The telescope adapter has a diameter of 1.25 inches for insertion in place of a 1.25 inch telescope eyepiece; allows the camera to be put into standard 1.25 inch eyepiece holders The opposite end of this adapter has a C-Mount "1 inch diameter with 32 threads per inch" standard for all "C" inama photography movie cameras For a 1.25" adapter to use C or CS mount CCTV type cameras with telescopes; threaded to accept standard 1.25" filters For one high quality C-mount "male thread" to 1.25" "fit any telescope with 1.25" eyepiece tube" eyepiece convertor for telescopes Watch the moon or even local scenery from your living room HDTV with a C-Mount Video Cam or any device with a VGA socket; the inside of the adapter is fully threaded to minimize reflections; made of black anodized aluminum This adapter make you attach your video camera with a 'C' or 'CS' thread to your telescope for prime focus photography. C-threads are commonly used in most consumer video cameras whereas the CS thread is mostly used in security cameras. The camera must have a standard removable C-mount lens, which is a 1" diameter thread. Will not work on video cameras with a fixed lenses or bayonet mount.Useway: attach the camera to the C-thread of this adapter. Then slide the 1.25" barrel into your 1.25" telescope focuser and you are ready to shoot.Internally grooved to reduce stray light reflection for maximum contrast.