Filter Size

[Regulus]

Can anyone tell me the actual size of the filter thread for both 2" and 11/4 inch ep's?
The thread diameter is different to the outside measurement of the barrel of these ep's, being internal. I am wondering if it is a 'standard' filter size thread that was used for camera filters also. After all, there were some odd sizes back in the day, with twin lens reflex models, like the Mamiya, having varying size lens apertures and filter requirments for different lenses.
Trev

[MrB]

M48 x0.75mm and M28.5 x 0.5mm I believe.

[cometcatcher]

The 48mm is. Standard 48mm camera filters will screw into 2" eyepieces.

[Regulus]

Thank you guys. That helps.
Trev

[ausastronomer]

Hi Simon,

You're only 1/2 right on this occasion. The thread pitch on 1.25" filter threads is definitely .6mm not .5mm.

2" eyepieces have always been threaded to accept standard 48mm photographic filters, which as you correctly mention is 48mm x .75mm pitch.

The standard threading on photographic filters for decades has always been with a metric .75mm pitch irrespective of filter size. For instance a 37mm photographic filter is threaded 37mm x .75mm pitch, a 52mm filter is threaded 52mm x .75mm pitch and even 82mm filters are still threaded at 82mm x .75mm pitch. Obviously there are some odd bods that slip through the cracks, which are not consistent with the general standard.

The threading on 1.25" eyepieces on the other hand evolved outside of the metric and photographic environment. The original internal threading on the vast majority of 1.25" eyepieces is 1⅛th inches (1.125") with a 42 turns per inch pitch. Converting this to metric gives you 28.575mm x .604mm pitch. Lumicon in the USA still produce their 1.25" filters with the original thread size of 1⅛th inches (1.125") with a 42 turns per inch pitch. Many of the European and Asian manufacturers have morphed this into a thread size of 28.5mm x .6mm pitch. I am guessing this slight diameter variation combined with machining tolerances is the primary reason some filters are a very snug fit and some can be quite loose. The same applies to 2" eyepieces but in this scenario it can be attributed to machining tolerances only.

Cheers,
John B

[Regulus]

Thx John. Very helpful.
Trev

[MrB]

Apologies, that was a typo. Can confirm it is 0.6mm.
Even got my thread pitch gauge out again to be sure :thumbsup: