Cassini Prism diagonals increase the viewing comfort and usability of Refractor Telescopes. True astronomical telescopes will produce an upside down image. No erecting lenses, mirrors, or prisms are added. Having these elements in the telescope can cause images to be dim and or distorted making terrestrial viewing (land viewing) difficult. Cassini Prism Diagonals will turn the image in your telescope right side up to enable easy, comfortable terrestrial viewing. The Cassini .965″ Mirror Diagonal Features and Benefits: Format: .965″