Construction: Maksutiv-Cassegrain-Rumak catadioptrical reflector
The MK66 features a catadioptric Maksutov-Cassegrain type design with two spherical mirrors and a full aperture meniscus corrector element.
This MK66 is a good example that the "startest" method of judging spherical aberration and optical quality by differences between the intra- and extrafocal out of focus patterns can't always be applied to more 'complex optical systems' like a maksutov cassegrain. There's a really informative essay by Roland Christen of Astro Physics about this topic: Star Testing Complex Optical Systems
My MK66 shows quite distinct differences between the intra- and extrafocal patterns which would indicate quite bad spherical aberrations for a newtonian telescope. However the quality of the in-focus image of this little Mak is without doubt.