2" filters: What thread pitch?

[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

[sculptor]

Hmm. Not getting the avalanche of keen and excited replies that I'd hoped!

To help trigger some replies, here's what I did: The female thread on the BinTel eyepiece has a (mean) minor diameter of 47.33 mm, although it is somewhat out of round. So I bored my brass tube to 47.33 mm. If the thread really is ISO, either metric or imperial, and close to 0.75 mm pitch, the depth of thread should be 0.406 mm by definition, with a "radius" of 0.108 mm in the valleys. If I'm using a non-radiused 60 deg carbide tool, I should use a thread depth of (0.406+0.108) = 0.554 mm because a 60 deg triangle sticks out one radius further than the enclosed circle. Therefore, I should bore to 47.33 mm, and then move the cross-slide a further 0.554 mm. Since the cross-slide is calibrated in diameter, not radius, I should change the reading by 1.108 mm. Did all of that. For what it's worth, the minor diameter is still 47.33, so I didn't overdo the threading step. The filter screws in (somewhat reluctantly, wanting to cross-thread), but only engages by 0.5 turns, whereas in the BinTel it engages by a full turn. What am I doing wrong?

Suspect I'm gonna have to cheat, find a dud filter from somewhere, scrap the filter itself, keep the frame, and superglue it in. Then screw the "real" filter into the female thread on the dud. Blecch!

[ausastronomer (John Bambury)]

Hi,

The thread is 48mm x .75mm pitch. Paint on the threads used for blackening often makes them difficult to attach. Clean the threads up with a metal scribe point and remove any paint globules and it should fit fine.

CS-John B

[gbeal]

And if you do go the cheating way, locate an old ugly 48mm camera filter and use the ring from that.

[sculptor]

Ok, right thread and right dimensions. Looks like I'm just a lousy machinist. I'll have one more go, and if that fails, then superglue and an old filter to the rescue.

Muchos Gracias!