Sunday, March 11, 2007

Brass Compression Rings

What, you ask, is a brass compression ring? My first telescope eyepiece focuser was a cheap Edmunds rack-and-pinion unit that was a press fit. To put an eyepiece into the focuser, you pressed it in, and the split in the upper part of the focuser tube spread just enough to hold the eyepiece. This wasn't ideal, because it tended to scratch the eyepiece barrel, and the force required often moved the telescope.

My first focuser upgrade was to a University Optics focuser that used a set screw in the side of the focuser tube to hold the eyepiece. Typically these were a 4-40 set screw that grabbed the side of the eyepiece barrel. They were zero effort to insert and remove (once you loosened the screw), but the set screw approach had two annoying side effects: the set screw, being steel, often scratched the finish of the eyepiece barrel; because the friction holding the eyepiece in place is exerted over a very small contact patch, very, very heavy eyepieces, or camera assemblies, might not be held in as tightly as they should.

And so, a few years ago, I started to see focusers that offered a "brass compression ring" instead. What was it? I thought that you turned something on the outside of the focuser tube, and it squeezed down against the eyepiece barrel. Nope!

Instead, you still have a set screw (or perhaps two of them) on the outside of the focuser tube, but instead of directly squeezing on the eyepiece barrel in just one spot, the set screw (or screws) press down on a piece of brass that sets in a channel inside the focuser tube. The brass started out straight, and was bent to fit into the channel. It is not a complete ring; perhaps it is an 1/8" or 1/4" short of making a complete circle inside the channel.

The brass ring's desire to straighten out again prevents it from easily slipping out of the channel. When you put pressure on the brass ring with a set screw, the set screw presses the brass ring against the eyepiece barrel--and because of the confining channel, the ring is pressing against the eyepiece barrel across a big chunk of the circumference of the eyepiece barrel.

Because the pressure is spread out across perhaps 1/4" by 2" of area, you don't have a single point that cuts through the chrome surface of the eyepiece barrel. Because the total contact area is substantially larger than a single set screw (even though the pressure per square inch is less), a brass compression ring can exert more force to hold an eyepiece in place than a single 4-40 set screw.

Anyway, why am I yammering on about brass compression rings? One of my customers has a very high end Takahashi mount, and he doesn't want the 1/4"-20 bolts that hold the ScopeRoller caster assembly onto the legs to scratch up the surface. My solution? I'm going to bore a channel perhaps 1/8" or 1/4" deep inside of the sleeve that goes over the leg, and put a brass compression ring in that channel. The 1/4"-20 screws will not be directly impinging on the legs. Instead, the brass compression ring (which is softer than steel) will be doing the pressing.

Of course, along with ordering some 3" Delrin stock tomorrow (why aren't these ever a size that I already have around?), I have to go brass hunting.

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