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

"matt" wrote:

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 ?

Not as far as I know.

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.

It's a fast guider. Adaptive optics in a sense, but it doesn't correct
for every error.

Here's some stuff from the guys who designed the system, which gives a
good insight into its capabilities and limitations:

http://voltaire.csun.edu/ao.html

In short: it aint perfect, but it helps.

If you've got a couple million to spend rather than a couple of
thousand I'm sure the Keck guys would sort you out with something
better


Tim
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