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# Adaptive Optics in the ITC

The ability of the AO system to correct the wavefront depends on the brightness and off-axis angle of the wavefront reference source (the AO "guide star"). The Strehl ratio of the AO-corrected core is approximated in the ITC by:

where *S _{fit}* is the Strehl due to the system fitting error (i.e. limited number of actuators),

*S*is the Strehl loss due to the limited number of photons from the guide star and

_{noise}*S*is the Strehl loss due to anisoplantism.

_{aniso}*R*is the R-band guide star magnitude and

_{GS}*theta*is the off-axis angle in arcsec.

_{GS}The total signal from a point source is the sum of the AO-corrected core and the uncorrected (seeing-limited) halo. For moderate or high Strehl ratios the core dominates, for poor correction (low Strehl) the halo dominates. As in the non-AO case, the ITC defaults to an "optimum" aperture which gives a reasonable approximation of the best S/N ratio; we have taken this to be *1.18 * FWHM _{AO}*, where

*FWHM*is the width of the corrected core:

_{AO}

*Last update August 29, 2003; Francois Rigaut and Phil Puxley*