Stop down Mask for Telescope - ES102 Explore Scientific  SLIDE ON, cap fit, External fit, 120mm diameter fit. 2.25 inch 55mm center hole . Meant to reduce brightness of an object  like the moon or planets on fast reflectors. It also increases the contrast of bright celestrial objects. It would also reduce chromatic aberrations in a fast refractor telescopes. Condition is New. Shipped with USPS First Class. Software Designed and tested. Works great with these and similar models. 3d printed and designed by Spencer 3d. As always fast USPS SHIPPING. 2 TO 3 DAY SHIPPING. BLACK . Other colors and types available just ask.