#1  
Old 08-02-2014, 12:44 PM
alpal's Avatar
alpal
Registered User

alpal is offline
 
Join Date: Jan 2012
Location: Melbourne
Posts: 2,808
Adaptive optics.

I had a look at the latest SBIG Adaptive optics page here:

https://www.sbig.com/products/adaptive-optics/ao-x/

Just look at the results as a live version:


1 Hz tip/tilt............................... .. 10 Hz tip tilt

https://www.sbig.com/site/assets/fil...ao_mov_1hz.gif https://www.sbig.com/site/assets/fil...o_mov_10hz.gif

The results of GIF images show up in my preview but not when I post.


Those results make me green with envy -
it makes me realise that I've been guiding on moving rubbish.
I would buy an SBIG or some other AO system tomorrow but I don't see how
I could ever get it working at 10Hz.

I have a Lodestar guide camera with a TS9 OAG & although there
are sometimes more than 10 guide stars to choose from
there are sometimes only one or two & I need to use minimum 1 second capture to guide on them.
I often have to use 2 second captures as there are no suitable guide stars otherwise.
I could never get 10Hz working unless I was super lucky with a bright enough guide star.

Is there any solution to this problem?
Reply With Quote
  #2  
Old 08-02-2014, 02:21 PM
Peter.M's Avatar
Peter.M
Registered User

Peter.M is offline
 
Join Date: Sep 2011
Location: Adelaide
Posts: 914
Quote:
Originally Posted by alpal View Post
I would buy an SBIG or some other AO system tomorrow but I don't see how
I could ever get it working at 10Hz.

Is there any solution to this problem?
With the gear you list you would probably have to consider starting with a fresh camera (from SBIG) or using the SX AO unit as the SBIG one only works with Sbig cameras/guiding systems.

As for the issues with star selection, to get the most out of AO units from what I have heard you may have to frame your image where there are suitable guidestars and this may involve camera rotation which might be a pain. I believe Peter Ward dosent do too much star chasing for his AO, but he is also using a 12 inch f3.8 which will gather more starlight than your scope and f3.8 gives the guider a fairly wide field of view to sample.
Reply With Quote
  #3  
Old 08-02-2014, 02:24 PM
rat156's Avatar
rat156
Registered User

rat156 is offline
 
Join Date: Aug 2005
Location: Melbourne
Posts: 1,653
Hi Alpal,

Of course there is a solution. If I had enough back focus I would be using AO, I don't have the back focus until they invent one of these things <6 mm thick, same for rotators. I don't really need it anyway, as at my imaging FL there would be little to gain. But, if you're imaging at long FL, AO is a must IMHO. If you're imaging at long FL you will probably have the necessary back focus for an AO unit.

So now you're convinced you need AO, what's the solution. You MUST be guiding in front of your filters if you want to use narrowband filters and AO, but if you're not concerned with this, guiding after the filters is OK, but the AO is superfluous for anything other than luminance. An OAG in front of the filters, preferably with a focal reducer in the OAG (though not necessary) will give you some bright stars on the guide chip, depending on the f-ratio of your scope, they may be bright enough to guide at 10 Hz, obviously depends on the guide star chosen. Remember that the example given on the Sbig website is optimised for advertising purposes, even 3 or 4 Hz can give a noticeable improvement though. You'll also like to have a rotator so you can pick your guide star from a wider field, or some of the OAGs have adjustments so you can move the pick off prism to shift the guider FOV.

Here's what you need;
1. Camera/guider system that sends AO commands to an AO unit (I think that Sbig, Orion and Starlight Express make the only AO units I know of), this usually consists of an OAG and AO unit combined, the Sbig system can accept guide commands from either and internal or external guide chip (or both, differential guiding, coming sometime in the future).
2. Enough back focus to put all this and possibly a rotator in as well, the best rotator is one where the guide chip can be rotated relative to the imaging chip, only possible with an external guide chip for Sbig, other manufacturers should be OK.
3. The need for AO
4. The cash to pay for all this

But do you need it?

If you can't get guide stars near you imaging target to guide faster that 0.5 - 1 Hz, normal guiding will work as well as AO as long as you have it sorted, in this case longer guiding exposures or a larger minimum guide movement will help to stop chasing the seeing. AO is not very useful for imaging at short FL, take my equipment for example, image scale is about 1"/pixel. The advantage of AO is to make fast, small adjustments, but if the adjustments are less than 0.5 -0.75 of a pixel, you just won't see it in the final image. Now look at the example on the Sbig website, a 20" F/8.3 RC using an STL11k, 0.46"/pixel. The FWHM comes in from 3.1 to 2.2", a gain of 0.9" or about 2 pixels. The seeing around here is rarely better than 2", so you'd think AO would be great, but, in practice, better guiding setup can give you most of the advantage.

Of course for colour subs (RGB or even SII and OIII to some extent), gain in resolution is a moot point as you won't be using it anyway. For some objects rich in OIII it may make a difference.

Wow, that's a long answer to a seemingly simple question, hope this discussion helps.

Cheers
Stuart
Reply With Quote
  #4  
Old 08-02-2014, 05:21 PM
alpal's Avatar
alpal
Registered User

alpal is offline
 
Join Date: Jan 2012
Location: Melbourne
Posts: 2,808
Thanks Stuart & Peter for the advice.

I have an 8" f6 Newt - the focal length is 1220 mm = .91 arc seconds per pixel at bin 1x1 on my QHY9.
You haven't convinced me that I could find good guide stars.

I have the programs Stellarium & WorldWide telescope.
Is there any way I could put my frame size into those programs &
rotate it to see if there would be any suitable guide stars at the edge of the field of view?
Is there any other program that can do what I need?

That way I could pick a guide star before I start a night's viewing.
My TS9 OAG also has 3 different slots in which to put the guide camera.
That could be organised well before imaging -
in other words - planning the night properly.

cheers
Allan
Reply With Quote
  #5  
Old 08-02-2014, 05:41 PM
Shiraz's Avatar
Shiraz (Ray)
Registered User

Shiraz is offline
 
Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,787
hi Allan.

The big elephant in the room with AO is that it only works on seeing when you are within the "isokinetic patch" - this is the region around the guide star where the atmospheric tilt is consistent from one place to another. If you have the guide star too far from the object you are imaging (eg maybe further away than 1 arc minute for a smallish scope) then AO will not help and can actually make things worse, since it can be correcting one way while the target is being shifted in the other direction by a different bit of atmosphere. Note that 1 arc minute rules out anything but ONAG - you can't do it with an off axis tracking sensor unless you have a very small field of view.

This is not to say that AO is no good - it will do a great job of removing wind induced jitter and residual mount errors up to a few Hertz, but don't expect it to tidy up bad seeing in your system unless you have a very narrow field of view with a big enough aperture to have a reasonable kinetic patch and be able to follow dim guide stars at high frequencies - maybe start thinking 20+ inches.

regards Ray

Last edited by Shiraz; 08-02-2014 at 09:49 PM.
Reply With Quote
  #6  
Old 08-02-2014, 06:26 PM
gregbradley's Avatar
gregbradley
Registered User

gregbradley is offline
 
Join Date: Feb 2006
Location: Sydney
Posts: 15,441
Perhaps a reducer put in the path of the guide camera on an OAG may help with guide star selection.

Greg.
Reply With Quote
  #7  
Old 08-02-2014, 09:31 PM
allan gould's Avatar
allan gould
Registered User

allan gould is offline
 
Join Date: May 2005
Location: Brisbane
Posts: 4,481
Quote:
Originally Posted by alpal View Post
Thanks Stuart & Peter for the advice.

I have an 8" f6 Newt - the focal length is 1220 mm = .91 arc seconds per pixel at bin 1x1 on my QHY9.
You haven't convinced me that I could find good guide stars.

I have the programs Stellarium & WorldWide telescope.
Is there any way I could put my frame size into those programs &
rotate it to see if there would be any suitable guide stars at the edge of the field of view?
Is there any other program that can do what I need?

That way I could pick a guide star before I start a night's viewing.
My TS9 OAG also has 3 different slots in which to put the guide camera.
That could be organised well before imaging -
in other words - planning the night properly.

cheers
Allan
Allan
The planetarium program C2A will allow you to place your camera field of view as well as an OAG to pick guide stars for an image. I was amazed at how well this performed to give a good selection and it's free.
Allan
Reply With Quote
  #8  
Old 08-02-2014, 10:32 PM
alpal's Avatar
alpal
Registered User

alpal is offline
 
Join Date: Jan 2012
Location: Melbourne
Posts: 2,808
Quote:
Originally Posted by Shiraz View Post
hi Allan.

