1.25" and 2" filter thread sizes

[sheeny (Al)]

Does anyone know the actual specification for the threads for 2" and 1.25" EP filters?

I measure the 2" filter thread at 48mm (assumed*) x 0.6mm pitch and the 1.25" filter thread at 28.4mm x 0.6mm pitch. 28.4mm is an odd metric size, but it is very close to 1 1/8" which makes sense, but 0.6mm equates to 42.333 threads per inch, which is a strange imperial size. Maybe it's 42 TPI? 0.6mm thread gauge fits but it could be a better fit...

*ID measured at 47.5mm, but I know 2" filters are often talked about as 48mm. Unfortunately, I don't have a 2" filter to measure the OD of the external thread.

The reason is I want to make an adapter so I can screw a 1.25" extension tube into the back of my 1.25" to 2" EP adapter... this will allow me to push my Staranalyser up the baffle tube of my C11 to maximise dispersion while maintaining focus. At least that's the plan...

Al.

[sheeny (Al)]

Hmmm... I might have answered my own questions:

Q: What are the thread sizes on Lumicon filters?

A: The 1.25" filters employ industry-standard 1.125" x 42 tpi threads (except for Questar/Brandon, which use a thread pattern unique to Questar). The 48 mm filters employ industry-standard 48 x 0.75 mm threading, which thread into 2" eyepieces. All larger filters use the given size with 0.75 mm pitch (e.g. a 72 mm Minus Violet filter has 72 x 0.75 mm threads).

I found that on the Lumicon FAQ page: http://www.lumicon.com/faq.php

The only problem is the thread in the back of my 1.25"to 2" adapter is definitely not .75mm pitch... it's 0.6mm pitch.

Any one have some thread gauges that can verify? (Or doesn't this thread fit a 2" filter???)

Al.

[kinetic (Steve)]

Al it's probably not much help but I will try and dig out the speil I wrote
down when I cut some 1.25 filter threads in the lathe for my filter wheel.

I have thread pitch guages and I vaguely recall that with the metric gauge
none were spot on but 0.6 was closer than 0.75 or vice versa.
And I think that I cut them with the 0.75 setting and not the 0.6
on the thread cutter gears. I think! i will check that.
I found that with just one or two bites in the thread at that scale
it didn't matter too much.

My Baader 1.25" only just grabbed in the thread I cut and the DSI
Meade set were snug. Can't help at all with the 2" size though.

Steve