Is there a 2" filter adaptor for a 1.25" thread?

[mental4astro (Alexander)]

Hi all,

There are these 2"/1.25" hybrid eyepieces. No problem there. BUT if you only have 2" filters, you can't always use the typical 2" to 1.25" reducing bushing tu use them. The 2" sleeve say of the Nagler 12mm doesn't have a 2" thread, so you are limited to using 1.25 " filters. In my Newtonian I can't use 2" filters as I wouldn't get the in-travel needed in the focuser.

Is there an EXPANSION bushing to thread into a 1.25" thread and allow the use of 2" filters? The expansion bushing/filter combo would then slide into the 2" focuser ahead of the 1.25". It would mean needing fewer filters just to be able to use these 2"/1.25" hybrid EPs.

Mental.

[ausastronomer (John Bambury)]

Hi Alex,

I get around this by having a 52mm to 48mm step down ring threaded to the bottom of my Feathertouch focusers on each scope; and loctited permanently in place. I just screw the filter on to the bottom of the focuser from the front of the scope then change between 2" and 1.25" eyepieces at will.

Maybe you can make something up to fit on the bottom of your focuser drawtube.

Cheers,
John B

[mental4astro (Alexander)]

Interesting solution John. It's got me thinking of another idea I saw if a filter slide that sits in front of the focuser. I can use a step down ring as you suggest and mount it to slide into a little frame that sits in front of the focuser. Thanks mate!

This would calm my nerves of dropping a filter onto the primary!!!

[mental4astro (Alexander)]

One question about measuring the diameter of the focuser & selecting the correct step down ring, do I quote the inside diameter of the focuser's thread or do I add 1mm to the measurement I get from using the vernier calipers?

[ausastronomer (John Bambury)]

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

[mental4astro (Alexander)]

Thanks for that John.

I've measured the inside of the focuser which is threaded. I also found a 52mm thread and it did wind on. I then found a 52/48 step down ring on eBay for $1.80 delivered. I'll see how this ring goes before taking it further.

My first idea of a 2"/1.25" adapter still does interest me as it would again reduce the need for multiple filters. I wonder if anyone knows of someone who could machine one? Please let me know & I'll contact them.