Archive for March 16th, 2012

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Thinking about cheap finders–and cheap finder mounts

March 16, 2012

This all started because Orion’s Maks are “wrong-handed” for their VersaGo II alt-az mount. Here, I’ll show you what I mean. Here’s my Apex 127 on the VersaGo II.

If the scope is sitting on its dovetail bar, the dovetail shoe for the finderscope is on the left side of the scope, at about 10:30 (viewed from the eyepiece end). So when the scope is mounted sidesaddle on the VersaGo II, where the dovetail faces left, the finder ends up at about 7:30. The eyepiece of the finderscope is a bit below the eyepiece of the main scope (the effect is exaggerated in this picture, which was taken looking down at the mount). This isn’t terribly inconvenient, it just looks weird, and it can cause some slight balance problems when the scope is aimed up high.

What I’d like is to have the finder sticking straight out sideways from the scope. That way the eyepieces would be at the same height, and the altitude axis of the mount would run through the centers of mass of both scopes so there would be no balance problems. I could achieve that by moving either the dovetail shoe for the finder or the main dovetail rail, but that would require drilling holes in the scope and I’m not willing to do that. A better solution is just to get some tube rings, so I can orient the scope and the finder shoe however I want.

Thinking about that led me to think about how nice it would be to have a small refractor mounted alongside the 5″ Mak. Something in the 70-80mm range could function as both a “superfinder” and rich-field telescope, so on one mount I’d have a low-power, widefield scope and a planet-killer.

Stellarvue sells an 80mm superfinder that some folks use as a stand-alone rich-field and spotting scope, but that runs something like $250. I’m sure it’s nice, Stellarvue gear is top notch, but as always I am interested in less expensive options. Celestron’s Travel Scope 70 is not much smaller,  it’s gotten generally good reviews (at CN, for example), and it can be found for $60-80 (the Amazon price fluctuates a lot, but other vendors usually have it for $60). That would work for a finder, but I’d have to mount it somehow. I could just buy some mounting rings, but adjustable mounting rings for a 70mm scope would cost more than the scope itself. There has to be a better way.

So that’s the first thread: moving up from a 50mm finder without breaking the bank.

The other issue is that I have several scopes that I use regularly, and only one 9×50 RACI finder. So I keep moving the finder around, and this is kind of a pain, because I have to realign it for every scope. It would be nice to just park it on one scope, but that means I’d need finders for the other scopes. As before, I could just buy some more RACI finders, but the 6×30 models are about $60 and the 9x50s, which I really prefer, are $90 or more.

Now, I could build my own finder. I have spare 50mm objectives from some cheap binoculars, and I have an erecting prism diagonal, and I could build the tube out of plumbing parts. But that still leaves the problem of mounting, and as before, the mounting rings would set me back almost as much as a new finder anyway.

That’s the second thread: adding 50mm finders without breaking the bank.

It’s been a while since I’ve bought a new finder, and I have to admit that the prices kind of took me aback. I can’t shake the thought that the Celestron Travel Scope 50 runs about $45 and the Travelscope 70 is $60. If only I could find some way to mount them, I could have easily focusable luxury finders for less than new RACIs of smaller aperture! And that’s really the rub in both of the threads of thought outlined above: building a finder-quality scope is not hard. Mounting it solidly, reliably, and conveniently is hard. Part of what you pay for in a commercial finder is a sturdy, easily-adjustable finder stalk with a standard dovetail foot.

Well, what if I built my own finder stalk?

There are examples out there. My favorite, because they look easy to fabricate, are what I call the “half-pipe” mounts that consist of two half-cylinders mounted back to back. Here are a couple from the “Frugal Astronomer” thread on CN:

This one is by CN user Grendel, and is made from cardboard tube–as is the finder, as shown in this post.

I think this one is from the same thread, but derned if I can find the original post now. Anyway, it’s not my photo, I’ll credit it properly if I figure out where I found it,  if it’s yours please chime in, etc. The nice thing about this one is that it’s easily adjustable, thanks to the combination of thumbscrews through the half-pipe and rubber bands pulling the finder against them. Note the zip ties holding the half-pipe mount to the main scope.

So these got me thinking about the possibilities of the half-pipe mount. Here are some sketches I knocked up in GIMP.

The one at the top is simplest, just a V-slot, essentially the same as in the previous photo. The finder would have to be held in with rubber bands, elastic, velcro straps, or zip ties. Alignment could be done with bolts (you could either tap threaded holes or drill simple holes and epoxy nuts on the outside) or shims.

At lower left everything is the same except a trough has been added to cradle the finder, which might make it easier to use. I don’t know that, obviously, just kicking ideas around here.

At lower right is a full ring holder. I figured, if you’re putting alignment bolts through anyway, just make two more holes and you’ve got a six-bolt alignment system just like on the commercial rings (see an example in the photos here).

The key thing isn’t finder alignment, though, since even rubber bands and shims would work there. The key thing is convenient and repeatable mounting and unmounting to the OTA. I got to thinking: with an inverted V-shaped foot, like all of these have, is there any reason it coulnd’t be cut and sanded to fit into the existing dovetail shoes, so that the dovetail retaining screw tightens on one side of the inverted V? If that could be made to work, this kind of finder base could be mounted and unmounted and moved between scopes just as conveniently as one of the commercial jobs.

The end of all of this thinking? I got a piece of ABS pipe when I was at the hardware store to get parts for my sun funnel. I’m going to play around and see what I can come up with. If I find a workable solution, I’ll post it.