User's Guide to the Adaptive Optics System

Table of Contents

Observing Information
Preparing for an AO Run
Graphical User Interface
Other Software
Gallery of Aberrations
Other Hardware
AO Song

Mt. Hamilton Homepage

Observing Information

Introduction to AO Observations
AO Run Planning
FAA and US Space Command Restrictions
AO Performance
Wavefront Sensor and Tip/Tilt Camera Sensitivity
Sky Coverage
IRCAL Sensitivity
AO Tutorials

Introduction to AO Observations

Adaptive Optics (AO) is used to correct atmospheric blurring of images in real time to create near-diffraction limited images. The Lick AO system is used at the Cassegrain focus of the Shane 3-m Telecope. It can be used in either natural guide star (NGS) or laser guide star (LGS) mode. Under good conditions the AO system routinely reaches the diffraction limit (0.15 arcsec) at 2.2 microns.

NGS mode requires a natural guide star brighter than 13th magnitude under normal conditions, within 55" of the science target. See WFS Sensitivity for more details.

LGS mode requires a natural guide star brighter than 16th magnitude for tip/tilt correction, within 55" of the science target (see Tip/Tilt Camera Sensitivity for more details. Laser operations are also restricted by FAA rules to elevations greater than 45 degrees and times from 11pm to 5am. LGS science targets must also be cleared by US Space Command in advance(see FAA and US Space Command Restrictions for more details).

The AO system feeds IRCAL, an AO-optimized IR camera that uses a 256x256 Rockwell PICNIC array which is sensitive from 0.9 - 2.5 microns and provides Nyquist sampled imaging at 2.2 microns (0.076 arcsec/pix). The internal image quality is excellent over the entire 19.4 arcsecond field of view. IRCAL is equipped with standard near-IR photometric filters and some narrow-band filters. User supplied filters can be installed in a warm external filter wheel. IRCAL also has grisms for medium resolution (R=500) H and K spectroscopy. It is strongly recommended to refer to the IRCAL manual in addition to the IRCAL Sensitivity Plots below before applying for 3-m time and planning your observations.

AO Run Planning

Section under construction - please forgive the disorganization/lack of information You may also want to refer to Preparing for an AO Run.

Most important to planning AO observations is determining what quality data are necessary to get the desired science results.

Issues to discuss?
1) Science targets: Do they have acceptable guide stars within 55 arcsec of the scence target?
2) Can the predicted Strehl under good conditions from guide star magnitude give you the necessary science results?
3) What is the backup plan for poor conditions?
4) Is it an NGS or LGS starget?
5) AO observations are best during periods of good seeing. In general, the best seeing occurs in the late summer and early fall.
6) Schedule observations so that your science targets are as close as possible to zenith during exposures. Less airmass means less turbulence.
7) Observed isoplanatic angles at Lick range from 15 arcsec to 40 arcsec (as measured with the near infrared camera. The isokinetic angle for laser observations is typically 60 arcseconds.
8) An AO system typically produced a point spread function (PSF) consisting of a diffraction-limited core plus a diffuse halo. The halo is typically the same size as the seeing disk, but contains much less light than it would in the absence of AO. For quantitative imaging, photometry, and spectroscopy is is usually necessary to observe a PSF star before and/or after observing the science object. You should select PSF reference stars to have the same magnitude, color, and elevation angle as the science object's guide star so that AO system conditions duplicate, as nearly as possible, the conditions during science object exposure.

FAA and US Space Command Restrictions

There are no restrictions for NGS observing from the FAA or US Space Command.

LGS mode observing does have restrictions because the moderate powered sodium laser used to create the guide star is a hazard to pilot's (and passenger's) eyes and sensitive cameras on satellites. To minimize the risk to pilots, the laser is only operated from 11pm to 5am when the traffic at the local airports is light. In addition, the laser is propagated at a maximum zenith distance of 45 degrees. To ensure that aircraft are not illuminated with the laser, we have a boresighted radar system that, when triggered, closes a fast shutter to cease laser propagation. We also employ plane spotters who sit outside the 3-m telescope dome. These spotters have headsets so they can communicate with the laser operator in the 3-m control room in case a plane approaches the laser and also have kill switches to shut down the laser in case a plane is approaching the beam to quickly for a more controlled shut down by the laser operator.

To protect astronauts and satellites from harm, all targets to be observed with the laser must be cleared in advance by US Space Command at Cheyenne Mountain in Colorado. Currently the avoidance cone diameter for Lick Observatory LGS targets is 10 arcminutes. This leads to few satellite shutdowns of the laser.

see ??? for instructions on how to submit LGS targets lists, etc..

AO Performance

Performance of the AO system depends on both the magnitude of the guide guide star and the seeing conditions. The below plots show what Strehl one can expect for different magnitude guide stars and under different seeing conditions in NGS mode.

Wavefront Sensor and Tip/Tilt Camera Sensitivity

Photometric data were acquired during late 2005 and early 2006 to evaluate the sensitivity of the WFS and Tip/Tilt cameras. These plots show the number of counts per subaperture one can expect on a good seeing night for stars of various colors and magnitudes at the lowest permitted rate for closing the AO and TT loops. Effective AO correction can be done with 100 counts per subaperture on the WFS. Tip/tilt correction can be done with 50 counts per quad. Going below these thresholds is not advised (though it is possible) because the signal-to-noise of the measurements is low and performance degrades significantly.

The WFS can have either the Mirror in place or the Dichroic. The mirror is used for NGS mode observations, the dichroic for LGS mode. In case you are doing NGS observations before or after LGS mode observations, you may request to have the mirror installed if you have a faint natural guide star (the swap takes about 10 minutes, so please plan ahead).

Sky Coverage

Odds of finding a natural guide star (necessary for either NGS or LGS observing) near a science target depends on both the galactic latitude of the science target and desired magnitude of the guide star. The plot below shows the density of natural guide stars at various galactic latitudes. The Lick AO system requires a ~13th magnitude (NGS) or ~16th magnitude (LGS) or brighter star within 55 arcseconds of the science target for corrections.
Density of natural guide stars at various Galactic Latitudes:

IRCAL Sensitivity

Below are plots depicting the sensitivity of the IRCAL camera for both point sources and extended sources. For more information about the IRCAL camera, see the IRCAL manual.

AO Tutorials

Novice AO users may find it useful to refer to the following web sites for basic general information about adaptive optics.

There are certainly other quality AO tutorial sites out there. If you feel a particular site should be included here, email Elinor Gates
Elinor Gates
Last modified: Mon Apr 28 18:06:37 PDT 2008