Integration Time Calculator - Altair Properties

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_noise is the Strehl loss due to the limited number of photons from the guide star and S_aniso is the Strehl loss due to anisoplantism. R_GS is the R-band guide star magnitude and theta_GS is the off-axis angle in arcsec.

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_AO is the width of the corrected core: 

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