From the ManufacturerReduces an astrophotography refractor's effective focal length by a factor of 0.8 x, providing a shorter, more optically fast focal ratio. Designed for use with any refractor telescope equipped with a 2" focuser. Enables a refractor to both acquire a larger field of view and to capture bright, detailed astrophotos in considerable less time. Features fully multi-coated, 38mm clear aperture optics housed in a sleek, machined and anodized aluminum body. Requires 55mm of back focus