The big elephant in the room with AO is that it only works on seeing when you are within the "isokinetic patch" - this is the region around the guide star where the atmospheric tilt is consistent from one place to another. If you have the guide star too far from the object you are imaging (eg maybe further away than 1 arc minute for a smallish scope) then AO will not help and can actually make things worse, since it can be correcting one way while the target is being shifted in the other direction by a different bit of atmosphere. Note that 1 arc minute rules out anything but ONAG - you can't do it with an off axis tracking sensor unless you have a very small field of view.

This is not to say that AO is no good - it will do a great job of removing wind induced jitter and residual mount errors up to a few Hertz, but don't expect it to tidy up bad seeing in your system unless you have a very narrow field of view with a big enough aperture to have a reasonable kinetic patch and be able to follow dim guide stars at high frequencies - maybe start thinking 20+ inches.

regards Ray

Yes - an 8" f6 scope won't really have the light collecting power
unless there is a fluke guide star.
20" - now you're talking.

My FOV is about 0.5 degrees - same as the moon.
Reply With Quote
  #9  
Old 08-02-2014, 10:32 PM
alpal's Avatar
alpal
Registered User

alpal is offline
 
Join Date: Jan 2012
Location: Melbourne
Posts: 2,808
Quote:
Originally Posted by gregbradley View Post
Perhaps a reducer put in the path of the guide camera on an OAG may help with guide star selection.

Greg.
On a TS9 OAG - do they have reducers for them?
Reply With Quote
  #10  
Old 08-02-2014, 10:34 PM
alpal's Avatar
alpal
Registered User

alpal is offline
 
Join Date: Jan 2012
Location: Melbourne
Posts: 2,808
Quote:
Originally Posted by allan gould View Post
Allan
The planetarium program C2A will allow you to place your camera field of view as well as an OAG to pick guide stars for an image. I was amazed at how well this performed to give a good selection and it's free.
Allan
Thanks - I'll check that program out.
With a lot of preparation & searching -
suitable opportunities could be found even with a small aperture scope..
Reply With Quote
  #11  
Old 08-02-2014, 11:13 PM
Shiraz's Avatar
Shiraz (Ray)
Registered User

Shiraz is offline
 
Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,787
Quote:
Originally Posted by alpal View Post
Yes - an 8" f6 scope won't really have the light collecting power
unless there is a fluke guide star.
20" - now you're talking.

My FOV is about 0.5 degrees - same as the moon.
At 0.5 degrees fofv with an 8 inch aperture, AO will not help with seeing, even if you can find a really bright guide star.

The main lunar image in the link shows the problem http://www.footootjes.nl/Astrophotog...hy_Seeing.html - the turbulence induced displacements on the left hand side of the image are not in any way synchronised with those on the right hand side. Even within the main craters, the central peaks seem to be moving independently of the motion of the crater walls. If, instead of the moon, you had a guide star in the left hand part of the field and a target in the right hand bit, an AO tracking the guide star will regularly see it moving in a different direction to the motion of the target - an AO correction in the wrong direction will result in worse effective target motion than with no correction at all. The "isokinetic patch" for this set of turbulence (the region over which points are moving in synch), is smaller than the main lunar craters shown. With typical conditions it will only be one or two arc minutes with your scope and AO simply cannot help with seeing if the guide star and target are separated by much more than this - in fact AO is likely to increase the star sizes due to seeing - they will probably be round, but bigger.

AO at relatively low update rates should help tidy up residual mount errors and possibly even wind induced tracking jitter, so it is probably worth doing. However, do not expect performance gains like those shown in the SBIG videos.

Last edited by Shiraz; 09-02-2014 at 12:07 AM.
Reply With Quote
  #12  
Old 09-02-2014, 12:01 AM
alpal's Avatar
alpal
Registered User

alpal is offline
 
Join Date: Jan 2012
Location: Melbourne
Posts: 2,808
Quote:
Originally Posted by Shiraz View Post
At 0.5 degrees fofv with an 8 inch aperture, AO will not help with seeing, even if you can find a really bright guide star.

The main lunar image in the link shows the problem http://www.footootjes.nl/Astrophotog...hy_Seeing.html - the turbulence induced displacements on the left hand side of the image are not in any way synchronised with those on the right hand side. Even within the main craters, the central peaks seem to be moving independently of the motion of the crater walls. If, instead of the moon, you had a guide star in the left hand part of the field and a target in the right hand bit, an AO tracking the guide star will regularly see it moving in a different direction to the motion of the target - an AO correction in the wrong direction will result in worse effective target motion than with no correction at all. The "isokinetic patch" for this set of turbulence (the region over which points are moving in synch), is smaller than one of the lunar craters shown. With typical conditions it will only be one or two arc minutes with your scope and AO simply cannot help with seeing if the guide star and target are separated by much more than this - in fact AO is likely to increase the star sizes due to seeing - they will probably be round, but bigger.

