It is normal for refractor type telescopes to exhibit field curvature. What this means is that the focal plane of the scope is actually a small section of the surface of a sphere. In case you are not aware the sensor in most cameras is completely flat, so you might imagine that depending on the radius of the field curvature it can be impossible to have all the stars in the field in sharp focus at the same time.

A field flattener is a corrector lens which acts to increase the radius of the curvature effect, thereby reducing the difference in optimal focal distance between the centre of the camera sensor and its corners. The larger the radius of curvature, the less its effects can be seen in the final image. When the system is properly optimised, the difference in focal position for the centre compared with the corners is smaller than the Critical Focus Zone for the scope. In order to work correctly though, the spacing between the corrector lens and the camera sensor has to be precisely adjusted.

More information about this field flattener can be found here.