"One of the most frequently used optical terms in both, professional and amateur circles is the Strehl ratio. It is the simplest meaningful way of expressing the effect of wavefront aberrations on image quality. By definition, Strehl ratio - introduced by the German physicist, mathematician and astronomer Dr. Karl Strehl at the end of 19th century - is the ratio of peak diffraction intensities of an aberrated vs. perfect wavefront. The ratio indicates the level of image quality in the presence of wavefront aberrations..."
"...expressing vital property of the single most important optical indicator, the PSF, building stone of nearly all intensity distribution forms..."
"Wavefront deviations from perfect spherical are directly related to the size of phase errors at all points of wave interference that form diffraction pattern. In other words, it is a nominal wavefront deviation from spherical that determines the change in pattern's intensity distribution."
"It is the root-mean-square, or RMS wavefront error, which expresses the deviation averaged over the entire wavefront. This average wavefront deviation determines the peak intensity of diffraction pattern and, hence, numerical value of the Strehl ratio (note that the RMS error itself is accurately representing the magnitude of wavefront deviation only when it is affecting relatively large wavefront area, which is generally the case with the conic surface aberrations)..."
"For relatively small errors - roughly 0.15 wave RMS, and smaller - the RMS wavefront error, and the resulting Strehl ratio, accurately reflect the effect of overall change in energy distribution, regardless of the type of aberration. With larger errors, the correlation between the RMS error and the Strehl vanishes..."
"As a general rule for aberrations below 0.15 wave RMS, the relative drop in peak diffraction intensity indicates how much of the energy is lost, relatively, from the Airy disc."
My simplified take on Strehl is derived from the above.
There may be no actual standard and different ways to measure Strehl, both mono and ploy chromatic, there are many factors that give us cause. However, every telescope will have a Strehl of some measure even if we do not know what it is. It is related to the peak diffraction intensity of an actual aberrant wavefront Airy disc to that of a perfect aperture (which we do know). Diffraction at the aperture is unavoidable and known, and therefore can be factored out. Diffraction effects can be increased with an obstruction. To my understanding, the Strehl ratio is not affected by added obstruction diffraction or shading other than blocking the central zone from measurement of any wavefront deviation.
Whatever the Strehl, what seems to be implied is an intensity distribution from the central disc to the diffraction rings due to aberration. The central peak intensity is reduced, and the surrounding ring intensity is increased. This causes a loss of contrast on small scales near the first diffraction ring and slightly beyond, and quickly normalizing at larger scales. The intensity distribution to the diffraction rings is increasingly detrimental to contrast performance on small scale planetary detail (and tight double stars) as aberration increases. Thus, small scale planetary resolution is reduced but not the Rayleigh and Dawes resolution limits.
An obstruction also compounds matters by further redistributing light from the Airy disc to the rings through added diffraction changing the way the waves interact in phase. Obstruction shading has no affect on the intensity distribution, however. Theoretically, the obstruction changes the pattern of diffraction, and it /may/ cause an increase in performance at high spatial frequencies near the angular diameter of the Airy disc. (In practice, I find this tiny realm very difficult to observe even in excellent lab-like seeing. I have tried to see this theory in action and failed. )
I understand Strehl to be a measure of peak diffraction intensity of an aberrant system, and it already factors in the necessary and unavoidable loss of intensity due to diffraction at the aperture. Some argue the same should apply to the peak diffraction intensity including the obstructed (negative) aperture, which is less than the maximum 83.8% light in the central disc. This offers a way to compare relative performance between obstructed and unobstructed types, which is nice to know if this is what we want to do, but it looses it's beauty of speaking to performance due to aberration alone.
Edited by Asbytec, 21 January 2019 - 06:57 AM.