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Questions SBIG AO-7 and Other Adaptive Optics - Long
Hello everybody,
I'm contemplating purchasing a SBIG AO-7 as being the only so called adaptive optics device currently on the market . I'd like to know : 1 - are there any other alternatives ? I found some references to a discontinued Stellar Products device and a higher order correction device , but their website doesn't show if this is a current product or just a future project . I'd appreciate any advice, personal knowledge or experience with these . 2 - the SBIG AO-7 is marketed by SBIG as a tip/tilt adaptive optics device. Allegedly it corrects for mount fast tracking errors plus atmospheric turbulence induced tip/tilt. However, it works only with the SBIG dual head cameras . It uses the guiding head to generate the tip/tilt error signal, and hereby is the crux of the problem. Given the considerable off axis position of the second CCD (the guiding TC-237 CCD) , the separation between the (larger) imaging CCD center and the guiding CCD is too large and the guiding CCD is so to speak out of the isoplanatic angle . In other words, the guiding CCD is not seeing the same turbulence as the imaging CCD , but a different tip/tilt motion due to its large off axis distance. This is true for any reasonable image sampling ,unless using an extremely (unusually) long focal length instrument, in which case the image sampling is much too fine , exceeding by far the scope resolution, and narrowing the FOV to almost nothing , reducing image birghtness and increasing exposure time tremendously . 3 - based on well known very reputable references related to atmospheric turbulence (and on my own measurements with a DIMM setup ) , the incoming wavefront tip/tilt needs a sampling frequency in the hundreds of Hertz. I can post links to these papers stemming from research and researchers at the world's largest telescopes . The AO-7 is much slower, although how slow exactly is debatable due to the imprecise and fuzzy specs on the SBIG website . Here's exactly what they say "Utilizing the second guiding CCD as an imaging sensor the guide star's position is read out at rates up to 40 times a second, and the tip-tilt mirror adjusted to hold the star on the designated pixel for the length of the imaging CCD's exposure. The tip-tilt mirror moves and settles to within 20% of the commanded position in a mere 10 milliseconds". The above quote is not easy to understand , as it seems to be either the product of marketing/management (as opposed to engineering or scientific truth) , or the product of marrying 2 devices that were shoehorned into a product without actually being suitable for it . Let me explain. If the guide star position is read out 40 times a second, it means the guiding camera has a frame rate of 40Hz. The frame duration is 25 ms (milliseconds) .. If the guide camera is read by the Windows PC software, this operating system introduces non trivial delays as well, at least adding a few milliseconds , let's say 5ms to pick a number (which is in the usual range , depending largely on the PC speed, type, operating system , installed software, shoe size etc) . The overall delay from the moment a frame started being exposed to the moment the PC calculated the tip/tilt correction will be 25ms+5ms=30ms . To satisfy Nyquist, the signal bandwidth must be less than half, meaning a signal period of 60ms and correction rate of 15Hz . Then SBIG adds the puzzling part, claiming the tip/tilt mirror settles to within 20% in a mere 10 milliseconds. They must be claiming here strictly the mirror +actuators settling time and NOT the overall system settling time after a wavefront tip/tilt is introduced . How much faster the mirror moves is irrelevant, since the correction will take place always at the 15Hz rate , not at the implied higher frequency due to the 10ms settling time . Where does that leave the system? Lower on the webpage, there's a claim that "This remarkable system promises to have a profound effect on CCD imaging by reducing the atmospheric turbulence, wind induced vibrations, and eliminating the remaining periodic errors in most telescope drives." It is true that most scopes will have or can be reasonably forced into having wind induced vibrations in the under 15Hz range, so the AO-7 corrects no doubt for wind gust induced vibrations. It is also true that most mounts will exhibit slow changing PE , and a correction bandwidth of 15Hz will take care of that. Atmospheric turbulence on the other hand, at less than 15Hz correction, no way . That is the pure marketing hype, and that's what gives the device the name of AO . To recap, AO-7 is NOT AO because: -the guiding ccd is too far off axis and images a different atmospheric turbulence pattern than the main imaging ccd . As a matter of fact, it probably corrects for a totally different atmospheric turbulence than the main CCD is seeing . -the sample rate is too low by an order of magnitude to correct for atmospheric turbulence . I'm sure the AO-7 is a great device, I'm sure it helps with mount vibrations , wind induced vibrations etc, improves images etc, but is NOT an AO device. Please correct me if I'm wrong, since I'd like to buy an AO device . If it's not going to be the SBIG AO7, then there's nothing else on the market . Help , please. Thanks, Matt Tudor |
