Vixen's SD Reducer HD Kit consists of a field flattener lens, a focal reducer and connecting tube rings. These are the components required for prime focus astrophotography with the new SD81S, SD103S and the SD115S optical tubes. The SD Reducer is made to be used in combination with a wide photo adapter (sold separately).All lens surfaces have AS coatings delivering over 99.9% light transmission. The Flattener lens improves images to the edge of the field of view while closely maintaining the original focal length of the telescope. This covers a full frame imaging sensor for prime focus astrophotography. The Reducer HD reduces the focal length to .79x while keeping the image flat and sharp to the edge of the field of view.