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Ready for a Weekend!

Posted by Tom Benedict on 08/09/2012

For a short work week, it’s been a long week at work. But it’s been a good one! Most of the external parts for the cameras are done. If fortune continues to shine on us, it’s looking like we might send out all the external parts for anodizing some time in the next few weeks. We still haven’t settled on a color for these things, but it’s looking like the original orange I used for making the CAD renderings may win out in the end. The only requirement is that the cameras be brightly colored. One of them will be located almost knee-height when it’s on the telescope, in an area where forklifts are in use. If I could make it fluorescent orange with tiger stripes, I would! But I’d settle for fire engine red or hot pink, too.

This is, I hope, the last rendering I post of this camera design. The next picture I post should be the real deal:

Sitelle Camera September, 2012

I’ve still got some internal parts in the queue, though, so the work is far from done. I’m working on the vacuum feed-throughs next week, and hope to make a start on the cold finger/foot/strap assemblies as well. That is a story all its own:

Any time you build a cryogenically cooled camera, you need some way to isolate the cold bits so they don’t let heat from the room-temperature camera body leak in. Along with this, you need some way to attach your cold source to the cold bits so any heat they generate or collect through ambient radiation can be taken away. The first is accomplished by using some thermally insulating material to support the cold bits. In these cameras I’m using G-10 fiberglass tabs to separate the warm parts and the cold parts. The second is accomplished by using some thermally conductive material to draw the heat off. In these cameras that’s done through an unfortunately complicated assembly of copper and aluminum.

Usually these assemblies are comprised of three parts: the cold head (the cold source), the cold strap (the bit that connects the cold head to the rest of the assembly), and the cold foot (the bit that actually connects the cold to the back of the detector chip). In this case our cold head is a Polycold PCC cold head. Our cold strap is a piece of copper 0.20mm thick x 12mm wide x 90mm long. Our cold foot is a block of copper about 35mm square and about 15mm thick.

The trick is that you have to be able to remove the cold head to access the rest of the camera. So somewhere in there you need some sort of a removable joint. In one camera I’ve worked on, this was done using a spring and two pressure plates. One of the plates is connected to the cold head. The other is connected to the cold strap. Push the two together, and the force of the spring keeps things in good solid contact. Voila! The cold head / cold strap / cold foot circuit is complete!

The original design for this camera also used a spring, but concerns about mechanical vibration coupling between the cold head (which does vibrate) to the detector chip through the spring ruled that out. I had to come up with something else. Over the course of the last three months I tried all sorts of ideas, none of which panned out. The best approach I had used a single bolt to bolt the copper cold strap directly to the cold head. Only problem? There was no way to reach the bolt with a wrench! No matter what I did, there was no way to get a tool in there. Time and again, I found myself back at the drawing board, starting over from scratch.

Then one of my co-workers came up with a brilliant idea: Why not use magnets? Stick a magnet on the cold head and a magnet on the cold strap, and let the magnets do the work of holding the two in contact. The idea is similar to how the clasp on a lot of handbags work these days. We tried it on the bench in the lab, and it worked great. So yesterday I mocked up a set of hardware to try this in a real cryostat. Today I installed everything and started pumping it down.

Installing the cryo head was dead nuts simple. As soon as the two magnets got close enough, “Clack!” they came into contact. The magnets are self-centering, so some of the off-center contact problems we had on the spring designs are also solved. Even before getting things cold, I could already tell the idea was working. In order to drive some of the water off the cryostat walls, I stuck a heater on it while it was pumping down. Both the cold head and the cold foot registered the heat being applied. Under vacuum, the only path that would let both parts see the same heat is the cold strap. Only way for that to work? For that joint to be solid. Yaaaay!

I plan to start cooling Saturday morning and see how it cools through the weekend. By Monday we should have good numbers for how it’s likely to behave. If it all works out well, I’ll start cutting parts for the real cameras on Tuesday.

But enough work! It’s time for a weekend!

And speaking of weekends, I finally ordered the tools and parts to replace the main halyard on our boat. My plan now is to finish drawing up the new main halyard winch in CAD, put the old winch back into the mast, and get the boat ready for when the halyard bits all show up. We should be sailing by next weekend. Meanwhile I can start machining the new winch, which is being designed not to let the cable jam ever again. When it’s ready, swapping them out will be about an hour’s work: the perfect project for a weekday evening with the promise of a weekend sail at the end of it.

Time to play.

– Tom

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