Hi Ray,
I can understand your logic.
How does it explain the pictures on SBIG's page?
https://www.sbig.com/products/adaptive-optics/ao-x/

The star size is smaller.

Also an ONAG only passes 5% of the light to the guide camera.
Reply With Quote
  #13  
Old 09-02-2014, 12:24 AM
Shiraz's Avatar
Shiraz (Ray)
Registered User

Shiraz is offline
 
Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,787
They could have obtained the results shown if they used a very big scope and arranged the guide star and target to be close together - they don't say what they did, but it sure wasn't using an 8 inch f6 with an OAG. Furthermore, most of the SBIG graphical data seems to be the results of a simulation - it's not specified what was simulated, but it doesn't look like it included de-correlation of the guide and target errors. Undoubtedly AO can help a bit, but we are not going to get results like those shown by bolting one on to our existing systems - SBIG used a specialised system to get those results. EDIT: according to Stuart, they used a 20 inch f8.3 scope....no details on distance between guide and test stars.

Interestingly, SBIG have previously noted that tip/tilt AO has a limited field of view - to quote from the SBIG AO7 manual: The isoplanatic patch commonly referred to in the literature is the angular extent over which the
higher order aberrations are correlated, and is typically only a few arc seconds. The odds of
having a suitably bright star so close to an object of interest is small, which is what has
motivated the development of laser guide stars, which can be put where they are needed. The
tip-tilt component of the aberration is correlated over a larger extent, minutes of arc, improving
the odds of finding a suitable guide star.


As I understand it, the ONAG uses a dichroic mirror to allow near IR to pass to the guide camera - sensitivity depends on the star spectrum and the guide cam spectral response, but it could be more than 5%. I'm not saying that ONAG is necessarily an effective option, just that it is the only option for smaller scopes if you want to tackle seeing using an AO.

Last edited by Shiraz; 09-02-2014 at 09:03 AM.
Reply With Quote
  #14  
Old 09-02-2014, 08:05 AM
torsion (Bram)
Registered User

torsion is offline
 
Join Date: Nov 2012
Location: Canberra
Posts: 87
What about the remaining question about the correction frequency (up to 10 Hz), ignoring the location of the guide star wrt the object of interest.

If you have a 2 seconds capture time per frame, then your max correction frequency is 0.5 Hz (ignoring the nyquist freqs ect).

Would you really be able to go for 0.1 s per frame?

I am using Metaguide (and QHY5L-ii), with a capture time of 0.125 s/frame, but then I stack 5 or 7 frames to provide a single correction signal. I am able to find guide stars with my OAG but it is pushing the limits already. Not sure if I would be able to lowering the stack to 1 or 2 frames and still have a decent correction signal.

So how would you go about to increase this? Larger diameters, even more sensitive guide cameras (or even a 'proper' camera used for guiding)?

cheers,
Bram
Reply With Quote
  #15  
Old 09-02-2014, 09:29 AM
RickS's Avatar
RickS (Rick)
PI cult recruiter

RickS is offline
 
Join Date: Apr 2010
Location: Brisbane
Posts: 10,582
There is some interesting discussion with experimental results here: https://www.sbig.com/about-us/blog/differential/

I also did a quick comparison with and without the SX AO-L on a GSO RC10: http://www.iceinspace.com.au/forum/s...ad.php?t=85338

Cheers,
Rick.
Reply With Quote
  #16  
Old 09-02-2014, 09:46 AM
torsion (Bram)
Registered User

torsion is offline
 
Join Date: Nov 2012
Location: Canberra
Posts: 87
Thank you Rick, very interesting.

May have seen the thread, but forgotten it. Will have a reread, and may revive the thread

cheers,
Bram
Reply With Quote
  #17  
Old 09-02-2014, 10:26 AM
Peter Ward's Avatar
Peter Ward
Galaxy hitchhiking guide

Peter Ward is offline
 
Join Date: Dec 2007
Location: The Shire
Posts: 6,639
Quote:
Originally Posted by Shiraz View Post
- in fact AO is likely to increase the star sizes due to seeing - they will probably be round, but bigger.
I have both AO-X and A0-8T systems...the latter being more user friendly due the placement of the guide sensor.

But this has never been my experience when using an AO.

Stars always have better FWHM's and intensities when I can get a 5-10 Hz guide star, compared to guiding via the mount.

