Maksutov lenses are catadioptric systems of lens and mirrors that allow a large (and fast) diameter to be designed at a lower cost respect to lenses. Also compactness is better. This solution unfortunately has drawbacks, such as the quality of defocused objects, that shows a ring of light. This is not important in astronomy however, being all stars at infinity. No stopping is allowed, so here below you can find only the focusing test.
I've tested this lens with my Canon 5D (full frame) using a 42x1 to EOS ring adapter.
Focusing is very critical and in my sample I've found the best sharpness to be 7 mm at left of infinity mark. Focusing is hard and a Live-View camera is highly recommended. Also focus stability is critical and I recommend to frequently check the focus.
Optical quality is quite good all over the field, and even vignetting is very good.
Moreover this lens can be used as a spotting scope or small telescope thanks to it's 79 mm diameter and the provided eyepiece. I've tested under a very dark sky, and I've found the images to be quite dark respect to equivalent telescopes, so I advise to use it in the daytime.