Authors

R. Davies

Affiliations

MPE

Abstract

The design study for MICADO, the E-ELT AO imaging camera, has shown that the commonly used performance metrics of Strehl ratio and encircled energy are unsatisfactory for astronomers. The reasons are simple. Strehl ratio that can be achieved is tied to sky coverage; and to an astronomer, observing a specific object or field is usually more important than achieving a high strehl ratio. On the other hand, encircled energy is too vague: it provides no information about resolution and is often given within a diameter that is too large to be scientifically useful. I propose an alternative metric, consisting of 2 parts, that should be of practical use to both astronomers and AO system designers. Making use of the characteristic core+halo profile of the partially corrected PSF, it comprises the fraction of flux within the core and the diameter of the core. From the astronomer’s perspective, the first part is akin to strehl ratio (for perfect tip-tilt correction), while the second part allows one to widen the sky coverage at the expense of resolution. For AO designers, it enables one to separate the performance requirements of the high order and tip-tilt correction.