#21  
Old 25-05-2015, 08:41 AM
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PRejto (Peter)
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I think it's possible to take advantage on near IR guiding without an ONAG if the proper IR filter is placed in front of an OAG camera. If I remember correctly Greg Bradley tried this and saw some improvement. I cannot find the thread but Gaston (inventor of ONAG) comments here:

http://www.iceinspace.com.au/forum/s...2&postcount=28

Peter

PS I have it on very good authority that a very well known company in astronomical gear will shortly release a kit of sorts to allow Sharplock type of focusing with OAG + IR Filter. Think of the possibility of never focusing for the night! Boggles the mind.
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  #22  
Old 25-05-2015, 11:27 AM
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I have used a 720nm IR filter I got off ebay. Also one I got from Edmunds Optics (better).

You need brightish guide stars as it does make them dimmer. I think I saw an improvement sometimes. Its hard to quantify but sometimes I don't think it did. It was subtle if there was an improvement.

Greg.
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  #23  
Old 25-05-2015, 01:02 PM
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Quote:
Originally Posted by ericwbenson View Post
Actually the original rule of thumb for the AO-7 was 10/10/10. 10th mag star on a 10 inch scope could correct at 10Hz. And remember this was in 1997-8 with the TC-211 guide chip, hardly a QE barn burner. What a giant leap when the TC-237 came along!

In my notes from 2007, my tests with a C11f/10 + ST8XME guide chip (the TC-237): exp time=0.1 sec, SNR=16 for 10.1 mag star using 3x3 binning and autodark.
So this is close to the 10/10/10 rule, although I would say the rule was a bit optimistic since you would likely be using a guide star with SNR~3. Below what I would consider a robust choice (I set ACP's minimum automatic guide star finder SNR to at least 7). But then again 10th mag guide stars are not falling on to your small guide chip all the time!

As mentioned by Eden and others previously, I think the actual elephant in the room is the isoplanatic path. While I believe it is larger than a few arcseconds (IIRC on the order of arcminutes depending on the type of deformation), even the most serious pro system today cannot overcome it, no matter how many kHz or higher orders are corrected. The input information, the guide star motion/deformation, is not correlated to the main image motion/deformation. The only way around this is an artificial guide star (e.g. a laser), or a bright star as the target.

One thing that always bothered me with the AO implementation: the bump algorithm. When the AO mirror/plate thingy gets past a predetermined point in its range of travel (say 50%), the software activates the normal guider relays to push the star back towards the middle of the guider chip, so that the AO deflection can be reduced below the threshold. Sounds ok at first but...the original SBIG implementation, and the MaxIm one I did, did not bump smartly.

The bump would activate the one relay (of N/S/E/W) that would push the star in the direction closest to back in the middle, and the relay activation time was a configurable, but constant number of seconds, 100 msec the default IIRC? With the AO7 running at high speed this actually worked ok since you would get multiple AO corrections while the guide star was slewing from the mount motors, hence the guide star sorta holding steady on the chip while the deflection percent was being reduced. So you wouldn't notice it too much.

With the somewhat slower AO8/AOL, or with fainter guider stars, where you are stuck at <2Hz update rate, the bump is noticeable. Plus it happened in the same direction for most of the exposure (DEC drift or RA drift is fairly constant for <1hr exposures). Additionally if the drift was at 45 deg from the RA or DEC axis, you would get two bumps in short order.

If you picked a larger bump time, you would get fewer bigger bumps. A small bump time meant lots of little ones as the deflection percentage straddled the threshold. What was better? I never really figured that one out...

So ideally the bump would be in the exact direction and the correct magnitude to get to deflection 0%, and slow enough for the AO to compensate for the motion, call it a drift correction slew. Or to be really smart the AO software would know the bump is happening and correct simultaneously without waiting for guide star exposures - this last one might be too smart and make things worse due to backlash etc but it looks good on paper

Best,
EB

Yes - that bump action is not good.

A nested loop as I explained earlier would need 2 guide cameras -
one on an ordinary OAG guiding the mount & the other
on a flip mirror system at 10 x the frequency.

I reckon that would work much better.

cheers
Allan
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  #24  
Old 25-05-2015, 05:48 PM
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Well it should be here in a couple more days.

