Quote:
Originally Posted by Shiraz
...
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
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Well, I'm an empiricist
. I know (because I've
watched it) that seeing contains both high (>10Hz) and low frequency (around 1Hz and longer?) components.
If I have a bright guide star that allows 10Hz corrections then a properly tuned & calibrated AO unit will reduce the amplitude of seeing induced guide errors of
less than 10Hz. I know it won't
remove the errors (especially the near 10Hz components) because the guiding is reactive, not predictive, but it
has to attenuate them. You can monitor this to some extent by watching wander and guide error graphs. The limitation with this monitoring is the sampling frequency of these graphs.
If I can only get 1Hz corrections (a fainter guide star) then at least I know that the low frequency seeing components will be attenuated and I don't have to rely on mount corrections for any lower frequency wander. There's no way an ME can correct faster than about 0.5Hz anyway and even that is pushing it with a heavy scope and probably meaningless from an imaging perspective since most seeing is much faster than this.
The only way to verify an AO's performance (which I don't feel compelled to do because I've seen it working
) is to compare residual error graphs for multiple runs with and without 10hz guiding. If a guide error graph has increased amplitude after turning on an AO then it's likely the AO was not properly tuned.
Cheers, Marcus
PS: Naturally, seeing is not just about guide star displacement (ie residual error or wander). Bad seeing is usually accompanied by "boiling" in which case, I always switch off, pack up & go to bed!
AO will
only reduce wander
.
PPS: I make no claim about the
source of the wander I'm trying to minimise - there's no way for me to know that - although at frequencies greater than 1Hz it's unlikely to be from the PME.