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Old 07-08-2014, 08:57 PM
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Shiraz (Ray)
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Join Date: Apr 2010
Location: ardrossan south australia
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Quote:
Originally Posted by clive milne View Post
Ray there may be some benefit orienting the off axis guider such that it is pointing to the sky in the upstream direction (of the air flow) relative to the imaging chip. (typically west)


I could elaborate, but I am sure you can understand why this would be preferable.
Hi Clive. Now that is lateral thinking - will try it if I have time.

Quote:
Originally Posted by gregbradley View Post
I am surprised by these results. I thought the whole point of an AO unit was to help get tight stars in weak seeing. If it doesn't do that and adds some CA then that is pretty pathetic.

Martin Pugh is getting amazing results from an SBIG AOX unit at Sierra Remote. Peter Ward's recent Eagle shows AO benefits even at moderate focal length.

So is this more the SX unit is not very good?

Greg.
Hi Greg.

all single element refractive AO devices add CA. However, it appears that it is not a major problem even at f4 and with the relatively thick plate of the SX, so it is not an issue for your system.

As far as I can tell, the main benefit of an AO is to give even better results in good seeing by tidying up residual mount errors - surely that is not too "pathetic". Rick hit the nail on the head - AO helps most when you are pushing against the mount limits (eg the seeing is <1.5arcsec and you are imaging at much less than an arcsec/pixel).

The penalty of sampling the guide signal from a star that is a long way from the image means that the total seeing error increases. All amateur AO systems do this - even the SBIG paper referenced earlier shows that to be the case. So current AO does not really fix poor seeing - because the geometry is wrong and the update rates are too slow. "Fixes the seeing" still seems to be used for some marketing purposes though. At least SX only claim "Overcomes rapid gear errors to stabilise even difficult mounts".

AO can tidy up mount errors due to lower build quality or wind induced motion, even in poor seeing - lots of evidence that it can provide a nett gain, even with the inherent reduction in resolution.

I would certainly not say that the SX AO is "not very good" - it works as expected. I have no idea how it compares to the SBIG alternative.

As an aside, the errors introduced by using a single off axis guide star are apparently well understood in SBIG. According to the paper referenced by Rick, they are working on a way to use multiple guide stars and an on-axis guide scope, which will get around the problem of decreased resolution - but I don't think they are there yet. If they can pull it off, they probably won't be able to improve resolution by much in poor seeing, but at least they will not make it worse.

Last edited by Shiraz; 07-08-2014 at 09:55 PM.
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