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A Minor Thing (But…)

Posted by Tom Benedict on 01/12/2012

More progress on the cameras I’ve been building at work! I spent the past several weeks testing and building the cameras’ getters. (Yes, a “getter” is a noun. “Gettering” is the verb. Even after having done this for years that still feels funky when I say or type it.) Anyway, the getters have finally joined the ranks of the parts that have gone from CAD to reality. It’s show and tell time!

A getter is a type of sorption pump. It typically includes some source of cold (a cryogenic gas, a closed-cycle cryocooler, or magic – your pick), a gettering material (activated charcoal in this case), and the bits and pieces to bring the cold to the gettering material. By cooling the gettering material down to cryogenic temperatures, it becomes a cold trap for residual gases in a vacuum system. Get the thing cold enough, and as molecules of residual gases come in contact with the getter, they tend to stick to it. Voila! Pump!

These cameras were originally designed to take a commercially available getter. But as the project progressed, we realized we needed more performance out of them than the commercial units offered. So we designed and built our own.

The first design criterion was that it pump faster than the commercial unit. The commercial getter we originally intended to use in these cameras is basically a cup with a bunch of activated charcoal glued in it. The easiest way to double the pump speed is to double the amount of surface area that’s doing the pumping. Answer? Use two cups and glue charcoal to the other cup, too! But space is limited, so we stuck the two cups back to back with one facing up and one facing down.

The second design criterion was that we didn’t want carbon dust getting all over the inside of the camera. For starters it’s dirty, and we’re building these things in a clean room for a reason. But carbon dust is also conductive. This is not what you want to have rain down all over a circuit board! Answer? Add covers!

So here’s the design we came up with:

Sitelle Getters - Rendering

Two getters are shown. The one in the foreground is right side up. The one in the background is upside down, showing off the counter-bored screw holes that allow the getter to be bolted to the cold head. Inside are the cavities that take the activated charcoal. On top of each cavity is a cover plate that’s meant to hold filter material in place to keep the carbon dust from leaking out.

Having a design is nice, but it doesn’t do you any good if it doesn’t actually work. The last two weeks have been spent testing different charcoal loads to see what works best. Monday we pumped down our latest setup, and so far it looks good. Our recipe at the moment is 2-3mm of Stycast 2850FT epoxy, poured into the bottom of each cavity with as much activated coconut charcoal dumped in as will fit. Once the epoxy has cured, the excess charcoal is poured out and the cavity is sealed with a sub-micron fiberglass filter to catch any residual carbon dust. This is then followed by a 60 mesh .0065″ diameter wire stainless steel screen to protect the filter (which is quite fragile), followed by the cover plate as shown in the above rendering. So far the lab tests look good. So do the getters:

Sitelle Getters

As exciting as it is to see each part of these cameras in finished form, it’s just as exciting to see the various bits and pieces on my workbench slowly disappearing as they’re incorporated into final form. The cleaner my bench is, the closer I am to done. I can’t wait.

But I also can’t wait to see this instrument in action. The cameras are only a tiny tiny part of what this thing will be. It’s exciting.

– Tom


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