[sculptor]

I've made an adaptor for my camera so it is parfocal with my 2" eyepieces. It is just a bit of brass 2" tube machined to accept a Canon T-adaptor and fit in a 2" Crayford focusser. I've been trying to machine in a thread to accept a 2" Oxygen-III filter. The filter fits in a Celestron or BinTel 2" eyepiece, and, taking a wax impression and measuring 20 turns with vernier calipers, it SEEMS to have a 0.75 mm pitch. But when I put a metric (60 degree) 0.75 mm pitch thread on the inside of my brass tube, the Oxygen-III filter doesn't QUITE want to go in, even if I make it quite a bit over-size (ie regardless of radius).

Does anyone know what the thread pitch, profile, and diameter really are? Are they perhaps imperial threads, eg 32 or 36 TPI (or perhaps 0.7 mm, although I thought I could measure better than that).

The filter and I are dangling on your word!

Mike