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Sitelle Getter II

Posted by Tom Benedict on 01/03/2013

Two posts ago I alluded to a design change to the cameras at work. Since I posted pretty fab pictures of the first rev of the getter, it’s only fair that this one has its turn to shine.

The original design was novel in a couple of ways. It had two chambers, one on top and one on bottom. Each was filled with its own charge of activated charcoal, and each had its own filter, protective screen, and polished aluminum cap. This made for a fairly high volume / high throughput getter in a neat little hockey puck sized package.

Sitelle Getters

Unfortunately two problems killed this getter. The first was that it simply didn’t fit. There’s a fair bit of material between where the cold head bolts on and where the cold strap attaches. In the original design of the camera, the flex circuit for the electronics feedthrough passed quite close to the bottom surface of the getter. In practice, though, the flex wound up being longer than expected, which caused the flex to touch the getter. The getter is maintained close to -190C, and the flex is maintained with one end at -100C and the other at room temp (typically 0-5C). Having a thermal short to -190C right in the middle was going to cause problems. To get past this, we machined off the bottom chamber of one of the getters, and moved on.

The second problem that killed this design was that the cold strap attached through the force of a magnet embedded in the getter attracting a second magnet, embedded in the top of the cold strap. Unfortunately the cold strap was too stiff to let this arrangement work well. The magnets were too weak to overcome the springiness of the cold strap, no matter how carefully we arranged things during assembly. And moving to stronger magnets would’ve meant the camera was harder to disassemble. So instead we ditched the magnets and moved to a conventional bolt-together design.

All of which completely invalidated this design for the getter. So I started over. I was planning to make another CNC-heavy design with lots of sexy curves, vented screws, stainless mesh, etc. But right as I got ready to fab the thing, I found out our CNC mill was completely tasked to making new filter frames for one of our other instruments. Whoops!

So I re-designed it (again) to be made entirely using conventional hand-crank tools. And I got rid of the screws. And used thinner material – a good thing in a getter since it means the getter is cooling faster, and is pumping before nasty stuff like water vapor can condense on the face of the CCD. The new design is made using 2mm plate and the same 0.010″ thick copper strap we use for our cold straps. They’re not as nice looking as the original design, but they should work.

Sitelle Getter II

And truth be told, they’re probably better than the original design in a number of ways. The overall cavity size is larger, and I managed to pack almost twice the carbon inside as the original getter. There’s a third copper strip hidden inside the getter that provides a thermal path up inside the bulk of the carbon, so it should cool quite quickly. I kept the filter, so still no chance of carbon dust getting into the camera body. And considering I made four of these in three days, it was a quick solution to a nagging problem that’s been hanging over my head for months.

The only real drawback to this design is that there’s no way to service the thing. If the filter tears or finger oils get on the top, there’s no real way to fix it. Toss it and grab another. Which is why I made four of these for two cameras. Spares!

I probably won’t have a chance to test these until next week. Meanwhile I need to make the bolt-together cold strap for the two science cameras. The prototype has been cooled down twice, and is working perfectly.

So now all I have to figure out is what to do with the two surviving first generation getters. They work! They just don’t fit inside these cameras. And it’d be a shame to turn them into paperweights.

– Tom

P.S. Yes, in case you’re wondering, all those holes were drilled by hand. On four parts. Nothing fancy, just a sacrificial plate bolted to the mill with the part clamped on top. The rest was all trig, and a lot of triple-checking my position so I didn’t screw anything up! All four turned out perfectly. But I swear, if I can skip drilling any holes for a while I’ll be ecstatic!

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