As said in previous posts, Strehl is a complex thing. And optical quality is more than a mere matter of Strehl: mono/poly Strehl, add to this surface roughness, class quality, quality control, spherochromatism, control of internal reflections, differences in focal ratios, and whatnot, and you end up having a complex multivariate problem which is hard to give a simple answer to. IME the most reliable predictor of a refractor's performance is its price tag.
Actually, for amateur level optics, its not that complex. Respectfully, what makes it complex are those folks who do not understand what a Strehl value is; how such measurements are taken; and what a high (or low) value really means to the goodness of a lens or mirror.
Strehl is the measure of how much monochrome (single wavelength) light an optic is putting into the Airy disk of a star delivered to the image/focal plane. Polychromatic Strehl is the estimation of how much light all wavelengths the optic is placing into the Airy disk. Polychromatic Strehl will always be lower (worse) then monochromatic Strehl. For us hobbyists, the generalization can be made that the more light a lens (or mirror) can place into the Airy disk, the better, higher quality it is.
What does Strehl really mean? A theoretically perfect Strehl of 1.00 means that ~84% of the available light is placed into the Airy disk with the remaining ~16% of the light placed into the diffraction rings. The more common .950 Strehl means that 95% of the ~84% of the light is placed into the Airy disk with the rest going into the rings. A barely diffraction limited telescope with a Strehl of .80 means only 80% of the available ~84% of the light goes into the Airy disk
For me, for a Strehl value to be meaningful the overarching point is that it must be generated by an honest-to-goodness interferometer and data reduction software. You may come across official looking test reports (mostly accompanying mirrors) that use the term "Calculated Strehl". This is a tip-off letting me know that the data comes from some qualitative test method (like Foucault or star test) and not an interferometer. Some other important things to know:
(1) Wavelength the optic was tested.
(2) Experience of the operator
(3) How many data points were used - this ranges from under 100 to many thousands
(4) Did the results "jive" with other test methods used. Very important. Multiple test methods should agree if any one method is to be deemed accurate.
Its important to note that most interferometers use a red laser. This presents a problem for us because most (if not all) our refractors perform best in green light. Red and green come to focus at different points so a Strehl measured in green will not be the same as one measured in red. Red lasers are more common and if the wavelength is not listed assume its red. And, to make matters worse, you cannot simply use algebraic math formulas to scale from red to green when evaluating a lens. This is because of spherochromatic aberration (spherical aberration as a function of wavelength). Mirrors do not suffer from spherochromatism because they do not refract light, so you can scale from red to green using simple math. For mirrors, I have found red lasers to be sufficient and usually do not go to the trouble to scale to green.
You mentioned some other things like surface roughness and glass quality and these are good to know. Over the last couple of years, I have grown fond of the Root Mean Square (RMS) metric. Basically this is an expression of the smoothness of the lens surface and is usually one of the measurements listed on the Interferogram.
Get a telescope from a manufacturer with a long history of excellence. The price will usually hint at the optical quality.
Yes, agree. You often get what you pay for. You will not get what you do not pay for. Learn to test the scope yourself - you don't need an interferometer.
Edited by peleuba, 04 May 2021 - 09:07 AM.