I was digging through some of my old gear a couple days ago, and came across yet another nifty Spiratone device -- a telescope adapter for T-mount thread telephotos. It threads onto the T-mount threads, and has a built-in eyepiece with diopter correction. Works pretty well as I dimly recall --been years. I have an equally old 500/5.6 preset telephoto that I've used it with to observe the moon.