Sbig AO-8 User Manual

SBIG

Model AO-8

Adaptive Optics

Installation and Operation

Manual

SBIG Astronomical Instruments, A Division of Diffraction Limited.

Tel: [phone] | Fax: [phone]| E-mail: [e-mail] | www.sbig.com

© 2015 Diffraction Limited. All rights reserved. The SBIG wordmark and logo are trademarks of
Diffraction Limited, All other trademarks, service marks and tradenames appearing in this brochure are
the property of their respective owners.

59 Grenfell Crescent, Unit B, Ottawa, ON Canada, k2G 0G3

Introduction

Congratulations on your purchase of the AO-8, a second-generation
Adaptive Optics device for your dual sensor ST series camera. Please
take the time to read these instructions and carefully review your
software’s operating instructions for an SBIG AO. Each software
program that supports the AO will have its own unique menu and set
of commands. We therefore limit the instructions in this manual to
installation of the device and an overview of its operation.

IMPORTANT: Read Appendix A for Cautions before connecting
the AO-8 to your camera. Remove the bottom cover and take any
foam or packing material out of the unit before operation.

AO-8 Design

consumed quite a bit of back focus due to the use of a mirror at 45
degrees. It’s alignment was quite critical since it could shift the
optical axis considerably if misaligned, and it was also somewhat
delicate due to the use of a
membrane under tension in two
axes to support the mirror. In
addition, it had an issue with
image rotation at large
deflections, which becomes
serious for large CCDs, but was
not a problem for the smaller
CCDs of the ST series. For
these reasons SBIG designed a
new Adaptive Optic device for
high speed beam steering called
the AO-L (Adaptive Optic –
Large Format), which could
cover the larger CCDs. The
same approach has been
extended to the ST camera line with the AO-8, a less expensive unit
for smaller CCDs, but operating in much the same way. It is shown
in an exploded view above.

SBIG’s previous adaptive optics accessory, the AO-7,

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Incoming light passes through a 6 mm thick plane parallel plate that
can be tilted by the action of two geared stepper motors. The total
amount of tilt in each direction is +/-9.6 degrees. The light beam is
deviated by 36 microns per degree of tilt, so the maximum deviation
is +/- 346 microns, or +/-38 pixels with an ST-7/8. We have done
careful ray tracing to convince ourselves that no focal shift or
significant aberration occurs as the plate is tilted over these small
angles, nor is there any distortion, rotation, or change in
magnification. Our testing has verified these results. The window is
AR coated with the same high quality coating on our ST chamber
window, and has less than 1% reflection per surface from 400 to 900
nm wavelength.

The AO-8 is somewhat slower than an AO-7, since the motor and
gearbox can only tip the motor at 18.75 degrees per second, or 75
pixels per second. In this design moves are slew-rate limited. What
this means to the user is that the user should use a focal length that
does not produce too large of a star image. For example, if the seeing
is bad and the star is 9 pixels across, it is probably jumping around by

4.5 pixels or so with each look, which would take 60 milliseconds to
move. If under such conditions you reduced your focal length to
where the star was only three pixels across, only 20 milliseconds are
spent making the move, and you can achieve a higher update rate with
the AO-8. The total overhead in the software for an AO-8 move is 45
milliseconds. If, for example, you are taking 50 millisecond
exposures with an optimized focal length (~3 pixel Full Width Half
Maximum stars), the total cycle time is 45 + 50 + 20 ms, or 115 ms (8
updates per second). The AO-7 advantage was that a move of any
length is 20 milliseconds, but the exposure and overhead times were
similar. Based on our experience and knowledge, the corrections
made by the AO-8 help with reducing ground layer atmospheric
turbulence, and are quite effective in reducing guide errors and wind
buffeting. The smallest increment of move for the AO-8 is 0.075
degrees per axis, or 2.7 microns, sbout a third of a pixel. A spring is
included to take up the backlash in the motor gearboxes.

AO-8 Installation

The AO-8 is designed to be bolted directly to the front of an
ST series camera with or without a filter wheel. In the case of the

CFW8A filter wheel, an adapter is required to provide the correct bolt
pattern.

To attach the AO-8 to an ST camera without a filter wheel:

1. Remove the “D” block from the camera (4 Phillips head screws).

2. Remove the bottom plate of the AO-8 by removing the 4 hex head
screws at the corners of the AO-8 housing. Remove any pieces of
neoprene or foam that were inserted between the window paddle and
the bottom plate during shipment. Save these in case the unit ever
needs to be returned to SBIG.

3. Using the four 4-40 x 1/4 screws and washers provided, attach
the bottom plate of the AO-8 to the camera. Appendix C, Fig 1.
Note that the smooth, featureless side of the bottom plate faces the
camera and the side with the cut out faces away from the camera.
Once the AO-8 bottom plate is attached, re-assemble the AO-8
housing to the AO-8 bottom plate.

5. Plug the cable from the AO-8 into the I2C-AUX port on the
camera. If the CFW-9 or CFW-10 is already plugged into the I2C­AUX port on the camera, remove its connector and plug the AO-8
into the camera instead. The filter wheel will then connect to the I2C­AUX OUT port on the AO.

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