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  #21  
Old 22-01-2012, 01:11 PM
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Only picked up on this thread recently... hopefully this may give some more insight to the topic.

Amateur AO systems (and they are *adaptive* as they correct the overall wavefront tilt ) are not all equal. They vary in:

1) maximum tilt
2) slew rate
3) optical quality
4) positioning accuracy
5) tracking accuracy
6) jitter (internal noise)

I have used both mirror and refractive based systems, sadly due to calibration difficulties, mirror based systems were discontinued. More the pity as they were *fast* , as with a bright guide star they could easily guide at 30Hz.

The practical limit on AO systems nowdays seems to be about 15Hz

SBIG's implementation is quite strong on points 4 & 5 above as the system is looking at a patch of sky *very* close the what the imaging sensor sees.

Rick, your contradictory FWHM's may not be a problem. Use of AO will often give much taller stellar profiles, albeit with a slightly wider sigma spread at the base.

That said, when the seeing is poor, tip-tilt AO systems don't help much, as wavefront errors easily swamp any positional error. When the seeing is good however, AO can and does bring out very faint stars and structures
that will otherwise get lost in the noise.

Just my 2 cents worth.....
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  #22  
Old 23-01-2012, 12:59 AM
cfranks (Charles)
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Well, my AO-LF test wasn't too successful as I discovered one of the internal motors wasn't working. Terry from Starlight-Xpress has offered to send me a motor to see if that will fix it. Can't but praise their customer support!

Charles
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  #23  
Old 24-01-2012, 01:00 AM
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Thanks for the comments, Peter. I've been intending to do some more testing but there was a severe shortage of clear skies and now I'm away on holidays for a while (I'm sending this post from a warm hotel room in a very chilly Oslo!)

Sorry to hear of your problem, Charles. Hope it gets resolved quickly.

Cheers,
Rick.
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  #24  
Old 24-01-2012, 10:20 AM
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Originally Posted by RickS View Post
Thanks for the comments, Peter. I've been intending to do some more testing but there was a severe shortage of clear skies and now I'm away on holidays for a while (I'm sending this post from a warm hotel room in a very chilly Oslo!)

Sorry to hear of your problem, Charles. Hope it gets resolved quickly.

Cheers,
Rick.
Hope you see the Northern Lights - saw an article today saying "Biggest solar storm since 2005 pummels Earth"

DT
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  #25  
Old 24-01-2012, 04:14 PM
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Hope you see the Northern Lights - saw an article today saying "Biggest solar storm since 2005 pummels Earth"

DT
Me too, thanks David! We're heading up north to Alta tomorrow. Looks like we just missed a spectacular display, but with luck there will be more over the next couple of weeks while we're here.

Cheers,
Rick.
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  #26  
Old 29-03-2015, 06:54 PM
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Rick, sorry to dig this up from early 2012- but did you persist with the SX OAG? What were your conclusions? Cheers
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  #27  
Old 29-03-2015, 08:46 PM
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Rick, sorry to dig this up from early 2012- but did you persist with the SX OAG? What were your conclusions? Cheers
Hi Logan,

I moved onto big sensors and haven't used the AO with them because I was expecting vignetting and issues with flexure. I probably would have persisted if my mount was less capable. I think AO is of most benefit for improving the performance of a weaker mount. The AP900 might benefit a little but it's already doing a pretty good job...

Cheers,
Rick.
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  #28  
Old 29-03-2015, 09:29 PM
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Good news Logan. I have just gotten an SX AO unit and will be implementing into a Proline 16803 soon. I'll let you know how it goes.
It may be a week or two.

Greg.
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  #29  
Old 30-03-2015, 12:41 AM
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Hi Greg,

I'll be interested to see your findings. Do you have the large format or standard version?
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  #30  
Old 30-03-2015, 05:58 AM
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Simpler than Adaptive optics?


I have a good idea which is so simple yet I don't see anything available.

A shutter on the camera set up to close whenever the guide graph goes over a certain preset limit e.g.
more than half a pixel or division.
I wish PHD guiding would have such a feature to control a shutter.

Sometimes when I am guiding a gust of wind might hit my mount -
if I could close the shutter automatically I wouldn't have to
discard the sub frame - just open the shutter when the mount had stabilised.