I suspect there is a statistical process happening, as with mount corrections, you are moving the system back to a datum up to several seconds after an unwanted displacement. With AO you can cut that back to a 1/10th of a second.

But being a tip-tilt system, AO doesn't help with, as I call it "fuzz ball" seeing. Deformable optics for wavefront correction would then be required.....but, if the last 20 years is anything to go by, never say never...
Reply With Quote
  #18  
Old 09-02-2014, 10:28 AM
alpal's Avatar
alpal
Registered User

alpal is offline
 
Join Date: Jan 2012
Location: Melbourne
Posts: 2,808
Quote:
Originally Posted by Shiraz View Post
They could have obtained the results shown if they used a very big scope and arranged the guide star and target to be close together - they don't say what they did, but it sure wasn't using an 8 inch f6 with an OAG. Furthermore, most of the SBIG graphical data seems to be the results of a simulation - it's not specified what was simulated, but it doesn't look like it included de-correlation of the guide and target errors. Undoubtedly AO can help a bit, but we are not going to get results like those shown by bolting one on to our existing systems - SBIG used a specialised system to get those results. EDIT: according to Stuart, they used a 20 inch f8.3 scope....no details on distance between guide and test stars.

Interestingly, SBIG have previously noted that tip/tilt AO has a limited field of view - to quote from the SBIG AO7 manual: The isoplanatic patch commonly referred to in the literature is the angular extent over which the
higher order aberrations are correlated, and is typically only a few arc seconds. The odds of
having a suitably bright star so close to an object of interest is small, which is what has
motivated the development of laser guide stars, which can be put where they are needed. The
tip-tilt component of the aberration is correlated over a larger extent, minutes of arc, improving
the odds of finding a suitable guide star.


As I understand it, the ONAG uses a dichroic mirror to allow near IR to pass to the guide camera - sensitivity depends on the star spectrum and the guide cam spectral response, but it could be more than 5%. I'm not saying that ONAG is necessarily an effective option, just that it is the only option for smaller scopes if you want to tackle seeing using an AO.

Yes Ray,
you're right -

" they don't say what they did.
Stuart said - they used a 20 inch f8.3 scope....no details on distance between guide and test stars."

The SBIG advertisement is therefore misleading people & has mislead me.
You'd expect them to say how far away the FOV in question was from the pick off mirror.
They don't say that either.

An ONAG might still work but obviously you'd need a large primary mirror
and a sensitive guide camera - at infra red frequencies -
maybe even cooled to give higher S/N.
Reply With Quote
  #19  
Old 09-02-2014, 10:36 AM
alpal's Avatar
alpal
Registered User

alpal is offline
 
Join Date: Jan 2012
Location: Melbourne
Posts: 2,808
Quote:
Originally Posted by RickS View Post
There is some interesting discussion with experimental results here: https://www.sbig.com/about-us/blog/differential/

I also did a quick comparison with and without the SX AO-L on a GSO RC10: http://www.iceinspace.com.au/forum/s...ad.php?t=85338

Cheers,
Rick.
Thanks for those inputs Rick,
I had read your thread before posting.
At least SBIG is still working on adaptive optics.
Reply With Quote
  #20  
Old 09-02-2014, 10:38 AM
alpal's Avatar
alpal
Registered User

alpal is offline
 
Join Date: Jan 2012
Location: Melbourne
Posts: 2,808
Quote:
Originally Posted by Peter Ward View Post
I have both AO-X and A0-8T systems...the latter being more user friendly due the placement of the guide sensor.

But this has never been my experience when using an AO.

Stars always have better FWHM's and intensities when I can get a 5-10 Hz guide star, compared to guiding via the mount.

I suspect there is a statistical process happening, as with mount corrections, you are moving the system back to a datum up to several seconds after an unwanted displacement. With AO you can cut that back to a 1/10th of a second.

But being a tip-tilt system, AO doesn't help with, as I call it "fuzz ball" seeing. Deformable optics for wavefront correction would then be required.....but, if the last 20 years is anything to go by, never say never...

Yes Peter -
I have followed your posts carefully.
You've certainly had good results with AO.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 09:42 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
NexDome Observatories
Advertisement
SkyWatcher Australia
Advertisement
Celestron Australia
Advertisement
Meade Australia
Advertisement
Bintel
Advertisement
Lunatico Astronomical
Advertisement
OzScopes Authorised Dealer
Advertisement
Astronomy and Electronics Centre
Advertisement