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STL-Adaptive Optics Accessory
Update: April 6, 2006

ao4.jpg (93642 bytes)

New AO-L Adaptive Optics Accessory for the
Large Format Research Series Cameras

The smaller AO-7 Adaptive Optics accessory was designed for the CCDs in the ST-7/8/9/10/2000 cameras. It is not recommended for use with the larger format STL series cameras. A larger moving element is required to eliminate vignetting of the larger CCDs and also to maintain good image quality due to corrections over the entier field of view. The 50mm diagonal mirror of the smaller AO-7 would be placed too far away from the CCD in the large format designs to be effective over the entire field of view. For these reasons, a new larger AO device has been developed for the STL series cameras. This new AO is placed in line and has a refractive element that can be made to respond more quickly than the moving mirror of the AO-7. It is larger, however, and the new AO will not make as many moves per second as the AO-7, it will however, respond faster when making a move.

The pictures above show the prototype AO-L. The production model (below) has a slightly smaller housing to reduce back focus to approximately 1.4" (~3.5cm). The apreture is larger to accommodate the STL-11000, and the glass element is increased in size and made rectangular (1.5" x 2") rather than round, also to accommodate the STL-11000. The optical glass is hard AR coated on both sides for less than 1% reflectivity and it can easily be removed from the case for cleaning.

Production AO with rectangular optical element

This AO is operated via the I2C port on the STL camera, and it may be operated with other I2C accessories such as the new STL 8 position 2" filter wheel. It is also compatible with any ST-7/8/9/10/2000 camera having an I2C port. This product is currently in production.


STL with 5-position filter wheel	STL with 5-position filter wheel and AO	STL with 8-position filter wheel	STL with 8 position filter wheel and AO

Atmospheric Refraction Compensation: In addition to motion compensation, we are working on an optional set of prisms that will fit the new AO-L to compensate for atmospheric refraction. We will post more about this as the product develops.

Review of the AO-L by Ron Wodaski posted
on the SBIG User's Group March 31, 2006:

From: Ron Wodaski
Date: Fri Mar 31, 2006 2:47 pm
Subject: The Cone Nebula with AO-L

I've been using the AO-L with my STL-11000 for a while now, on the 20" RC,
and have been very happy with operation and most importantly, results.
However, I recently wrote software for the instrument rotator that's been on
the scope, that allows me to control it remotely, so I was finally able to
test the AO-L under just the right set of conditions:

* Good seeing
* Able to put a very bright guide star on the guide chip
* Able to guide using 0.01-second exposures.

On the down side, there were thin high clouds and sky brightness was much
higher than normal, so the S/N isn't as good as it could be for the exposure
time.

I do not have a monitor on that computer, so I can't tell you what the exact
correct rate was - the overhead from Remote Administrator software tends to
drive the correction rate down when I watch. While I was logged in, I was
getting just under 10Hz corrections with this configuration. I took two
30-minute exposures of the Cone Nebula with this configuration.

Our seeing monitor reported seeing of 1.5-1.6" (it is a prototype of the new
seeing monitor coming from SBIG) at the time of these exposures. Normally,
with the 20", I get a FWHM that is around 0.2 to 0.5 arcsecond larger than
the seeing numbers. On this night, with very fast exposures, I pretty much
got exactly what the local seeing was at the time - the FWHM of the stars in
the image was 1.5-1.6 arcseconds. This is interesting for several reasons:

* Number one, it is interesting because I recorded the seeing pretty much as
it was for the site. Without high-speed AO, that just doesn't happen. I
might get within a few tenths of an arcsecond of the actual measured seeing,
but no closer.

* Number two, and quite remarkable, is that I got those kinds of numbers in
such a long exposure. We use such long exposures here because of the dark
skies, and typically, the longer the exposure, the more the FWHM of the star
grows. It is very rare to get a FWHM that isn't at least .5" fatter than the
seeing on 30-minute exposures.

Here is an average of the two 30-minute exposures, with minimal processing
in Photoshop using the Zone system:

80% of full size, 421k:
http://www.bb-obs.com/img/rc20/11000/Nebulae/cone_highRes_80.jpg

50% of fill size, 180k:
http://www.bb-obs.com/img/rc20/11000/Nebulae/cone_highRes_50.jpg

The dim stars are so small they look like hot pixels. <G>

I noticed an interesting side effect of using the AO-L for fast corrections
on such a large field. Typically, as seeing improves, the stars in the
corners not only elongate due to field curvature, but they also start to
show odd artifacts - gull wings, spikes, fans, etc. With the AO-L in line
and operating at high speed, the corner stars show less elongation than
normal, and virtually no artifacts. I talked with Brian Lula about this
today (he's visiting his observatory in NM), and he came up with an idea: if
the AO-L compensates for some of the wavefront tilt, it could reduce
off-axis problems. It will be interesting to monitor this effect and perhaps
quantify it at some point. It appears to be the high speed of the
corrections that makes the difference; we do not see this benefit from the
AO-L when exposure time are in the 0.5 to 2 second range.

I would have to say I'm nothing short of ecstatic about the results from the
AO-L. This is a nice piece of equipment.

Ron Wodaski
http://www.newastro.com

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