Celestron 2" Mirror Diagonal with XLT Coatings for SCT TelescopeThere are few telescope accessories that work as hard or matter as much as a diagonal! An integral part of the optical chain, a good diagonal can make a big difference in the quality of the image you see. A 2 inch diagonal is required if you want to use 2 inch eyepieces when observing with your telescope. The larger 2 inch barrel allows you to see a bigger percentage of the telescopes light cone, which works wonders for capturing faint deep sky