Tips for AO Operation

This section includes various tips and techniques for optimizing your AO experience.

Before you Start

• Make sure you can guide with the Guide Tab before you try AO guiding.  If you can't guide normally then you are not going to be able to AO guide.
• Pay special attention to how the guide commands are sent to the mount (Guide tab / Settings / Control Via).
• The Guide tab / Move feature is useful for verifying that all four mount guide direction commands are reaching the telescope mount.
• Your typical optical assembly, starting at the telescope, will be AO, Guider, Filter Wheel, then Main Camera.  Guider may be an off-axis guider mirror assembly and guide camera, off-axis guide cameras such as SBIG StarChaser, or in some cases it is part of the main camera (the disadvantage of being behind the filter wheel is lower light levels due to filter passbands). Both the guide camera and main camera must view the sky through the AO.
• When setting up, focus the main camera first, then focus the guide camera.  Once the guide camera is focused, it is generally not necessary to refocus it, because it is parfocal with the main camera - when you focus the main camera, you're also focusing the guide camera.  
• Don't expect the guide camera stars to be perfect, because they are off-axis.  Some distortion is normal and is tolerated by the guider algorithms.
• Don't forget to set up File menu Settings / Site and Optics.

Focal Length and Resolution

The primary purpose of the AO unit is to optimize resolution, which typically means it is used on long focal length / high resolution imaging systems.  Optimizing resolution is achieved by:

• Eliminating mount dynamics from guiding operations - precise movements, no stiction, delays, etc.
• Speeding up correction rate - depending on model an AO can achieve 10 Hz to 20 Hz with a sufficiently bright guide star
• Reducing the effect of wind movement, mechanical drive errors, equipment flexure, and "slow seeing" that is often caused by the ground layer or observatory thermal effects

AO is particularly effective in good seeing conditions at high resolution. In some cases it can improve the FWHM of your stars by 20% to 30%.

When to Use Binning

• If your guide stars are big blobs, turn on binning.  It will not reduce the precision of AO guiding; more likely, it will improve it.  Do not hesitate to use 2x2 or 3x3 binning.
• If your guide stars are tiny single-pixel dots, use 1x1.

When to Calibrate AO

AO Calibration needs to be done:

• The first time you ever use it
• If you unbolt it from the camera and then reattach it
• If you move to a new computer and don't copy your MaxIm DL settings over

Once the unit is calibrated, it never needs to be calibrated again unless you somehow mechanically change the hardware, e.g. rotate the AO with respect to the camera, change camera, change spacing to the camera, etc.

When to Calibrate Drive

First of all, if your mount tracks accurately enough that the tip/tilt never runs out of range, you don't even need to enable the mount controls.  If you have slow drift or large periodic error, then you will want to enable mount bumping.  The purpose of the bumping is to make sure the guide star never drifts to the point where the AO hits the end stop.  It does not need to - and usually will not - move the AO back to center.  It simply needs to avoid hitting the end stops.  

You need to calibrate the drive:

• The first time you use the unit
• If you flip over the pier on a German Equatorial mount (some add-on programs like ACP can adjust the drive settings automatically without recalibrating).
• If you rotate the camera/AO assembly relative to the telescope:
• If you are using an instrument rotator, then you will need to calibrate after rotating.  Note that some programs such as ACP can compensate automatically.
• If you are not using a rotator but manually rotate the camera assembly in the focuser, then you will need to recalibrate
• If you change something that requires recalibrating the AO

Watch Out For Guide Cables

Unfortunately the "ST-4" style tracking interface cable is a somewhat loosely-defined standard. RJ-12 connectors can be flipped in two possible orientations. Some mount and camera manufacturers use the "wrong" orientation, so the cable that came with the autoguider camera may not work with your mount.  If the cable is flipped, a common result is that the mount continuously moves in declination; sometimes it will move only on one axis.  

In addition these cables are relatively fragile and can be easily damaged.

It is generally safer and easier to use the ASCOM PulseGuide or Telescope option if available (Control Via under the Guide Tab).

Tips for Calibration (AO and Drive)

• Avoid crowded fields.  
• It is best to identify an isolated, bright star and calibrate on that.  
• If you pick a star to calibrate on, and the movement causes it to exit the field, calibration will fail
• If you pick a star to calibrate on, and another brighter one enters the field, the software may lock onto the wrong star on subsequent exposures and produce an invalid calibration
• Use an exposure that produces a bright star image without saturating

Update Rate

The update rate is measured in cycles per second, or Hertz (Hz).

• Update rate varies by AO model:  
• SBIG AO-8 is slower but has the largest tilt distance. Update rate of 3 to 5 Hz can be achieved.
• SBIG AO-X and AO-7 are fastest.  They can readily achieve 10 Hz, and under ideal conditions can achieve 20 Hz.
• Other AO models will have to be evaluated by the user
• An old rule-of-thumb (based on AO-7) is that you should be able to achieve 10 Hz using a 10th magnitude star on a 10-inch telescope. Mileage may vary!
• Obviously a high update rate is not possible with a long exposure.  If you are using a 1 second guide exposure, you can expect to update the AO at best once a second. To achieve 10 Hz you must have an exposure a little shorter than 0.1 second (some time is needed to move the AO actuator).
• On older equipment, using a smaller Track Box will improve the update rate.  On newer equipment, which can read out the sensor much faster, this may have no impact.  In that case using a larger track box may reduce the odds of losing the star, especially if you are using Dither.