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Hi Alex,
It's a bit more complicated than that. Just knowing the ID of the drawtube isn't going to help much. You need to know the thread pitch and all that as well. Is the inside of your focuser drawtube threaded at all?
The inside of the Feathertouch focuser drawtube is correctly threaded to take a standard 52mm camera filter. These have a 52mm x .75mm thread. Standard astronomy filters have a 48mm x .75mm thread. This is a standard 48mm photography filter thread. Therefore it is a simple process to use a standard 52mm to 48mm photographic filter step down ring to allow astronomy filters to be fitted to the inside of the Feathertouch focuser drawtube.
If your focuser drawtube is threaded you will need to get a special adaptor machined to match the internal threads on the drawtube and then accept 48mm filters on the other side (48mm x .75mm)
I think this is a job for a machining shop. You could take your focuser there and let a fitter and turner, or a toolmaker, loose with it. I would be inclined to get it made out of brass which is more expensive than aluminium but a lot more durable across the thin edges of the threads. Anyone proficient in using a lathe could do it.
Cheers,
John B
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