What ring mount for a 3" diameter lens

I have a horrible 500mm mirror lens that I want to use as a spotting scope and need a means of mounting it on a tripod. The cheapo ring mounts describe their size in terms of (mostly L) lens types rather than the diameter.

So can anyone tell me what lenses would be 3" diameter at the point you'd attach a tripod ring ?

Thanks,

Steve.

if it's a mirror lens...then isn't it tiny? at least compared to a real 500mm lens...just mount the camera to the tripod

Yes, it is tiny. I want to use it without a camera body. Hence the need for a ring mount to attach it to a tripod.

Steve.

ah...my bad...i didn't think of that until after i had posted...no clue on how to answer that question....good luck though