they are needed. The
tip-tilt component of the aberration is correlated over a larger extent, minutes of arc, improving
the odds of finding a suitable guide star.
As I understand it, the ONAG uses a dichroic mirror to allow near IR to pass to the guide camera - sensitivity depends on the star spectrum and the guide cam spectral response, but it could be more than 5%. I'm not saying that ONAG is necessarily an effective option, just that it is the only option for smaller scopes if you want to tackle seeing using an AO.
Last edited by Shiraz; 09-02-2014 at 10:03 AM.
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