SOAR Guider and Active Optics Performance

SOAR Active Optics Description and Operation

SOAR’s optics are controlled by an active options system (aO) that manages the configuration of the primary, the location and orientation of the secondary mirror, and supplements the telescope tracking using a tip-tilt mechanism on the tertiary mirror.

For best imaging performance, the active optics must be correctly adjusted. These adjustments are made in three ways:

• At the start of the night, the operator “tunes” the primary using observations of a bright star by the SOAR Calibration Wavefront Sensor (CWFS), which incorporates a 32x32 element Shack-Hartman sensors. These observations are used to adjust the 120 actuators of the primary mirror.
• Lookup tables are used to adjust primary mirror for other telescope positions (as an alt-az telescope, only elevation matters); the positioning of the secondary mirror is also controlled to compensation for flexure in the telescope.
• The guiders – both at the IR and optical foci – have the ability to adjust focus and astigmatism in addition to guiding. The focus and astigmatism corrections are done using imaged averaged over tens of seconds, not the shorter time periods used for guiding.

Signals from the guiders are used to drive both the tertiary mirror (fast tip-tilt corrections) and the telescope mount (slower, larger corrections). The tip-tilt correction can adjust for higher frequencies.

The higher-order corrections may not work under particularly adverse conditions (faint guide stars, clouds) but should be operating by default.

Recommendations for Best Performance

Observers may find the following recommendations helpful in getting best imaging performance:

• Consider where you are pointing; performance will be worse at high airmass or into the wind (when it is windy). This may seem obvious but we mention it nonetheless.
• Make sure (check with the operators) that the autofocus and astigmatism corrections are turned on. With faint guide stars or variable clouds, sometimes they won’t work and need to be off, but most of the time these corrections will help.
• If the images look unusual (distorted), it may be beneficial to re-tune the optics with the CWFS. Sometimes the telescope optics don’t fully stabilize until an hour or two into the night – or conditions may change – so re-tuning can be beneficial. The process takes 20 minutes or less under reasonable sky conditions.
• The seeing monitor (DIMM) is available as an aid in estimating expected performance. If for some reason the Pachon monitor isn’t working, the Tololo monitor is useful. At reasonable zenith distances, SOAR performance should match the DIMM. If it doesn’t, one or more of the steps above may help.

The telescope operators should be paying attention to conditions as well, and should feel free to offer suggestions to observers, or to make adjustments themselves when operating the AEON queue.