Is there any system like that?

cheers
Allan
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  #31  
Old 30-03-2015, 09:12 AM
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Allan, that is certainly an interesting idea. I can think of 2 issues straight away.

1. When the shutter normally closes the image downloads. So, to use the camera shutter one would need to do a modification of the camera electronics. Probably the easiest way would be to have a separate shutter somewhere in the image train.

2. The total light time would vary, though perhaps only by a matter of seconds. I guess darks/flats would work OK, but the signal would vary...perhaps like thin clouds reduce values in some subs.

Peter
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  #32  
Old 30-03-2015, 07:52 PM
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Originally Posted by PRejto View Post
Allan, that is certainly an interesting idea. I can think of 2 issues straight away.

1. When the shutter normally closes the image downloads. So, to use the camera shutter one would need to do a modification of the camera electronics. Probably the easiest way would be to have a separate shutter somewhere in the image train.

2. The total light time would vary, though perhaps only by a matter of seconds. I guess darks/flats would work OK, but the signal would vary...perhaps like thin clouds reduce values in some subs.

Peter
Yes Peter,

1) Yes - the shutter in the camera could be modified - to have an external control or another shutter could be fitted.

2) If you wanted say a 300 second exposure & there was a bit of wind around you could set a 400 second exposure but download it when only 300 total seconds had expired.
That would give you up top to 100 seconds where the shutter could be closed.

It would be nice if it was all automatic - that's where the software writers like PHD could help.

Why isn't this done already?
I think it's such a good idea -
you could also have the shutter closed whenever the guide graph passed a pre-set limit.

cheers
Allan
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  #33  
Old 30-03-2015, 08:18 PM
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Quote:
Originally Posted by RickS View Post
... I think AO is of most benefit for improving the performance of a weaker mount. The AP900 might benefit a little but it's already doing a pretty good job...
Thanks Ric. Your initial write up was very good. If you revisit, please keep us posted.
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  #34  
Old 30-03-2015, 08:18 PM
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Quote:
Originally Posted by gregbradley View Post
Good news Logan. I have just gotten an SX AO unit and will be implementing into a Proline 16803 soon. I'll let you know how it goes.
It may be a week or two.

Greg.
Thanks Greg. Look forward to it.
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  #35  
Old 30-03-2015, 08:33 PM
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Originally Posted by alpal View Post
Yes Peter,

1) Yes - the shutter in the camera could be modified - to have an external control or another shutter could be fitted.

2) If you wanted say a 300 second exposure & there was a bit of wind around you could set a 400 second exposure but download it when only 300 total seconds had expired.
That would give you up top to 100 seconds where the shutter could be closed.

It would be nice if it was all automatic - that's where the software writers like PHD could help.

Why isn't this done already?
I think it's such a good idea -
you could also have the shutter closed whenever the guide graph passed a pre-set limit.

cheers
Allan
While a good idea in theory, its application would be limited I think. If you are imaging broadband data, say luminance, by the time your guider has picked up that the star has exceeded the tolerance the frame is already smeared. If you were aiming to get the sharpest data, the aim would be to not have the star go outside your tolerance in the first place.
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  #36  
Old 30-03-2015, 09:12 PM
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Quote:
Originally Posted by Peter.M View Post
While a good idea in theory, its application would be limited I think. If you are imaging broadband data, say luminance, by the time your guider has picked up that the star has exceeded the tolerance the frame is already smeared. If you were aiming to get the sharpest data, the aim would be to not have the star go outside your tolerance in the first place.
Of course that would be better.
It's a question involving timing - however -

if I look at a guide graph I can see a sharp excursion starting from the normal graph
& if I had a shutter button I could close the shutter & wait till
the excursion had reached it's peak & started heading towards zero before opening it again.
Surely the image would be better?

We're looking here for any improvement without spending megabucks.

To get good results in a closed loop system you need what's called a " nested loop. "
One loop could be at 1 second every second e.g the guide camera &
another much faster at 10 times a second such as adaptive optics.
Yes you'll get better results that way as you have a nested loop.
The loop at 1/10th of a second refines the first loop to increase the accuracy.

cheers
Allan
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  #37  
Old 01-04-2015, 07:16 PM
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Hi Allan,

I swear I had this conversation in an astro group a decade ago...I just can't find the email trail or yahoo group thread. As I recall the answer was it might help a bit with resolution, however the many artefacts it could be produce might nullify the gain.

