Astromania Dovetail Mounting Base with 2 Holes for M4 Screws Fits any diameter telescope tube except Schmidt-Cassegrain telescope optical tubes, The base is made of die-cast metal and features a large thumbscrew lock. Includes two M4 screws. Compliable with finder scope, Laser Pointer bracket or reflex sight bracket on telescopes, great for adding a second or third aiming device to your telescope. Drilled (but not threaded) to accommodate screws up to 4mm in diameter. Center-to-center screw hole spacing on your telescope must be 20mm (0.79") in order for this dovetail base to be compatible. Made of high strength Aluminum with Anodizing Aluminium Process. Not intended for use on Schmidt-Cassegrain telescope optical tubes. Ideal for custom installation of any finder scope, EZ Finder reflex sight, and Green Laser Pointer bracket on any telescope.