Vixen's SD Flattener HD Kit consists of a field flattener lens, 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, with a 52mm filter thread, is made to be used in combination with a wide photo adapter and T Ring (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. Not usable for visual observing.