The performance of an AO system is usually described in term of Strehl ratio. The Strehl ratio is the ratio of the psf maximum intensity to the theoretical diffraction-limited psf one; the latter is the Airy pattern in the case of a clear circular aperture telescope of diameter D. We present the steps to compute an unbiased estimation of the Strehl ratio from any AO image of a point source provided that the Nyquist criterion is fulfilled for the effective spatial frequency cutoff.

For a theoretical diffraction-limited psf centered, sampled
by a Shah function, and with energy normalized to one,
the discrete peak value *M*_{0} at zero position is
given by the formula:

where is the ratio of the Shah function sampling frequency to
the Nyquist frequency (), and *U* is the linear central
obstruction of the telescope. Let be an AO
image sampled by a array of detectors. In order to
compare with the previous psf, we
first normalize the image energy to 1.0 and defilter from the
pixel function (for a square pixel with a filling factor *g*, one
divides the discrete Fourier transform of the image by the
function sinc sinc).
The defiltering operation is not needed in case of smooth peak or
oversampled images ().
The peak position in the image is then located from
a spline function interpolation. The image is shifted to this
position via the Fourier transform and not via a spline interpolation.
Using Eq. (A1 (click here)), the Strehl ratio of the AO image is thus given by:

where *M* is the peak value in the shifted image.
Of course, if we use a non point source
image, the Sr value will be under-estimated all the more as
the source is resolved. Assuming data without noise, the error source
in this technique comes from the position estimated from
the spline interpolation. For synthetic psfs which have been shifted
by fractional pixel values in *x* and *y*, the maximum position error is
always pixels which yields to a marginal relative
error for the Strehl ratio .

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