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Old 16-08-2014, 11:10 AM
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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 marc4darkskies View Post
Interesting thread - thanks Ray. I won't dispute your findings because I've never done any scientific-like testing!! However, I have used an AO-L quite a bit (only at F11.7 on my Tak) and never experienced a degradation unless the AO guide parameters weren't correctly tuned (aggressiveness and, to a lesser extent, slew rate).

I set aggressiveness depending on guide star brightness (guide frequency) and seeing. The higher the guide frequency, the lower the aggressiveness and I moderate this based on seeing. The point being I never try to correct any one guide error with a single correction because that likely means you're chasing seeing (regardless of how good that might be). The objective is to minimize "wander". Wander is a measurement that indicates the amount of position error that has not been corrected (and it can never consistently be zero). Using TheSkyX and an AO-L it's possible to monitor Wander and adjust the AO settings to ensure wander is minimised and also diminished from the values observed without the AO switched on. The intent is to ensure there is at least some benefit to switching the AO on.

Slew rate just limits the maximum correction that the AO is allowed to apply (arcsecs/sec). The maximum correction is the slew rate divided by the guide frequency (Hz). Ie a setting of 10 at 10Hz limits any correction to less than 1 arcsec. This will help avoid over correction when the seeing is bad.

In your case, I'd be interested in seeing your guide error graphs with and without the AO.

Cheers, Marcus
Thanks Marcus - great post and a breath of fresh air.

I had been concerned that AO is being sold as the panacea that can "fix the seeing" - something it certainly does on pro systems - but I could not see how it could do that in the way we use it.

Amateur AO seems to be a bit like putting a generic shock absorber on your car - it may help a bit but it can also be worse than nothing. My test in calm seeing was with default settings for the AO and was simply to show that AO certainly can make things worse in some circumstances by introducing uncorrelated seeing noise into the tracking loop. It was never intended as a blanket bucketing of AO, although it seems to have been interpreted that way.

You provide a sound strategy for optimising AO parameters to get a balance between fixing the slow (mount?) errors without introducing excessive seeing noise. My attempts to optimise the AO were not as systematic, being limited to trying different update rates and setting the minimum error at which correction was attempted. I did not keep the error logs - did not realise that they would interest anyone.

Your strategy seems to suggest that you view seeing as the enemy and you attempt to keep it from doing too much damage while still getting some guiding gain. That raises the question of what the AO may actually be correcting for, if it isn't the seeing. Do you have any thoughts on that? Could it just be mount wobbles or could it be dome seeing or tube current effects?

regards ray

Last edited by Shiraz; 16-08-2014 at 01:28 PM.
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