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Old 22-08-2014, 01:46 PM
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Shiraz (Ray)
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Location: ardrossan south australia
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Quote:
Originally Posted by bert View Post
Just for the record I have used an Sbig ao @32hz on a core 2 duo machine... Which is hardly a powerhouse.

Currently I'm using an aol with an onag xt at f11, guided only in ir. That seems to work pretty well. I have read anecdotal evidence that guiding in ir may have some benefits... But still tuning.
Thanks for the heads up Brett.

As I understand it, the old model 7 was pretty fast, but the newer stepper designs can be a lot slower and update rate can vary with how much correction is required. The AOL manual states that the max update for AOL with a 4 pixel offset is about 7 Hz (driven by software overhead, imaging time and time to move the corrector). Does that sound reasonable? The SX (also stepper) with 100ms frame exposures yielded around 3-4 Hz, depending on the required correction, but I have not compared the SX side by side with anything else, so please don't take these figures as comparing apples to apples..

IR ONAG seems to me to be a very good idea for AO, but I don't think it is practical at f4 - there just isn't room.

Quote:
Originally Posted by marc4darkskies View Post
I actually don't understand what you mean Ray. I'm correcting two dimensional guide star position error by tip/tilting a piece of glass in two dimensions - that's what it's doing. I'm not trying to be flippant here. It's simple feedback control, essentially, to cancel position noise. There is no point trying to understand anything else IMO and certainly no "need" to understand the physics of seeing - that's for the pros who are deforming huge mirrors to compensate for wavefront error. If you accept seeing can cause say low frequency (1Hz) to high frequency (>10Hz) guide star position errors then cancelling out some of that error with simple feedback control will be increasingly effective as the tip/tilt frequency increases.



Pros have large deformable segmented mirrors and while I think they do use tip/tilt on mirror segments (??) for coarse position error corrections, the really cool AO happens because they can analyse the wave front distortions (generally using laser generated reference stars) and correct for them with very clever computation and fast actuators. Pros may also be doing feed forward control.

Amateur AO is limited to correcting position errors only and then only at a relatively low frequency. The effectiveness is completely dependent on the frequency of correction (guide star brightness) and tuning. The main assumption is that the guide star is experiencing approximately the same positional abberations as the rest of the field - a pretty safe assumption given the apertures we use.

Cheers, Marcus
quite a few issues Marcus, so another long post - sorry.

I did a limited assessment of the performance of an SX AO on an f4 scope, set up as recommended in the SX manual. It was established that CA was not a major issue at f4, that I could fit in the AO + guider + filter wheel, that the AO worked well in wind and that it produced slightly worse results in very calm conditions with average seeing.

I was not surprised that the AO degraded the results in some conditions - at f4, the guide star is well over a degree away from the centre of the field of view, so it seemed quite reasonable that uncorrelated seeing noise in the guide channel would cause problems. At the time my thought was that the situation could possibly be improved with tuning, but the SX documentation provided no info on how/if to do so. So the fact remained that I had set up the AO pretty much as suggested by the maker and it resulted in worse performance in one particular test environment - I posted the result.

You and Peter indicated that you always get some benefit from AO. My understanding is that you tune the filtering of the guide data to strongly suppress the fast fluctuations and guide on the remaining low frequency stuff. You stated that you do not know what causes this low frequency "wander". Some of it could possibly be due to atmospheric turbulence, but probably not much (eg from the SBIG AOL manual "Based on our experience and knowledge, AO-L and AO-7 rates help only slightly with reducing ground layer atmospheric turbulence, but are quite effective in reducing guide errors and wind buffeting"). Apart from guide errors and wind, some wander may be due to dome seeing, minor mount/hardware vibrations and tube/mirror seeing. I am very interested to know which of the possibilities dominates and under what conditions, because some issues can be managed more effectively in other ways. For example, dome seeing can be cured by cooling the dome interior ( FWHM have been ~ halved in this way by some pro observatories). Mirror seeing may be fixed by fitting front fans to break up the boundary layer. I hope to be able to isolate and measure the effects that are influencing resolution and then find ways to deal with them up front. Then, if it still makes sense, I will fit an AO to reduce what is left - at least that's the plan. Should be fun

As an aside, I have no interest in professional systems per se, it's just that the pros take the time to test and document what they do - we amateurs tend to rely on impressions. The pro systems that seem to best relate to our systems may be "ground layer AO" - there seems to be a fair bit of activity in that area and they face the same issues we do. The big differences are that they use multiple guide stars, that their guide stars are much closer to the fofv (or in it) and the bandwidths tend to be higher (all of which which sounds like a blueprint for SBIG AO R&D). Many use deformable mirrors, but some are based on simple tip/tilt like we use and these are usually applied for correcting tracking errors (eg due to wind or mechanical components).

Regards Ray

Last edited by Shiraz; 22-08-2014 at 04:24 PM.
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