The next idea was to try a local machine shop. I explained what I needed, they seemed to totally understand...and then they came back with a quote for $450. No thank you.
So the last resort was to just build one. I found one thread online showing how someone made their own, but I couldn't find a threaded ring I would feel comfortable just gluing in place...and I like building things and thinking through problems, so I built my own:
Here's how I did it!
First, you'll need to buy a 77mm->62mm step down ring. These are available online on amazon and ebay, or at local camera shops. They're much cheaper online; but I didn't want to wait, and I figured I'd support a local business, so I bought mine from a camera shop. It was ~$10.
Everything else I bought came from McMaster Carr, which is without question one of the coolest companies around. They have just about anything you could possibly need to build something from scratch.
Shopping list:
-One push-on round aluminum cap with a 4.5" inner diameter (scroll down for it; steel would be cheaper, but I was worried about it being too heavy)
-One pack of extra strong, 1/4-20, glass filled nylon thumb screws, 1.5 inch long (scroll down for it; the specific size and pitch of these screws isn't terribly critical)
-One pack of low-head metric socket screws, M3 size, 16mm length (scroll down for them; the size of these screws is important...if they aren't this small, then they will potentially hang out into the light path, or keep the etalon from screwing into place)
-One pack of M3 nylon insert lock nuts, M3 size
-One pack of nylon washers, M3 in size (these probably aren't critical, but I was hoping to minimize the chances of galvanic corrosion between the stainless steel screws and the aluminum cap...again, probably not a huge issue, but I figured I'd do it while I was in there.)
All of that came out to a little over $30.
Tools:
-A jig saw with a small blade for cutting tight circles
-A drill and a set of drill bits
-A 1/4-20 tap (this is for the nylon screws; if you want to use a different tap, just buy a different set of nylon screws)
-A small allen head and a small socket to tighten the nuts and bolts down (I don't remember off the top of my head what size you'll need here, but it's small!)
-A file or coarse sandpaper
Steps
1. Here's the metal cap we'll be using:
And here's the threaded ring:
We need to center the threaded ring on this cap and then trace the inner circle of that ring onto the cap. We'll then cut that hole out. To center mine, I started off with this whole complicated plan of using geometry to mark the center of both things, but came up with something much simpler. Use a pair of scissors and your hand as a depth gauge, and then just keep moving around the cap until the ring is pushed in equally in every direction.
Eventually, the black ring will be equally close to the edge of the metal cap in all directions, and voila! It's centered. Trace the inside of the ring with a sharpie.
2. Cut out the circle you just drew. Drill a pilot hole so the saw blade can go in, and then cut the rest of the way around.
(Ignore the other smaller holes for now.) After the hole is cut out, go around the fresh edge with a file to knock down any burrs and smooth it all out. Sandpaper would probably also work here.
3. Drill holes in the threaded ring so it can be screwed to the metal cap. This is really the only step in the whole process that requires a good amount of precision. If the holes you drill are too far to the outside of the ring, the head of the screw will keep the etalon from threading on correctly. If the holes you drill are too far to the inside of the ring, the holes will be very close to the wall of the ring, which could weaken it. The heads will also be hanging out in the light path.
The heads of the screws are about 5.4 mm in diamter, so we want to drill the pilot holes 2.7 mm from the inner edge (5.4 / 2 = 2.7). Measure in with a caliper, then use a center punch to mark that location. This will keep your drill bit from wandering.
The red arrow is pointing to the little dimple where I used the center punch.
Once you've punched a bunch of dimples, drill all the holes. Use a 1/8" or a 3 mm drill bit.
I wasn't too concerned with making sure everything was perfectly symmetric around the ring. The only thing that mattered was that the holes were very close to 2.7 in from the inner edge. Even with the lack of symmetry, it'll still be plenty strong, and nobody will ever see it.
4. Transfer the holes to the metal cap. Not too much to explain here.
5. Bolt the ring in place. I trimmed the nylon washers with some scissors so they weren't hanging off the edge into the light path (that's what all the little plastic bits are).
6. Drill 4 holes around the circumference of the metal cap to accept the nylon thumb screws. You can use pretty much whatever size screws you want here. I used 1/4-20 because I had that tap on hand. If you use a 1/4-20 tap, you'll drill a 13/64" hole. If you use a different size screw and tap, then read the instructions for that tap to find what size hole you need to drill.
Once the holes are drilled, tap the holes so you can thread the nylon screws in.
7. Paint the inside of the cap flat black to minimize reflections.
8. Enjoy your new adapter plate!
It's attached securely enough with the four nylon screws that I can pull my tripod over before the cap can be pulled off. I'm probably still going to add felt inside of the cap, add two more nylon screws, and add a little leash so that it's all tied to the telescope, though. It would really suck if it all fell off. :X
The nice thing about this is that if I wanted to mount it to a larger or smaller telescope, I could just buy a larger or smaller metal cap, and transfer the threaded ring over to it. This would mean another adapter for <$20.
If anything is unclear, let me know! :rockon:
