Sbig AO-7 User Manual

AO-7 Adaptive Optics Accessory

Operating Manual

Revised November 2003

Santa Barbara Instrument Group

147A Castilian Drive

Santa Barbara, California 93117

Phone: [phone]

Fax: [phone]

E-Mail: [e-mail]

Home Page: www.sbig.com

Introduction

SBIG’s adaptive optics package is designed to enable users of SBIG Dual CCD cameras1 (like
the ST-7) to achieve the ultimate in resolution when imaging deep sky objects. The package
consists of two components: a high speed tip-tilt mirror capable of correcting the telescope
pointing at rates up to 50 times a second, and a software package that implements the Lucy­Richardson deconvolution technique for image sharpening. Both items were developed by SBIG
in concert with Benoit Schillings and Brad Wallis, two advanced amateurs well known in the
film and CCD imaging field. This package can produce a 2X improvement in resolution over
normal self guided images, where corrections are applied only to the telescope drive. This
package truly advances the capability of the amateur astronomer, considering that most
professional observatories do not have comparable systems.

Theoretical Background

Under good seeing conditions, atmospheric turbulence causes the stellar image to wander about
the mean position, in addition to blurring occasionally. Three levels of adaptive optic correction
are possible. The simplest technique, which we have implemented, uses a high speed steering
device to suppress the wander and hold the star image fixed on the sensing device. A second
level of sophistication is to try to correct the lower level aberrations produced by the atmosphere,
in addition to stabilizing the image, while viewing a star. The ultimate technique, developed by
the military, uses a laser guide star to provide a point-like source high in the atmosphere that
provides enough light to enable accurate sensing of the wavefront, and therefore correction. The
laser guide star approach, however, can only correct the higher order aberrations; the image must
still be stabilized by a tip-tilt mirror viewing the object, not the laser guide star.

The success of any adaptive optic technique is limited by three important considerations:
First of all, the atmospheric aberration is not correlated spatially over a large extent. The
isoplanatic patch commonly referred to in the literature is the angular extent over which the
higher order aberrations are correlated, and is typically only a few arc seconds. The odds of
having a suitably bright star so close to an object of interest is small, which is what has
motivated the development of laser guide stars, which can be put where they are needed. The
tip-tilt component of the aberration is correlated over a larger extent, minutes of arc, improving
the odds of finding a suitable guide star.

The second consideration is that the atmosphere is constantly changing. With any
system, you have to correct the telescope or adjust the mirror based on the last image acquired,
which can be several milliseconds old by the time it is collected. For example, with a 10
millisecond exposure, the average position of the star is sensed as it was 5 milliseconds
previously. This consideration forces one to work at very high rates to correct the higher order
aberrations. The lower order effects, such as the tilt of the wavefront, change less rapidly and
can be corrected with slower systems.

The last major consideration is field of view. Systems which correct the higher order
aberrations typically can not improve the image over more than a 1 minute of arc field of view.
This corresponds to an ST-7 used with a 934 inch focal length (24 meters).

The importance of these considerations is that a typical amateur CCD camera with a field
of view of around 10 minutes of arc, using a guide star about 10 minutes off the center of the
field, is not improved by correcting more than the tip-tilt component of the aberration, and does
not need to run at excessively high rates to obtain all the possible improvement. The higher

1

The AO-7 is not compatible with the Large Format ST-L line of cameras. At this time a new AO unit is being

designed for those cameras.

order aberrations and fast image variation are simply not correlated between the two CCDs.
This is the philosophy behind SBIG’s AO-7 tip -tilt mirror design. Our correction rate allows
correction of stellar wander up to about 5 hertz (50% wander reduction), assuming a bright
enough guide star, and correlation between tracking and imaging CCDs. In general, deep sky
objects do not have good guide stars imbedded in them, so our off axis location of the tracking
CCD is essential for the user to acquire guide stars over a substantial area by rotating the camera.
We do not feel better reduction of atmospheric effects than that provided by our unit is possible
for amateur sized telescopes.

In general, adaptive optic techniques used for astronomy are always starved for light. We
have optimized our noise performance to where one can typically guide on a 10th magnitude star
at 10 frames per second with a 10 inch telescope. We call this rule the “10” rule of thumb!
Finding a good bright guide star is aided by a good star atlas program, such as Software Bisque’s
The SkyTM, and focal reducers or Barlow lenses to put the best star at the right distance off axis.
In general, even running at rates as low as 1 frame per second provides some improvement since
the AO unit makes an accurate small move, much better than most telescope drives, since it has
no backlash and a fast response.

In general, the AO module cannot be used to image the moon, since there is no guide star.
It cannot improve planet imaging unless a guide star is close by, such as might be provided by a
moon of Jupiter.

