This accessory is necessary because refractors provide a curved focal plane; stars are indeed not point-like near the edges in imaging. It provides a corrected image all the way to the edge of the field. A specific adapter will allow you to attach the reducer/corrector to the focuser of your telescope. It is rotatable to optimize your framing and accepts 50.8mm filters. Optimized for the Sky-Watcher Evolux 62ED f/6.45 (Doublet) telescope, this screw-on reducer/corrector reduces the focal length of the telescope by a factor of 0.9x (resulting focal ratio of f/5.8 - apparent field 5.09°). Using a focal reducer in astrophotography also allows for reduced exposure time. You will need to use a dedicated M48 wide field T-ring for your camera, available for Sony, Nikon, or Canon.