Tele Vue's 2" diameter extension tubes, available in 2" & 3.5" lengths, are handy for retaining the optical path length when replacing a star diagonal or when focus is moved out by eyepiece projection & Barlow photography. They accept 48mm filters and have anti-reflective threads & extra-long locking screws which aren't likely to fall out by accident. Tele Vue's adapters use a brass clamping ring to fully encircle the device you're attaching, as opposed to other's efforts which just use a set screw.