It cleared customs in Melbourne this morning. So now the long wait for the Aussie distribution system to snail its way here. It might make it to Adelaide in three days.
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Old 26-05-2015, 08:05 PM
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It arrived this morning. Not what I expected for delivery time or for the product either. It is smaller than I expected but has a considerable amount of weight to it. Apparently 800 grams. I am assuming that weight includes the cable and adapter that is supplied. I think I will be moving the scope further forward in the saddle now.

Adapter has been ordered today too.
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  #26  
Old 26-05-2015, 08:14 PM
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I think it would be interesting to see some tests done with and without AO. I do realize it can be tedious to pull gear apart, more to the point, put it back together, especially when it was all shimmed and square before hand
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  #27  
Old 26-05-2015, 08:24 PM
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Originally Posted by Joshua Bunn View Post
I think it would be interesting to see some tests done with and without AO. I do realize it can be tedious to pull gear apart, more to the point, put it back together, especially when it was all shimmed and square before hand
More to the point the guessing game on back focus I have had to do with this scope and flattener. The scope specs on back focus is not great. It took some time to get data from GSO on distances etc and then it took sometime to narrow down the problems. Nor has the back focus on the flattener been good too, I am not convinced that the flattener actually works either. I have tried four distances and the latest one is about the closest to correction as possible with still some field curvature in the corners.

So what is another adapter (god damn so many of them that I think I know have more than 10 of them) and pull apart and put back together again. In terms of shims, my new camera only had one shim in one corner, so that has made things quite simple.
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  #28  
Old 26-05-2015, 08:27 PM
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Quote:
Originally Posted by Paul Haese View Post

So what is another adapter (god damn so many of them that I think I know have more than 10 of them) and pull apart and put back together again. In terms of shims, my new camera only had one shim in one corner, so that has made things quite simple.
I await the comparison then Paul
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  #29  
Old 26-05-2015, 09:54 PM
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More to the point the guessing game on back focus I have had to do with this scope and flattener.
Hi Paul,
FYI beware that the mechanical backfocus will not be the same as the optical backfocus due to the thick glass plate. I imagine it will be explained in the user manual.

For the curious, the exact difference actually depends slightly on the focal ratio and wavelength. Years ago there used to be a webpage that had a relatively clear explanation of the derivation but it disappeared so a while back I copied the gist of it, fixed it up, and posted it on my website.
http://www.faintgalaxy.com/focusshift.htm

Best,
EB
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  #30  
Old 26-05-2015, 10:22 PM
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Originally Posted by ericwbenson View Post
Hi Paul,
FYI beware that the mechanical backfocus will not be the same as the optical backfocus due to the thick glass plate. I imagine it will be explained in the user manual.

For the curious, the exact difference actually depends slightly on the focal ratio and wavelength. Years ago there used to be a webpage that had a relatively clear explanation of the derivation but it disappeared so a while back I copied the gist of it, fixed it up, and posted it on my website.
http://www.faintgalaxy.com/focusshift.htm

Best,
EB
Hi Eric, I am using the Optical backfocus of the unit. It is stated in the paperwork. It is actually a bit shorter than the physical backfocus, by about 5mm from what I can tell.

Thanks for the link. Interesting reading.
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  #31  
Old 28-05-2015, 09:22 AM
gaston (Gaston)
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AO units are fundamentally limited by the isoplanatic path, which is just few arc seconds across in the visible band (most of the time, seeing being a stochastic problem by nature).
Most professional AO systems work in IR bands where it is "easier" to deal with seeing and "larger" isoplanatic angles.


A key figure of merit is the ratio N=D/r0 between the scope aperture D and r0 known as the Fried’s parameter, or Fried’s coherence length, which is a diameter (not a radius). Typically r0 stretches from 5cm to 10cm, with r0=40cm or more few exceptional sites.

