Strehl ratio
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The modern definition of the Strehl ratio is the ratio of the observed peak intensity at the detection plane of a telescope or other imaging system from a point source compared to the theoretical maximum peak intensity of a perfect imaging system working at the diffraction limit. This is closely related to the sharpness criteria for optics defined by Karl Strehl[1][2].
Unless stated otherwise, the Strehl Ratio is usually defined at the best focus of the imaging system under study. The intensity distribution in the image plane of a point source is generally called the point spread function. Obviously, characterising the form of the point-spread function by a single number, as the Strehl Ratio does, will only be meaningful and sensible if the point-spread function is little distorted from its ideal (aberration-free) form. This is valid for well corrected systems that operate close to the diffraction limit. This includes most telescopes and microscopes, but excludes most photographic systems, for example. The Strehl Ratio has been linked via the work of Marechal to an aberration tolerancing theory which is very useful to designers of well corrected optical systems, allowing a meaningful link between the aberrations of geometrical optics and the diffraction theory of physical optics. A major snag with the Strehl Ratio as a method of image assessment is that, although it is relatively easy to calculate for an optical design prescription on paper, it is normally difficult to measure for a real optical system, not least because the theoretical maximum peak intensity is not readily available.
The ratio is commonly used to assess the quality of seeing in the presence of atmospheric turbulence and assess the performance of any adaptive optical correction system. It is also used for the selection of short exposure images in the lucky imaging method.
[edit] References
- ^ STREHL, K. 1895, Aplanatische und fehlerhafte Abbildung im Fernrohr, Zeitschrift für Instrumentenkunde 15(Oct.), 362-370.
- ^ STREHL, K. 1902, Ueber Luftschlieren und Zonenfehler, Zeitschrift für Instrumentenkunde, 22(July), 213-217.
