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Adaptive Optics With Mid-Sized Refractor

04 Nov, 2018 13:01
I was wondering if anyone has experience with adaptive optics on refractors (or anything in the 300-500mm range) and can say or presume how much of a difference it makes in image quality over regular guiding. I'm mostly a backyard imager so I do deal with mediocre seeing a fair bit.

I currently have an SBIG STT-8300 with the AO-8T unit and am looking at getting a midsized petzval like the Tak FSQ106EDX4 or the new Borg 107FL F3.9ED. The reason im asking is with the AO unit I have quite a lot of backfocus and to use any fast scopes/reducers i would likely need a new focuser and a couple custom adapters which obviously add a lot to the total price tag, all of which could be avoided if AO is not used.

Any input will be very helpful,
20 May, 2019 07:38
AO will not make up for poor seeing no matter which OTA type is used. It can be helpful and deliver better FWHM even on a premium mount…but, only in excellent seeing.  If you have a premium mount it may or may not make a large difference to you. If your mount is only average the AO should give you superior guiding.

20 May, 2019 09:45
It will not improve your image quality.  It's more of a quick autoguider.  It responds quickly via tip tilting a refractive element.  When it reaches a threshold, then it will bump the mount to move and the process starts over.  It's reactive or chasing the seeing.  No predictive stuff going on.  At the focal length specified, I don't think you would see much of a difference.   When imaging with focal lengths that are over a couple of meters, then it could be valuable as your payload has increased, and moving a small element is far easier than a 50+ kg payload.  It may also help counter winds as the scopes will have a greater surface area (sail).
20 May, 2019 10:30
I agree with Terry. At short FL I doubt that you would see any advantage. At long FL in good seeing AO results can be quite significantly better as it just results in a much lower rms guiding. Better guiding certainly can lower the FWHM one measures. But, it will always be seeing limited.
20 May, 2019 17:04
I have used my AO8 in my FSQ106 and it doesn’t really accomplish anything.  For the most part, using a short FL your image scale is going to be north of 1a-s and often above 2 a-s depending on the camera.  Getting your guide error from 0.4 to 0.25 a-s won’t make any difference with that large image scale.

If you are using an AVX though, or a similar low end mount, the AO unit can be a great way to guide and correct large errors quickly.   One rub with the SBIG AO is that you must use the SBIG STI or built in guide camera. That tiny little chip can make it hard to find a bright enough star for really fast guide exposures.
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