r0 = 7cm being an average value for most of us.
If N<=1 the scope is essential diffraction limited, for N>1 it becomes seeing limited. For N <3 roughly the seeing is mainly of tilt/tip in nature (the guide star wander for short exposures), a large N requires higher order wavefront correction (beside tilt/tip). In any case the isoplanatic angle is quite small, unless atmospheric turbulences are limited to the boundary layer (near ground). The AO corrections rate needs to be in the few 100 Hz to truly correct seeing inside the isoplanatic patch (again most of the time). Unless you try to resolve double stars few arc seconds apart, AO for amateur astronomers should be seen as an imager stabilizer device for mitigating mount and other left over system errors, correlated across the all scope FOV.
Here is a link with an interesting example related to the isoplanatic angle from the Palomar AO system (credit R. Dekany, Caltec):

http://www.innovationsforesight.com/...eing-tutorial/

The best way to limit seeing in auto-guiding is to use long exposures (>10 to 30 seconds). Mount tracking error, flexure and atmospheric refraction can be deal with PEC and with system modelization such as Tpoint and Protrack.
However nothing truly replaces a good mount, AO helps to mitigate short time, “high” frequency, mount mechanical errors (noise) not corrected by PEC. On the other hand AO may require using quite bright guide stars (for short exposure) and may lead to more risk of “chasing” the seeing outside the isoplanatic angle, which in turn may blur the image more than without any AO.


Bottom line the seeing will eventually limit the guiding quality.

However NIR offers a unique opportunity to improve the tracking accuracy.
Longer wavelengths are less sensitive to seeing and its effect scales as the 6/5th power of the wavelength.
Let’s use 550nm for the average visible wavelength (classical guiding) and 850nm for the NIR one (for instance ONAG guiding), this leads to an improvement of:


[ (850/550)(6/5) -1] x 100 = 69%


Which translates to smoother tracking.
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  #32  
Old 28-05-2015, 10:00 AM
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Tried some more multistar guiding last night - seemed to yield a nice flat guiding graph.

What I did notice is that it is far more immune to passing cloud - scattered cloud had been drifting through for 5-10minutes, causing definite dimming of the subframes, before the star faded alarm sounded.

DT
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  #33  
Old 28-05-2015, 12:10 PM
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Quote:
Originally Posted by alpal View Post
Hi Paul,
Let me chime in with what I know:

I would suggest you try it out if at all possible.
I have over 20 years experience with industrial closed loop control systems -
which is exactly what a telescope mount with an OAG is -
in fact - it's a dual axis closed loop control system RA & DEC.

I have found that all the best industrial systems use what is
known as a nested loop:
this means that there is one closed loop operating at one frequency -
let's say 1 Hz and another one nested inside it operating at say 10 Hz.
The ratio is normally 10 to 1.
This gives much greater accuracy than a single closed loop axis.

Notice that the normal ratio is 10 to 1.
That is important otherwise the 2 loops fight against each other & produce poor results.
Therefore - if your OAG system is operating at 1 Hz & your AO is at 2 Hz there would not be an advantage.
You would need a bright guide star to get 10 Hz for Adaptive Optics.

cheers
Allan
Interestingly this is exactly what I do. I have 2 scopes side by side on the same mount. One has a spectrograph on it and I will guide on the slit using a STi at 1 or 2 sec exposures. This is fine for the spectrograph. Sometimes I will simultaneously image through the second scope for photometry. If the target is dim requiring long exposures I will use the AO unit to guide the photometry image usually with 0.2 -0.5 sec exposures depending on the filter. There is a bit of differential flexure between the 2 scopes and the AO corrects this very well. It would never have enough flexure to exceed the range of the AO unit.
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Old 28-05-2015, 01:37 PM
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Great video on your site about the ONAG.

Quote:
Originally Posted by gaston View Post
Let’s use 550nm for the average visible wavelength (classical guiding) and 850nm for the NIR one (for instance ONAG guiding), this leads to an improvement of:


[ (850/550)(6/5) -1] x 100 = 69%
Is there any point guiding at 850nm vs. 750nm taking into account the sensitivity of the guider as well? Also would you rather use a cut off filter rather than a NB filter or it doesn't matter?
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Old 28-05-2015, 06:26 PM
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Originally Posted by Terry B View Post
Interestingly this is exactly what I do. I have 2 scopes side by side on the same mount. One has a spectrograph on it and I will guide on the slit using a STi at 1 or 2 sec exposures. This is fine for the spectrograph. Sometimes I will simultaneously image through the second scope for photometry. If the target is dim requiring long exposures I will use the AO unit to guide the photometry image usually with 0.2 -0.5 sec exposures depending on the filter. There is a bit of differential flexure between the 2 scopes and the AO corrects this very well. It would never have enough flexure to exceed the range of the AO unit.

