Disappointed with this lens.
The problem though is not an optical one as the lens produces fine images for a reflex lens. The problem is with the design and build. Actually, there are 2 problems.
The first is that the rear mount is of the free rotating kind. To explain. The rear mount requires a T-mount adapter, which allows connection to whatever camera you have (you just specify your required T-mount type at time of purchase). The rear mount rotates so that when the lens is fixed on a tripod, it is easy to align the camera to the vertical or horizontal positions as required, which is very useful. However, what is missing is a locking mechanism to prevent further rotation when the camera is attached and aligned as required. The result is that the camera then immediately slews to one side or other because the rear mount continues to rotate as there is nothing to prevent it from doing so. The only improvisation at the time was to hold the camera to stop it rotating!!! On reflection, I think a long slider plate and some wedging material would do the job.
The second problem and much greater problem, was the focusing ring. This being a manual lens, focusing is quite a critical aspect. The focusing ring on this lens is so stiff and heavy that it required 2 hands to move it with any degree of accuracy. The problem with that is that the lens then moves around like crazy and makes focusing on a specific target extremely difficult even with focus peaking, which doesn't work that well because of potential lack of light and that the target subject is difficult to maintain within the frame whilst trying to focus on it because of the amount of force being exerted on the lens just to move the focus ring.
The result is that from a useability perspective, I much prefer the lighter and cheaper Vivitar 800mm Series 1, which is much easier to work with even though it has 200mm less reach than the Maksutov. Unless there is a way to greatly loosen the focusing ring, this lens will probably never see the light of day again.