That Teleskope-Spezialisten link is very intressting to scroll through.
It is interesting to see the actual star and Ronchi tests. One thing I find interesting is the MCT star tests, they look pretty ugly and different of both sides of focus. But, we have to be more careful when evaluating the star test of these complex designs because they do not test like a parabola which can form a more or less spherical wavefront with some classic primary spherical present. MCTs have a large amount of secondary spherical induced by the meniscus as a function of it's design. However, the combination of higher order SA from the meniscus cannot be canceled by lower order SA from the primary. They will not test in the more classic sense of primary spherical aberration alone including the shadow breakout (the preferred 33% obstruction aside). And if done well, the result is a very low RMS value and a high Strehl.
The result is not a spherical wavefront, but one that approximates a sphere by smoothly undulating around a reference sphere. The resulting undulating waveform affects the star test and they will not be the same on either side of focus as each zone comes to focus in a slightly different point. If you notice, almost consistently, the MCT has a brighter outer ring outside focus (marginal zone) and some indication of an inner ring inside focus (paraxial zone). It's almost and generally like an undercorrected condition with each zone focusing differently. This also seems to show in the Ronchi tests, but it is not a classic undercorrection of primary spherical alone and the star test should not be interpreted as such. I believe this is what Roland was talking about in his (in)famous article on complex designs. They can be made to "please the star test crowd" using an asphere and touching up some zones, but performance in focus is not really improved.
What the star tests seem to be telling me is the effect of balancing the higher order from the meniscus with opposite amount of lower order from the primary is working as deigned and it suggests no aspheric term was employed (often debated and seen in the Meade). The desired wavefront is being achieved somewhat consistently. Then performance is improved further with balancing with defocus to the point along the caustic of best diffraction focus (at the ~70% zone) where spherical aberration is again minimized.
The MCT has no higher order term built into the meniscus as an SCT does, and you may notice the SCTs tend to test more in a classical sense (with the higher order term dealt with) and any residual lower order SA can be easily seen. Not so in the MCT because of it's complex design. This ugly star test does not mean the MCT is a bad optic. In fact, there can be as much as 0.4 PV wave of higher order SA and the MCT will still be diffraction limited. This is because each undulation of the resulting wavefront closely approximates a reference sphere (perfect wavefront) and each zone covers only a small area of the wavefront and the RMS can be very good.
Fast APOs also have residual higher order spherical due to their steeply curved surfaces. In this sense, Maks kind of are refractor-like.
Some pertinent quotes from the link above:
"When the optical system gets more complex than a simple parabolic mirror, then there are inherent aberrations that affect the star test."
"Machines exist now that can lay down a 1/20 wave or better spherical surface on a piece of glass without any human intervention."
"In this pure form, the Mak-Cass has left over 5th order aberrations and, depending on design, these can be less than 1/10 wave on the wavefront."
"By the way, fast Apo refractors have these same aberrations also. The RMS value will be better than 1/50 RMS and the Strehl ratio will be exceedingly high."
"When tested on the night sky, the inside and outside diffraction patterns will be quite different."
"I can tell you that it is easy to do some rough compensation with quick local polishing at several zones to get more equal inside and outside star patterns, but the result will almost certainly be a loss of contrast."
More info here:
"An interesting aspect of the commercial Maksutov-Cassegrain is the question of its star test. There is a notion that its optics has special properties, making it sort of exception in that its intra and extra focal pattern are not supposed to be identical, even when it is near perfectly corrected. Or, put somewhat differently, that it doesn't need to have near-perfect star test for near-perfect performance.
The answer to this special status is in its higher order spherical aberration. Due to its steeply curved optical surfaces, especially those of the meniscus corrector, Maksutov-Cassegrain systems generate 6th-order spherical aberration that can't be cancelled (w/o aspheric surface terms), only minimized by balancing it with the 4th-order aberration. While roughly as much noticeable in the star test as the lower-order spherical aberration for given P-V wavefront error (FIG. 189), the balanced form is considerably less detrimental to image quality."