That's great - that means you have a true nested loop.

cheers
Allan
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  #36  
Old 28-05-2015, 06:34 PM
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Is there any point guiding at 850nm vs. 750nm taking into account the sensitivity of the guider as well? Also would you rather use a cut off filter rather than a NB filter or it doesn't matter?
The energy output of an M-Class star peaks between 750nm and 850nm, but remains high right past the sensitivity of mainstream guiding cameras. The further into the near-infrared, the better the seeing conditions and this is why a cut-off filter is preferable over a narrow-band filter, so that you can take advantage of whatever near-infrared light is being emitted by the star and whatever sensitivity the camera has to offer at that end of the spectrum, however small.
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Old 28-05-2015, 08:22 PM
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Quote:
Originally Posted by multiweb View Post
Great video on your site about the ONAG.



Is there any point guiding at 850nm vs. 750nm taking into account the sensitivity of the guider as well? Also would you rather use a cut off filter rather than a NB filter or it doesn't matter?

I used 850nm has the average wavelength in NIR to make the calculation simple, assuming the filter is a high pass with a cut off wavelength at 750nm (ONAG) and the sensor (silicon based) is limited around 950nm. To get a more precise answer one should integrate the sensor spectral sensitivity (QE) and the star power spectrum too. But this is a good enough approximation.

Here is further information in NIR for guiding:

http://www.innovationsforesight.com/...-infrared-nir/

I would avoid a NIR NB filter since you will loose too much energy, you are better off with a high pass filter as described above.

P.S: As a simple rule of thumb, the reduction in back focus coming from a window inside the optical path(like the camera or an AO) is a third of the its thickness, roughly. This means a 3mm thick window would decrease the back focus (relative to the mechanical one) by about 1mm.
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Old 28-05-2015, 09:01 PM
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Quote:
Originally Posted by gaston View Post
I used 850nm has the average wavelength in NIR to make the calculation simple, assuming the filter is a high pass with a cut off wavelength at 750nm (ONAG) and the sensor (silicon based) is limited around 950nm. To get a more precise answer one should integrate the sensor spectral sensitivity (QE) and the star power spectrum too. But this is a good enough approximation.

Here is further information in NIR for guiding:

http://www.innovationsforesight.com/...-infrared-nir/

I would avoid a NIR NB filter since you will loose too much energy, you are better off with a high pass filter as described above.

P.S: As a simple rule of thumb, the reduction in back focus coming from a window inside the optical path(like the camera or an AO) is a third of the its thickness, roughly. This means a 3mm thick window would decrease the back focus (relative to the mechanical one) by about 1mm.
In my opinion, the image comparison on your site is misleading. While it shows a marked improvement, the first image is guided horribly, stars are not even round in the first place. Secondly, is the only difference between the two setups the NIR guiding? Who is to say, this is not an example of how well SBIGS AO works since that is mentioned in the text.
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Old 28-05-2015, 10:31 PM
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In my opinion, the image comparison on your site is misleading. While it shows a marked improvement, the first image is guided horribly, stars are not even round in the first place. Secondly, is the only difference between the two setups the NIR guiding? Who is to say, this is not an example of how well SBIGS AO works since that is mentioned in the text.
Fair comments Peter,

In both cases (with and without the ONAG) the AO-L was used, the difference is only visible versus NIR guidance.
I think the text could be clearer on this matter, thanks for the question I'll complete it eventually.

I agree that the first image shows some elongated stars too, guiding at 14 degrees over the horizon was quite challenging even with the AO-L enabled, this is a bit an extreme situation.
Obviously I would not recommend to do so, the target was chosen so low above the horizon to test the seeing effect on guiding with visible and NIR under bad conditions.
But even if you correct for the star elongations there is still a significant difference using visible or NIR for auto-guiding (same scope, AO, and target).
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  #40  
Old 01-06-2015, 04:25 PM
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Atmos (Colin)
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One day, if I ever have the money, I would actually like to purchase an AOX or something of that nature to do a quantitative analysis as to the affects of this kind of "adaptive optics" and scientific data acquisition.

To my knowledge, most smaller professional facilities do not use any kind of tilt/tip adaptive optics. I figure that if it was going to help their optical system they would be using something of that kind. Although I have never actually done any real digging into it, I've never heard of it being used, someone can correct me if I am wrong.

Something I would like to do in the future
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