"I've read several articles about Maks and there was one, very involved and above my level that said because of their steep optical surfaces, the inside and outside focus patterns often show overcorrection. But, that is a design parameter, not a flaw."
Do you have a link? I am having a hard time with the star test image posted. Hard to make out, but it does not appear to be 1/8th wave. It looks to be between 4 and 8 waves defocus, I'd like to see the image at 8 waves. The outer soft ring is confusing. Is it a halo of some sort? I have seen very thin halos outside my defocused pattern in good seeing very close to focus (including the color image below.)
Now, in my experience, the Maks I have tested certainly appear to be better that 1/4th LSA, but likely not 1/8th. Even though my own patterns resembled Suiters version of a 33% obstructed, 1/8th PV LSA, when you add some residual astigmatism of coma, which are likely present in very small amounts (like the Skywatcher 150 link below), that drops the total wavefront to about 1/6th PV and the (estimated) Strehl remains relatively high. Maks tend to be very smooth and free of any obvious zones.
There is some question as to whether the smaller Maks are aspheric with an elliptical primary (which may be a form of balanced low/high order aberration by adding lower order of the opposite sign, i.e., lengthening the radius of the primary.) There's a good argument that they almost have to be to perform as well as they do. This complicates the star test, IMO, as we likely will not be seeing textbook LSA. The good news seems to be, inside focus we get a shadow breakout pretty early with energy near the shadow. This is indicative, IMO, of descent correction probably in the vicinity of about 1/6th PV total wavefront deviation, including some residual lower and higher order SA. (I am not sure how to test for HSA, specifically or in conjunction with other aberrations.)
The bottom line seems to be, Maks test pretty well and are likely better than classic 1/4 PV LSA. Below are my star tests of a 150 MCT. It's kind of difficult to know exactly how many waves defocus due to the focal ratio not being exactly f/12 as stated. But, if the math is right, that's near 8 waves defocus as Suiter uses in some of his illustrations. Notice the lower contrast outside focus indicative of under correction (after removing the baffle mod.) But, the important thing is the emergence of the inner ring inside focus which, to me, is not indicative of an obstructed ~30% 1/4th PV LSA pattern. So, I presume the correction is better than that, at least. In focus the patterns look very nice, too. The first bright ring is uniform to within my ability to detect otherwise in the most excellent seeing.
Got a better look at the focused pattern one night, Arcturus I believe.
Testing a complex telescope is, for me anyway, not that easy. Basically, the thing I like to see is energy near the shadow inside and not too far from focus like a more perfect optic has and unlike the dark central pattern of pure 1/4th LSA. This means, whatever they did, they managed to get a more uniform, even if dissimilar, star test pattern and good images.
Below is one I took at just outside (IIRC) focus. Thought is was quite beautiful during the most steady seeing. Not sure what it tells me, other than the seeing was most excellent. If you look close, the first bright ring that breaks out is actually comprised of very thin rings close together. I found that quite striking.
Edited by Asbytec, 14 August 2016 - 02:41 AM.