Monday, October 30, 2006

"Oh, No, Not Another Learning Experience!"

That's the poster in someone's cubicle at work. I've had days where I have had far more learning experiences than I deserve.

Today's learning experiences involved machining. Telescope eyepieces come in two common sizes: 1.25" and 2" diameter barrels. (There used to be a Japanese standard size of .965", but it seems to be going extinct.) Many newer telescopes have a focuser that accepts a 2" eyepiece--and usually, they include an adapter that slides into the 2" focuser so that you can use 1.25" diameter eyepieces as well.

One of the advantages of the 2" diameter eyepieces is that you have a much larger piece of glass, and a somewhat wider field. It's much more pleasant.

The other advantage of the 2" diameter eyepieces is that longer focal lengths are commonly available with the bigger pieces of glass, which is useful for getting lower magnification and therefore a wider field. (Trust me, there are times you want less power, and a wider field, especially on galaxies.) For example, I don't think I've ever seen a 1.25" eyepiece with a focal length longer than about 55mm. But I have a 2" eyepiece with a focal length of 85mm.

Now, my 8" reflector, wonderful piece of optics that it is, only a 1.25" focuser. I discovered that by holding some of these 2" eyepieces above the focuser, I could actually get a pretty respectable image--and a very wide field (which is useful when hunting for faint objects that don't show up in the finder scope). Yes, I could buy a 2" focuser and install it, but that's quite a bit of work, and might involve moving the mirror mount within the tube, which means drilling fresh holes and patching the old ones--too much work.

Now, I've seen adapters that let you put a 2" eyepiece into a 1.25" focuser, and no, they aren't made by Mobius Strip Enterprises. Obviously, the adapter goes into the 1.25" focuser, and the 2" eyepiece sits above the focuser. The downside is that the adapter might reduce the edges of the cone of light coming from the telescope's mirror, making the image slightly less bright. Of course, the cone of light is often quite small, anyway, to get into a 1.25" eyepiece, so the loss is usually pretty trivial.

But what the heck, I've got a lathe, I'll make the adapter that I need! This should be really quick! And that's where the lessons were learned.

1. Start out with a 2.25" diameter, 3.5" long piece of Delrin.

2. Face the ends so that you have a perfect cylinder, with 90 degree angles on both ends.

3. Drill a 1 7/8" diameter hole 1.25" deep in one end, using the drill press. This isn't perfectly centered, but the drill press is powerful, and it's fast.

4. Use the boring tool on the lathe to make that hole exactly 2.00" inside diameter--and the boring tool also corrects whatever slight discrepancies there were in centering of the hole in the drill press. The boring tool makes the hole very exactly centered--within .001" or .002".

5. Drill a 1 1/8" diameter hole in the other end of the cylinder using the drill press, until it cuts through to the 2.00" hole.

6. Use the boring tool on the lathe to smooth and center that 1 1/8" hole (not actually enlarging it more than .01").

7. Now, what should be easy: turn the outside of the end with the 1 1/8" hole down to 1.2450", so it will fit into the 1.25" eyepiece holder. I'm going to cut away the outer 1" of the cylinder for a length of 1.5". This should be easy, right?

Here are the lessons learned.

1. When you start turning a 3.5" long piece of plastic, it is very, very easy for the pressure of the cutting tool to pop the plastic right out of the chuck. (Remember that because the end in the chuck is only a 1/8" thick wall (2.25"-2.00"/2), you can't really tighten the chuck down too aggressively because the plastic bends slightly.). The plastic pops out of the chuck, again. And again. And again. If you support both ends of the plastic, for example, by using a live center in the tailstock for the end that isn't the chuck, this isn't really a problem. But it turns out that having already bored that 1 1/8" hole in the plastic, my live center was too big to grab it. I improvised, using a 3/8" drill chuck in the tailstock to hold that end, but this really wasn't dramatically better than leaving it unsupported.

2. It is vitally important that the cutting tool in your lathe, when turning down a piece of plastic, be exactly perpendicular to the cylinder (that is to say, horizontal). If the cutting tool is much above or below horizontal, it cuts very poorly. I knew this--but didn't bother to check if I was off horizontal until I had seen the plastic piece flying across the garage a number of times.

Next time, I'll turn down the plastic to 1.25" on end end before boring the holes on each end.

And yes, it works very nicely. I was able to use the 85mm eyepiece, giving 17x, and one degree field of view.

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