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Old 20-08-2014, 09:59 PM
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marc4darkskies (Marcus)
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marc4darkskies is offline
Join Date: Mar 2007
Location: Quialigo, NSW
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Originally Posted by Shiraz View Post
Thanks Marcus. Your empirical approach of "use it because it works" is eminently sensible. However, I have a different approach - I want to know what it is actually doing, not for academic interest, but because that may give some insight into how to improve the system in other ways, as well as (or even in place of) AO.
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.

Originally Posted by Shiraz View Post
... Amateurs use AO in ways that appear to be in conflict with the way that the pro's use them - I want to know why they work as well as they seem to and where the limits are.
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
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