How the AO-7 Unit Works

In SBIG’s AO-7, a tip -tilt mirror is used to make fine corrections to the position of a star to hold
the image fixed on the CCD during the exposure. The tip-tilt mirror has magnets on the back,
which interact with the current flowing through a set of voice coils on the AO module housing to
rapidly move the mirror. The technique is very similar to that employed in loudspeakers, except
there the magnet is fixed and the wires are on the moving speaker cone. We put the wires on the
fixed housing to better conduct heat away from the moving mirror, where it might distort the
optical element.
The mirror and magnets are suspended using a flexible beryllium copper membrane. A
needle pushes up against a jewel bearing mounted to the center of the mirror to hold the focus
constant. SBIG has developed a proprietary technique to rapidly damp the motion of the mirror,
so small moves are precise, with very little overshoot or ringing. The tilt of the mirror during
operation is very slight, and does not lead to any measurable defocus at the edges of the frame,
even on large format cameras like the ST-8. The correction range of the tip-tilt mirror is about ±
30 pixels with an ST-7
(± 250 microns); the software adjusts the telescope position to take care of drive errors greater
than this if they exist. The telescope position is adjusted while the AO is running, but this does
not cause problems since the AO corrects much faster than the telescope drive. The telescope
drive is adjusted to keep the AO-7 voice coil drive levels between 25 and 75%.
The AO-7 accessory utilizes the tracking CCD int egral to the Dual CCD cameras. The
PC controlling the imaging process uses the tracking CCD to measure the position of the guide
star, then sends a position correction to the tip-tilt mirror, waits for the mirror to move, and starts
the next tracking CCD exposure. The imaging CCD continuously collects light throughout the
cyclical process. In SBIG’s tip-tilt mirror the mirror can complete the move in 10 milliseconds.
Reading out the CCD and processing the information takes another 9 milliseconds, so wit h an
exposure time of 1 millisecond the unit can achieve update rates of 50 frames per second. With
an exposure of 10 milliseconds, the total time to go through the loop once is 29 milliseconds,
corresponding to a rate of 34 frames per second. The maximum sinusoidal components of the

stellar motion that are well corrected are only about 1/10th of the update rate. For example, a 40
frames per second update rate attenuates a four hertz stellar sinusoidal motion by about 50%.

Compatibility

The AO-7 can be used with SBIG’s ST-7/8/9/10/2000 cameras (parallel or USB) with or without
the CFW-8 filter wheel. The CFW-6 filter wheel is not supported.

Mounting the AO unit to the Camera and Telescope

1. Unpacking the AO module

The AO module’s tip-tilt mirror is somewhat delicate due to the flex member and the needle
bearing support. For shipping a piece of cardboard and a lens cloth is sandwiched between the
two halves of the module to prevent the mirror from vibrating or hammering against the jewel
bearing. Remove the long screws that hold the halves together, remove the cardboard, and
reassemble the unit. Save the cardboard and lens cloth in case you need to return the unit to
SBIG for service. If necessary gently clean the mirror with rubbing alcohol and cotton swabs.
Never reuse the swabs – wipe the alcohol on with one swab, dry it with another, and repeat the
process using new swabs until the mirror is clean. If fingerprints are present, reusing the swabs
just moves the grease around, and does not remo ve it from the surface. This is why the swabs
must be thrown away after one use. This is typically a good technique with optical elements.
It is a good idea to check the flexure when it is first unpacked. Gently depress the mirror
around the edge wit h a swab or fingertip about 0.04 inch (1 mm). The mirror should have a firm,
smooth spring action, with no gritty feel such as might result if the mirror was rubbing against
the support structure. Check it in four places, 90 degrees apart. Contact SBIG if it doesn’t feel
right, or tends to hang when released.
When reassembling the two halves of the unit, orient the halves so that the cable exits on
either the + or – X side of the AO-7. The X axis is the same as the long direction of the CCD.
The CCDOPS software includes an Exercise command under the AO menu. This
option allows one to drive the mirror to its limits in a rectangular pattern, or each axis
individually. You can select the axis or combination, and the update rate between moves. This
option is excellent for checking out the AO device after unpacking. Make sure it is connected to
the camera, and the combination powered up. (only connect and disconnect the AO module to
camera when both are unpowered!). Try an update time of 100 milliseconds initially, and look at
a distant light reflected in the AO module mirror. You should see the mirror deflecting. Next
increase the update rate to 20 milliseconds. The light should blur into a rectangular pattern of
light, with no more than 20% overshoot at each corner. It will probably not be square, and may
be slightly parallelogram shaped, with the axes not at exactly 90 degrees. This is satisfactory,
since the resultant error of the move is small. Hit ESC to terminate the mirror motion.

2. Connecting the module to the Camera

There are three ways to attach the AO module to the camera, depending on your configuration.
In all three you start by removing the T-Thread D-Block adapter from the front of the camera
using a Philips head screwdriver as shown in Figure 1A below:

• Method 1: Without a CFW-8, directly connecting the AO-7 to the Camera.

• Method 2: With a CFW-8, directly connecting the CFW-8 to the Camera and then using

the Male-to-Male T-Thread coupler to connect the AO -7 to the CFW-8.