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Old 20-08-2014, 06:31 PM
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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 gregbradley View Post
Now I am wondering even more if this is really the difference between a Starlight Express AO unit and an SBIG one. From what little I gather about the SX unit it seems 10hz corrections are unlikely although they do have the new Lodestar which is 2X more sensitive than the old one which was about the most sensitive guide camera around.

Greg.
Hi Greg.

I only did two tests of the SX AO, so I have no basis at all to compare it with an SBIG one. However, it worked well and behaved exactly as expected under two conditions: it made the stars slightly worse in short exposures, on a good mount in still conditions: it did a good job of cleaning up stars in bad wind. Others have found that AO is generally worth having in more usual conditions, particularly with lesser quality mounts, so I did not test under those conditions.

I used the SX with manufacturer-recommended settings (with minor tweaks). Marcus has pointed out that some software provides mechanisms for optimising AO performance to match the mount and conditions, but I did not have any such software. It may well be that the SX AO could have performed better with alternative software.

I ran the Lodestar at 0.1 sec exposure, but the actual update rate (of any AO) was less than 10Hz since it depends on a wide range of factors:
guide exposure +
USB download+
centroid and offset calculation+
serial comms to AO+
step AO to demanded position (variable depending on offset)+
serial handshake to PC+
begin next guide exposure

Plus there is unpredictable Win8 scheduling before it does anything new.



Quote:
Originally Posted by marc4darkskies View Post
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.
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. 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.

Quote:
Originally Posted by alistairsam View Post
I have the Lodestar X2 and its a lot more sensitive than the lodestar which I had previously.
with the X2 and my Orion AO, I get atleast 5 stars with good SNR almost anywhere in the sky with 100 to 250ms exposures. with bin 2x2, SNR improves. but its very clean and hassle free.

Here's a quick 3min test of my Orion AO, at 3Hz. Stars are elongated due to bad collimation.
https://www.dropbox.com/s/6pyiuhiq4u...-fnl1.png?dl=0
and this was without a single mount correction.

my OTA has a lot of flexure so the AO certainly helped reducing the size of the stars and dropped my fwhm, primary benefit that I can see are improvements of an average mount and bad OTA.
Hoping to do some more tests and exposures this week.

the Orion has a Maxim plugin so everything is done from within Maxim which helps a lot. Maxim appears to give you a fair bit of help in optimising the AO - what procedure do you use?

Cheers
Alistair
Hi Alistair. Good results and look forward to seeing more. Looks like AO is definitely worth doing with your system and conditions. Looks like Maxim helps with optimising the AO - what procedure do you use?

Regards Ray

Last edited by Shiraz; 20-08-2014 at 06:54 PM.
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