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A Semi-Kinematic Fiber Mount

Posted by Tom Benedict on 29/12/2011

As Designed As Built

I still haven’t wired up my 5.8GHz video downlink for my KAP rig. Life keeps getting in the way. Mostly I’ve been living in the shop at work, making parts. Today I finished the last project I had to get off my plate before building the two cameras for our new instrument. Just in time for the new year!

Before describing what this is, lemme back up a little…

We have an instrument at work that is fed light by a fiber optic bundle. An injection module lives on the telescope whose job is to get light into the fiber. The other end is a spectrograph that’s located in a temperature controlled room deep inside the building. The arrangement works quite well, but the performance of the whole thing depends on very accurate positioning of the fiber. Essentially the light of a star needs to be focused onto a fiber 100 microns in diameter, and positioned to better than a micron. Tight tolerances.

To make it even more fun, the bundle has three fibers in it, not just one. All three need to line up with optics in the injection module to better than a micron. So rotation is just as important as the position.

Ideally a bundle like this would have a precision ground rectangular ferrule with nice registration features on it. As you can see, the ferrule is round. And there aren’t any good registration features to set rotation off of. To be fair, the original design was copied off of another instrument that has given years of good service. But it only had a single fiber, so rotation wasn’t important, and it was installed once and never touched again. This one is removed and installed every time the instrument comes off the telescope. And each time it’s installed, the optics in the injection module have to be re-aligned to match the new position and rotation of the fiber bundle.

Clearly something had to be done.

The idea behind this new mount really goes to our master machinist, who has since retired. He took one look at it and said, “You should make something like the chuck on a ram EDM machine.” And that’s precisely what we did!

The heart of the thing is a stainless steel v-block that’s toward the rear in both pictures. I wound up making it out of some unknown 400-series alloy I grabbed out of the scrap box. I rough machined it, then surface ground it to size. This was the most critical part in the whole thing. I’m new to precision grinding, so it took me almost an entire day to make. It’s really not that complicated a part. I’m just slow. It has a bunch of tapped holes in it to boot, which is always an adventure with stainless. I’m happy to say I didn’t break any taps, for once!

The clamp block is made out of 330 stainless, and the collar and pin is a commercial shaft collar with a 3mm dowel pin pressed into it. That constrains the fiber in rotation. Depth is set by a shoulder in the aluminum base block. The v-block is recessed into the aluminum base block by 6mm, and is locked into place with a pair of button head screws. (I love it when I can work button head screws into a project. They’re so nice looking!)

To install the fiber you push it into the hole, snug down the clamp block screws, rotate the fiber until the two pins touch, then torque the clamp block screws down. That’s it. The fiber bundle is 7.5mm in diameter, and is held over 24mm of its length. That’s a 3.2:1 clamp, so the arrangement is quite stiff once things are cinched down.

Which is good for us! Because this mounts to the telescope, there’s no fixed direction for gravity. The fiber is protected by a stainless steel jacket, so there’s some weight hanging off of it. The long clamp on the v-block keeps it from shifting around as the telescope moves. (Remember that 1 micron tolerance on position and orientation? Yeah!)

The instrument this is intended for is already checked out for its upcoming run, so we won’t get to install this for almost a month. I hope to have good things to say about it once we put it into service.

Now on to my video downlink!

– Tom



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