Took it out this evening and did some testing. Star tests indicate some residual SA, but very nice airy disks of stars.
Frank, I just came off a wonderful 8 years of observing with my Orion 150 MCT. The older version. I picked it up for a steal on closeout and it's been my longest owned and most used scope ever for a variety of reasons. One if which was the fact it worked very well. The other two reasons are retirement and excellent tropical seeing. I have not seen a heat plume in years (in our modest tropical climate) by cooling the OTA with a frozen medical gel pack. Perfect star images straight out the door.
Well, not perfect. Some folks may differ, but I am convinced you will see some residual spherical aberration. It may look a bit over corrected. That is perfectly normal and by design (presumably without an aspheric primary). The reason is, well, you are not star testing a parabola. The MCT is a complex design, after all, so we cannot rely on Suiter's simulations of obstructed lower order spherical to guide us because that is not what we will see.
By design, the meniscus throws out a lot of higher order spherical and the primary's lower order spherical cannot correct for it completely. So, the wavefront will have some residual higher order spherical and this will show up in the star test making star testing a little less intuitive and a little more difficult to evaluate. Residual higher order can be quite high without affecting diffraction limited performance, something like 0.4 waves PV. You'll see some of that in the star test and likely with a low enough RMS to put you into the 90's Strehl, regardless.
Rest assured, it's very likely a good scope by design and it's an example where the view in focus is what matters. I am confident you will find sharp and high contrast in focus diffraction, that's what matters. So, don't rely on simulated images of lower order spherical to tell you much about how this scope is performing. That's not the design waveform of a Mak, it's more complicated than that. The star test can be deceiving.
If you want to read more: https://www.telescop...k_spherical.htm
"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."
Edited by Asbytec, 18 August 2019 - 07:58 AM.