This Field Flattener from Explore Scientific corrects and virtually eliminates the inherent spherical aberration at the edges of many curved lenses in refractor telescopes. Uncorrected, this aberration causes distortion in the field of view, often ruining astrophotography or CCD images. The optics on the field flattener are fully multicoated and it fits 2" focusers. This version comes with standard T-mount threads for optional camera-specific T-rings, which are required for proper use.
This field flattener is an excellent choice for cameras with cropped sensors -- APS-C and smaller. It will not provide a flat field when used with full-frame cameras.