A few obstacles I can think of off the top of my head:
-funny diffraction spikes caused by the shutter
-changes to the flat field i.e. star/bgd in middle gets more light than near the edges, unless using an sbig type sweeping shutter, which is btw usually slower than the iris types, which leads to the next point:
-in highly variable seeing your shutter open/close overhead time will seriously eat into your duty cycle.
-dark current accumulation (and the associated noise). If your effective duty cycle ends up being 50%, you dark current wrt the light signal just doubled.

Best,
EB
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  #38  
Old 01-04-2015, 08:18 PM
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Originally Posted by ericwbenson View Post
Hi Allan,

I swear I had this conversation in an astro group a decade ago...I just can't find the email trail or yahoo group thread. As I recall the answer was it might help a bit with resolution, however the many artefacts it could be produce might nullify the gain.

A few obstacles I can think of off the top of my head:
-funny diffraction spikes caused by the shutter
-changes to the flat field i.e. star/bgd in middle gets more light than near the edges, unless using an sbig type sweeping shutter, which is btw usually slower than the iris types, which leads to the next point:
-in highly variable seeing your shutter open/close overhead time will seriously eat into your duty cycle.
-dark current accumulation (and the associated noise). If your effective duty cycle ends up being 50%, you dark current wrt the light signal just doubled.

Best,
EB

Thanks EB,
There are bound to be a few problems but I think the overall result
would be an improvement.

If your total exposure time was identical the lights would be perfect.
However the dark frames would be not matching perfectly as they
would be a little longer than a frame where no shutter was used.
It would be better to have a proper Iris shutter than the sweeping type found in the QHY9m.

I wrote to Craig Stark & he suggests I write to this forum here:
http://openphdguiding.org/

He thinks that there is some merit to it & it's actually an old idea that was never implemented as most people would just get a better mount.

I have yet to write to that site but I will soon.

cheers
Allan.
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  #39  
Old 01-04-2015, 09:11 PM
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Originally Posted by alpal View Post
If your total exposure time was identical the lights would be perfect.
However the dark frames would be not matching perfectly as they
would be a little longer than a frame where no shutter was used.
Not quite, the lights and darks could still match in length (e.g. 10 min total exposure) i.e. the dark signal accumulation would be 10 min for the dark and 10min for the light frame (dark signal accumulates regardless of shutter state), but the object signal accumulation is only for 5 min (50% duty cycle on shutter let's say), so the relative size of the dark signal relative to the object signal is twice as big, that's all. This is no problem for bright objects, where you have lots of signal to throw away, but for faint small stuff, not so much.

BTW this is the same strategy as lucky imaging with zero read noise cameras... throw away all the bad frames and stack only the good ones, but if your duty cycle is low because your seeing criteria is tight then it takes too long to record the faint details.

EB
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  #40  
Old 01-04-2015, 09:27 PM
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Originally Posted by ericwbenson View Post
Not quite, the lights and darks could still match in length (e.g. 10 min total exposure) i.e. the dark signal accumulation would be 10 min for the dark and 10min for the light frame (dark signal accumulates regardless of shutter state), but the object signal accumulation is only for 5 min (50% duty cycle on shutter let's say), so the relative size of the dark signal relative to the object signal is twice as big, that's all. This is no problem for bright objects, where you have lots of signal to throw away, but for faint small stuff, not so much.

BTW this is the same strategy as lucky imaging with zero read noise cameras... throw away all the bad frames and stack only the good ones, but if your duty cycle is low because your seeing criteria is tight then it takes too long to record the faint details.

EB

No - I am thinking that the software would be smart enough that it
would give you say a 10 minute subframe even if the shutter was closed for say 3 minutes.

In that case the dark information would be 13 minutes long & the light subframe 10 minutes.
The light frames would therefore not match perfectly to the dark exposure
meaning that a 10 minute dark frame that you later took to cancel out the 13 minute frame would not match.

That's the problem.
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