Strehl ratios, FWHM, and Encircled energies

The point spread function of an adapative optics system can not be described with a single parameter (such as FWHM). This is due to the varying form of the PSF. Common performance measures are as follows:

Strehl ratio	The Strehl ratio is defined as the peak intensity in an image of a point source normalized by the peak intensity it would have if diffraction-limited. It is a common measure of the performance of optical systems. For moderate Strehl ratios (Strehl >~ 0.2) this parameter is close to the amount of energy in the core of the PSF.
FWHM	This is defined as usual Full-Width-at-Half-Max however, note that this parameter is a relatively insensitive measure of the fraction of energy within any specified aperture. This is simply due to the fact that putting a small fraction of the PSF energy into a small core quickly drives the FWHM to the size of the core.
50%EED	If the science program requires a significant fraction energy within a particular radius then the 50% Encircled Energy Diameter may be the parameter of choice.

Last update 24 August 2000; Mark Chun