When I purchased my 6-inch, f/15 achromatic refractor in 1990 it had a 2-inch focuser, but no 1-1/4-inch adapter. I called Jan Seyfried of University Optics and asked if he had any for sale. He said “No, but I’ll make you one.” Since he was making it, I asked him if he could put filter threads in it, and he did. He simply epoxied an empty filter ring in the backside of the adapter (see photos).
This adapter has been one of the handiest telescope accessories I have ever purchased. Consider the advantages. (1) I do not have to buy a 1-1/4 inch and a 2-inch filter since the 2-inch one now fits all eyepieces. (2) When observing at high power, I can switch eyepieces as the seeing permits, without constantly changing the filter. And (3) I can use my old 6mm University Optics Orthoscopic eyepiece which hasn’t any threads, and still use a filter. This eyepiece produces 381X in my scope and works great, but the few times I use it doesn’t justify buying a new one with threads.
I have never seen another adapter with threads like this one. What I cannot understand is why in this day and age, don’t all adapters have filter threads. It is such a simple concept. Perhaps there is an optical reason for keeping the filter close to the eyepiece especially in fast scopes, but this system sure works great for me and my long focus instrument. Maybe the opticians in the club can debate this topic. My guess is that un-threaded adaptors sell more filters. Van Slyke engineering is now making an update on Unitron’s Uni-Hex rotary eyepiece holder, which has a filter holder in the body, far away from the eyepieces. This holder suggests that the concept of an adapter threaded for filters cannot be totally amiss. Go make one, you’ll love it.