This accessory is necessary because the refractors (glasses) provide a curved focal plane, the stars are not point near the edges. The reducer / corrector provides a corrected image to the edge of the field. Screws directly to the bezel's crayford (M56 thread). Optimized for Sky-Watcher 80ED f / 7.5 Black Diamond glasses (Doublet), this screw-in reducer / corrector reduces the viewfinder's focal length by a factor of 0.85x (induced focal ratio of f / 6.375).
The use of a focal reducer in astrophotography also reduces the exposure time. You will need to use an M48 wide field T-ring dedicated to your camera, available from Sony, Nikon, or Canon.
CAUTION . Not suitable for Sky-Watcher ESPRIT 80ED glasses (triplet).
M56 female at the front
M48 male at the rear