Sbig AO-L User Manual

SBIG’s New AO-L High Speed Guider for the STL Camera Series

Alan Holmes

12/19/2005

!!! Important Note: Remove the front cover and take the neoprene foam out of the
unit before operation!!!

AO-L Operation:

SBIG’s previous high speed guider, the AO-7, is unable to be used with the large CCDs
of the STL cameras due to vignetting by the 50 mm diameter mirror it uses. At the 45
degree angle the mirror is used at it foreshortens to only 35 mm. It also had an issue with
image rotation at large deflections which becomes serious for large CCDs, but is not a
problem for the smaller CCDs of the ST series. For these reasons SBIG has designed a
new Adaptive Optic device for high speed beam steering called the AO-L (Active Optic –
Large Format). It does not have the range and speed of the old AO-7, but can cover the
larger CCDs. It is shown in an exploded view below.

Figure One: Exploded view of New AO-L

The light passes through a 6 mm thick plane parallel plate that can be tilted by the action
of two linear stepper motors in conjunction with a Delrin pivot point. The total amount
of tilt in each direction is about +/- 4.6 degrees. The light beam is deviated by 36
microns per degree of tilt, so the maximum deviation is +/- 165 microns, or +/-18 pixels
with an STL-6303. We have done careful raytracing 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 coating on our STL chamber window,
and has less than 1% reflection per surface from 400 to 900 nm wavelength. Our
assumption is that this device is being used with long focus telescopes where bright stars
in the field of view are unlikely, and is not used with short focus refractors. Short
refracters should not need a fast steering device like the AO-L.
The main advantage of the new technique is that it is thin, only using up 1.483
inches (38 mm) of back focus distance, whereas a mirror-based technique would
probably require 6 inches (150 mm) of back focus. It is somewhat slower, since the
motor can only tip the motor at 11.7 degrees per second, or 47 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 pixels or
so with each look, which would take 85 milliseconds to move. If under such conditions
you reduced your focal length to where the star was only three pixels across, only 28
milliseconds are spent making the move, and you can achieve a higher update rate with
the AO-L. The total overhead in the software for an AO-L 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 + 28 ms, or 123 ms
(8 updates per second). The AO-7 advantage is that a move of any length is 20
milliseconds, but the exposure and overhead times were similar. Based on our
experience and knowledge, AO-L and AO-7 rates help only slightly with reducing
ground layer atmospheric turbulence, but are quite effective in reducing guide errors and
wind buffeting.

AO-L Installation:

The AO-L is designed to be bolted directly to the nosepiece mounting plate on the front
of your STL camera. The motor portion of the AO-L hangs over the top edge of the
camera. The steps to attaching the AO-L are as follows:

1) Very important: Make sure the AO-L is unpowered so if a screw falls into it you
do not get an electrical short.

2) Remove the top plate of the AO-L by removing the 6 screws that hold it down.

3) Remove the pieces of neoprene or foam that were inserted between the window
paddle and the top plate during shipment. Save these in case the unit ever needs
to be returned to SBIG. They cage the window paddle, keeping it from flying
around when the shippers throw the boxes from truck to truck, and being
damaged.

4) Using the 4 screws provided, attach the housing of the AO-L to the camera
nosepiece mounting plate, which is 2.75 inches (7 cm) on a side. You can do this
without removing the window paddle. Do this while the camera is unpowered.
Be VERY careful not to let the screws fall into the camera. If they do fall in,
shake them out carefully or remove the camera chamber front plate to reach them
so as to not scratch the optical surfaces within the camera. You do not need to