I realize this is a very old thread now, but I wanted to provide an update with my experience using astronomy eyepieces on the ED50.
First of all, I want to thank Bill for offering to send me two of the threaded barrel adapters so I could use astronomy eyepieces on my scope. I started out with the 13-30 Nikon eyepiece, and that was fine, but I was intrigued by the idea of being able to use fixed-power eyepieces with a wider field of view.
My first eyepiece was the 20mm SWA recommended by Bill. I could quickly see why this was his favorite. Although only providing about 12x magnification, the huge field of view and bright, sharp image was just so pleasing to the eye!
My next eyepiece was a 15mm SWA, and that was also excellent. It was very small and provided a very good image. Comparing it to my 13-30x Nikon, the image quality was nearly as good as that eyepiece at 19x, but the field of view was quite a bit better.
Both of the eyepieces above were essentially "plug and play," needing only to unscrew the 1.25" barrel and fit the threaded collar provided by Bill. It couldn't have been much easier.
Recently, I wanted a little better quality eyepiece so I took a chance and ordered the Pentax 12mm, not knowing for certain whether it could be fitted to the scope with Bill's adapter rings. When I received the eyepiece, I was immediately struck by the quality and robust build. Compared the the less expensive 15 and 20 SWA's, the Pentax is in another class. Unfortunately, it was not "plug and play" because I could not easily remove the 1.25" barrel like I could on the SWA eyepieces. Nothing I tried would get it to unscrew... Until...
I happen to have the tools to do some amateur gunsmithing, and one of those tools is a barrel vise. As a last resort, I put the barrel of the Pentax eyepiece in my barrel vise (Yikes!), wrapped the top portion of the eyepiece in tape, and got out the pipe wrench (double Yikes!). I was either going to get the barrel off or break the eyepiece trying. After some effort and a few four-letter words, the barrel finally relented and unscrewed from the body of the eyepiece. The damage done was minor and mostly cosmetic. Nothing that couldn't be touched up with a black sharpie.
Upon removing the 1.25 barrel from the body, I realized that unlike the hollow barrels on the 15 and 20mm SWA's, the Pentax barrel has a lens inside of it! At that point, I wasn't sure if the eyepiece would even work, but I muddled on anyway with the hopes that I could salvage something out of this mess I had gotten myself into...
From there, fitting the Pentax body to the ED50 was fairly straightforward because (fortunately for me) the outside thread on Bill's adapter fit both the ED50 scope threads and the inside of the Pentax eyepiece body. Finally something went right!
Unfortunately, I wasn't done yet. The eyepiece was a thing of beauty and functioned perfectly, except that it wouldn't focus to infinity (Grrrrrr...). What to do, what to do... It was awfully close though, and I knew that if I could just reduce the distance from the eyepiece to the body of the ED50, I could get infinity focus. So out came the metal file! (Triple Yikes!!!). And I filed off about 1/16" from the bottom of the eyepiece body so when it was screwed into the scope, I could achieve infinity focus. AND IT WORKED!!!!!
So here's the final result. Infinity focus is just before the focuser limit is achieved. What that also means is that this scope/eyepiece combination will now focus to just about 10 feet as well! I'm not sure the situations that I'll need such close focus, but I'm sure I will find one.
I have no idea what the resulting magnification is, or how to measure it, but I don't think it's the 23x estimated by Bill at the top of the thread. I'm sure that's due to removing one lens that stayed with the 1.25" barrel I removed. However, the image quality is just SUPERB, and I estimate the final magnification to be something around 15-17x, which is really good for my needs, as I tend to prefer the brighter images that lower magnifications provide anyway. With just a 50mm objective lens, anything over 20x will be too dim to be useful in my opinion, especially at those "magic hours" of dawn and dusk.
At the end of the day, I'm rather thrilled with how this experiment worked out. I was comparing this ED50/Pentax 12mm combo to a Leica 62 spotter the other day, and the little ED50 more than held it's own against the Leica at 16x. Truth be told, I preferred the image from the ED50/Pentax combo a fair amount more. It was at least as sharp, if not sharper, it was considerably brighter, and the field of view was much better. As far as I'm concerned, I have found my eyepiece for this lovely little spotter! If you're brave enough to try this, and have the tools to remove the barrel from the Pentax 12mm, you will end up with an amazing eyepiece for your ED50.