Lens Cell

 This is the last component. It holds the lens in a cell that can be adjusted for tiny angles by set screws in the tube impinging on the outside of the cell.

The OD is 59mm to fit in a 59.28mm PVC tube. That lip near the bottom is 50.08mm ID (the original tube ID). The bored section above is 52mm ID.  This is just large enough for the 51.90mm OD lens to slide with slight encouragement (not a hammer) down to the lip. I will put a rubber O-ring at the lip to cushion the lens, then another O-ring on top of the lens, then a 52mm retaining ring to keep it in position.

Lessons Learned

I cut a 1.30" section off the tube.  Then I squared it and trimmed it to .875" long.  Then I turned the exterior to 60mm OD where I discovered that the workpiece was not exactly centered in the 3-jaw chuck.  Why? Aren't 3-jaw chucks self-centering? I had removed the jaws to reverse them. While reversing the jaws, I did not reinstall them correctly, so they were not centering the workpiece on the axis of rotation. The exterior trim had the jaws holding the interior of the tube and the 2.5" 3-jaw chuck cannot enlarge enough to firmly hold a 60mm tube. 

Fortunately, one of the useful aspects of a lathe is minor discrepancies like this are not terminal. Once properly centered, turning the tube to 60mm OD meant that I could now hold the tube on the outside and turn the interior to a properly centered bore.

Once turned, I discovered the lens cell would not quire go into the PVC tube which was actually 59.28mm. So back to the lathe to turn off another mm. (I need a little slop so that the set screws in the PVC tube can move the lens cell.)

Cutting the tailpiece tube adapter from a 1" thick piece of Delrin was a win. I am not sure how much of the inability of the extension tube to stay square was that the adapter made of CFC was 1/2" thick and how much was that I bored the entry hole slightly too large, but with the hole 0.05" larger than the extension tube, it is now a snug fit that can be pulled or pushed with just a little effort before locking down with an 8-32 thumbscrew.  (The extension tube is really just a coarse focus with a helical focuser in the end to provide fine focus, held by an 8-32 thumbscrew, so it does not need to move effortlessly.)

I had been worried that I might need to bore a compression ring channel on the inside of the tailpiece tube adapter. Telescope eyepiece holders can go cheap and use just a thumbscrew to hold the eyepiece in place. A little fancier model has a brass compression ring sitting in a channel inside the tube.

The thumbscrew squeezes down on the ring, applying pressure around the eyepiece barrel, thus avoiding marring the eyepiece barrel. These are boogers to do. You need to cut typically a 1/2" wide, .01" deep channel on the inside of the tube to accept the brass. I have done this once at special request for ScopeRollers for an Astro-Physics mount where the finish on the legs is so beautiful you do not want to scratch them. This required making a reverse rake boring bar.

The helical focuser is at lower left. Yes, the PVC tube needs to be painted bright glossy white again.