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On Wed, 27 Oct 2004 21:04:29 -0400, "matt" wrote:
1 - are there any other alternatives ? I found some references to a discontinued Stellar Products device and a higher order correction device , but their website doesn't show if this is a current product or just a future project . I'd appreciate any advice, personal knowledge or experience with these . Not unless you make them yourself. 2 - the SBIG AO-7 is marketed by SBIG as a tip/tilt adaptive optics device. Allegedly it corrects for mount fast tracking errors plus atmospheric turbulence induced tip/tilt... Your analysis is essentially correct. The AO7 does not correct for atmospheric effects in any significant way. It corrects for guiding and tracking errors in the mount and for some vibration in the mechanical system. I believe it is fair to call such a device an active optical element, however, since it does modify the final image by altering the optical path. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
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"matt" wrote
I'm contemplating purchasing a SBIG AO-7 as being the only so called adaptive optics device currently on the market ............ ......... Please correct me if I'm wrong, since I'd like to buy an AO device . If it's not going to be the SBIG AO7, then there's nothing else on the market .. Help , please. Matt, As far as I know there is nothing else currently for sale other than the AO-7, and soon its "big brother" for the SBIG STL cameras. SBIG also mentioned something about possibly selling a cheaper slower "tracker" version. Most people contend that the AO-7 does next to nothing for seeing but it is great for improving guiding for problems like mount glitches and shifting optics. As for your point about the guider-chip being off-axis: At one time about a year ago or so SBIG had some info on their website noting that the AO-7 would work much better on correcting seeing if it could guide on a star that was near the center of the imaging chip. They claimed to be working on an add-on to do this which I believe was (is?) a beam-splitter that would send IR over to the guide-chip. However SBIG's efforts must not have worked out too well because I've not heard anything on it for a while. Maybe it just went on the back burner. When I called SBIG and talked to them they basically said that the AO-7 works to improve seeing mostly on large telescopes, which is OK for me since I mostly use a 20-inch F/8 RC Cass. They said that there would be little improvement of 'seeing' on my 10-inch but it should help on the 20. In the manual for the AO-7 there is a section that says something like 'Take a very short sub-second exposure of a bright star. The AO-7 will not be able to improve the FWHM beyond what you measure in such short exposures". I've found that my AO-7 improves FWHM about 20% to 50% or more on the three telescopes I've tried it on. On a 60 second exposure with my ST-9/AO-7 on the 20" F/8, the FWHM will go from say 3.5 to 2.5 when using the AO. I'm not sure what it's correcting, but it works. On the other hand, I've found my AO-7 to be more delicate than the SBIG cameras and more prone to needing repairs. I would not want to drop it, and SBIG did have a certain number of them not survive shipping (mine being one - It was DOA and I had to send it back for adjustments). You also 'pay' for your better resolution with added set-up time and complexity when imaging, plus it's just another gizmo to take along to the observatory or out in the field. It also makes the camera heavier and more difficult to mount on a telescope. I now only use the AO-7 about half the time when I'm really concerned about improving resolution. It has also helped in the field on those nights I've been too lazy to do a full drift polar alignment. You might find interesting the following page of info offered by the guys who 'invented' the AO-7 and sold it to SBIG: http://voltaire.csun.edu/ao.html You can also download the manual at the SBIG website and perhaps gain some feeling on how it works from the section of the CCDsoft and CCDops manuals discussing it. George Normandin |
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I have heard that Starlight Xpress are currently developing an "Active Optics"
equivalent to the AO7. The SX product will be a straight through configuration (tip/tilt optical window) and will essentially correct the same problems as the AO7 - that is high speed guiding and image shift corrections . The preliminary specs are 10ms response, +/-75 microns image shift, and the image shift relationship to the input data is well defined, This should allow 'open loop' guiding with a guide telescope as well as conventional closed loop control with the guide device behind the AO unit. I hear it is due for release at the end of the year. Mark |
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