Designed specifically for their Esprit 150 ED APO f 7 refractor optical tube, the Sky-Watcher 0.77x Focal Reducer Flattener is an indispensable dual-purpose accessory for astrophotography and astroimaging. The field flattener corrects the inherent spherical aberration of curved lenses which, if left uncorrected, causes images to appear distorted at the edges of the field of view. As a focal reducer, it reduces the focal ratio of the scope which translates into wider fields of view with shorter exposure times—allowing you to capture more images in the same amount of time without sacrificing quality.