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Hi Alpal,
Of course there is a solution. If I had enough back focus I would be using AO, I don't have the back focus until they invent one of these things <6 mm thick, same for rotators. I don't really need it anyway, as at my imaging FL there would be little to gain. But, if you're imaging at long FL, AO is a must IMHO. If you're imaging at long FL you will probably have the necessary back focus for an AO unit.
So now you're convinced you need AO, what's the solution. You MUST be guiding in front of your filters if you want to use narrowband filters and AO, but if you're not concerned with this, guiding after the filters is OK, but the AO is superfluous for anything other than luminance. An OAG in front of the filters, preferably with a focal reducer in the OAG (though not necessary) will give you some bright stars on the guide chip, depending on the f-ratio of your scope, they may be bright enough to guide at 10 Hz, obviously depends on the guide star chosen. Remember that the example given on the Sbig website is optimised for advertising purposes, even 3 or 4 Hz can give a noticeable improvement though. You'll also like to have a rotator so you can pick your guide star from a wider field, or some of the OAGs have adjustments so you can move the pick off prism to shift the guider FOV.
Here's what you need;
1. Camera/guider system that sends AO commands to an AO unit (I think that Sbig, Orion and Starlight Express make the only AO units I know of), this usually consists of an OAG and AO unit combined, the Sbig system can accept guide commands from either and internal or external guide chip (or both, differential guiding, coming sometime in the future).
2. Enough back focus to put all this and possibly a rotator in as well, the best rotator is one where the guide chip can be rotated relative to the imaging chip, only possible with an external guide chip for Sbig, other manufacturers should be OK.
3. The need for AO
4. The cash to pay for all this
But do you need it?
If you can't get guide stars near you imaging target to guide faster that 0.5 - 1 Hz, normal guiding will work as well as AO as long as you have it sorted, in this case longer guiding exposures or a larger minimum guide movement will help to stop chasing the seeing. AO is not very useful for imaging at short FL, take my equipment for example, image scale is about 1"/pixel. The advantage of AO is to make fast, small adjustments, but if the adjustments are less than 0.5 -0.75 of a pixel, you just won't see it in the final image. Now look at the example on the Sbig website, a 20" F/8.3 RC using an STL11k, 0.46"/pixel. The FWHM comes in from 3.1 to 2.2", a gain of 0.9" or about 2 pixels. The seeing around here is rarely better than 2", so you'd think AO would be great, but, in practice, better guiding setup can give you most of the advantage.
Of course for colour subs (RGB or even SII and OIII to some extent), gain in resolution is a moot point as you won't be using it anyway. For some objects rich in OIII it may make a difference.
Wow, that's a long answer to a seemingly simple question, hope this discussion helps.
Cheers
Stuart
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