This image was taken from the back yard in the city and was done to check the star roundness and field flattness. The stars are quite good across most of the field. There is a small amount of curvature out at the edges of the field but otherwise the stars are basically tight and round like they should be.
The Maksutov-Newtonian telescope design uses a large corrector lens at the front of the telescope to create a flat field with no coma or curvature. This image shows that the design works quite well. The small amount of curvature at the edges is truly minor and is not noticable except at fairly high zoom.