Observer’s Information for the MMTAO

NEW An AO Guide Star Tool at the Keck AO Web Page is available to help you check to see if there is a guide star near your science target.

The goal for all AO systems is to obtain optimal diffraction-limited correction for any point on the sky. In order to do this, a method of measuring the wavefront distortions introduced by the atmosphere is required, and the most common method is to look at the measured wavefront from a distant source above the atmosphere and assume that the wavefronts incident on the atmosphere are plane parallel. AO correction improves image quality for wavelengths longer than H band, increasing significantly into the mid-IR bands.

Most AO systems use a star (a “natural guide star”) to provide the reference wavefront, and optimal correction is provided on this target. Ideally, a bright guide star would exist in the center of the scientific field of view. An AO corrected image has a region of significantly improved image quality centered on the guide star, which falls off with increasing angular distance from the guide star.

Laser guide stars (LGS) take the approach of projecting a bright spot of light into the atmosphere and determining wavefront aberration from the returned light. CAAO is developing a laser projection system for the MMT, with a prototype being developed for testing in Spring 2004, and meanwhile the system relies on natural guide stars for AO correction.

NGS AO Requirements

For the AO system to close its loop and begin AO correction, a natural guide star (NGS) of R < 10 mag is needed, although lower performance correction is possible down to 13 mag. The NGS itself should be less than 1 arcsec (FWHM) in extent, and significantly brighter than any surrounding structure or nebulosity.

The degree of image correction is specified in terms of Strehl ratio, which is the ratio of the peak intensity of the AO corrected image by the peak intensity of an ideal telescope point spread function. The expected Strehl ratios and isoplanatic patch diameters are listed in the table below.

Waveband Strehl Ratio Isoplanatic Patch diameter
K 30% 30 arcsec
M 95% > 20 arcsec

The image quality of an AO system degrades as the distance between the NGS and science target increases, and so the best correction is obtained when the science target can be used as the NGS. The region around the NGS with good correction is called the isoplanatic patch, and it is a function of wavelength, airmass, and degree of AO correction. For 1.2 – 2.5 microns, this is a region with a radius of ~10 arcsec, increasing with longer wavelengths. Science targets that are more than ~ 15 arcsec away from a natural guide star will have poor image correction.

Targets should also have an airmass < 2, and ideally lower than 1.5. The higher the airmass, the poorer the correction.

After further engineering runs, we hope to have more quantitative plots showing the effects of isoplanatic angle and guide star magnitude on the AO system’s Strehl ratio.

Telescope Location

(taken from

  • Elevation: 2606m (8550 ft)
  • Latitude: +31d 41′ 19.6″
  • Longitude: 110d 53′ 04.4″ W
  • Time Zone: +7 hours
  • Typical seeing: 0.8arcsec (FWHM)