The View Up Here

Random scribblings about kites, photography, machining, and anything else

Science Fair Projects for Big People

Posted by Tom Benedict on 18/11/2011

I have three kids in school. All of them do science fair projects. Sometimes they’re enthusiastic about them, but most of the time they treat them as if they were homework assignments rather than fun opportunities to explore the universe around them. On several occasions they’ve told me about things they’re doing outside of school and I’ve mentioned that they would make good science fair projects. “But that’s not our assignment.” ??! Science fair projects shouldn’t BE assigned. They should stem from curiosity! Unfortunately I’ve had a rotten time communicating that to them.

But the truth is over half of what I do at work would qualify as a science fair project. So that’s how I’ve started describing my work to them. “Hey, I worked on my science fair project again today!” Since we’re hoping to present our most recent work at the SPIE Astronomical Instrumentation conference next year, and since my stuff is likely to be done with a paper and poster rather than a paper and presentation, the whole science fair analogy is actually quite close.

So here’s my science fair project this year:

We run a number of instruments at the place where I work. All of these are run under vacuum, and are kept cold to varying degrees. One of our instruments lives around 75K. Others live at a balmy 150K. None of them operate warmer than -120C. Our goal with all our instruments is to get them pumped, get them cold, and leave them that way for as long as possible. Thermally cycling electronics and mechanics is a great way to break stuff and wear them out. The fewer cycles, the better. So the longer things stay pumped and cooled, the happier we are.

Two problems come up:

The first is we need to be able to monitor things like temperature and vacuum. Temperature is easy. Vacuum is more difficult. Vacuum gauges that measure vacuum near atmosphere are pretty cheap, and pretty easy to use. Gauges that measure higher vacuums like the ones we use are more expensive. Worse, all of them emit light. Since we’re talking about instruments that measure very VERY dim light levels, having something inside the instrument that’s blasting light poses something of a problem. So we tried to use low range gauges for reasons of cost, complexity, and light level.

That’s when the second problem came up: We found out that the high range gauges we’d been using were acting as fairly efficient vacuum pumps. This is nothing new. The cold cathode vacuum gauge is basically an ion pump. Dedicated ion pumps are just bigger. So we wanted to find out what the pumping speeds are for various gauges. We tested a number of gauges, and found the cold cathode gauge won hands-down. (Yes, we have numbers to back this up. No, I won’t present them here. Yes, they’ll be in the SPIE paper in 2012.)

Which brought us back to the first problem: How do you make it so a cold cathode gauge doesn’t spill light into your camera?

To test this we outfitted a small vacuum system with a window so we could look down the throats of the gauges we were testing. To give you an idea of how bad this problem is, this is what a hot ion gauge looks like, hooked to our vacuum system:

Ion Gauge Emissions

And this is what it looks like when you look into the window:

 

Gauge View Hot Cathode (Ion Gauge)

I’m sure some of you are thinking, “But ion gauges are basically light bulbs! Cold cathode gauges don’t emit nearly as much light.” And you’d be right. They emit a lot less. (Again, we do have numbers to back this up. Look for them in the 2012 SPIE paper.) But they still emit light. And if your camera is doing an hour long exposure, every little bit matters. This is what a cold cathode gauge looks like when you poke your head inside:

 

Gauge View Cold Cathode (Cold Cathode Gauge)

Imagine putting one of these inside your camera, pointed at the chip or the film, and ask yourself if you’d be able to use it for photography. Then consider astronomical instruments sometimes make exposures that are minutes to hours long. All the while the detector is accumulating the photons being emitted by the gauge. You can see it’s a problem.

The problem is, light baffles for vacuum gauges really don’t exist as a commercial product. We looked. We did find a vacuum baffle, but it was really designed to keep crap like coating chamber gunk from getting into gauges, not to keep gauge light from getting into a detector. It dumped the light by a factor of ten, but that’s it. We were looking for factors of millions or more. Since nothing existed, we started designing.

This project is a collaboration between two people. One of us is an optical engineer, the other (me) is a mechanical guy. The optics guy is doing the design analysis, I’m making the prototypes, and we’re both pitching in on the testing and data analysis. The optics guy went through a number of design ideas we came up with while brainstorming one day. All of them panned out, but not all of them could be built. One of my ideas seemed simple at first, but by the time he’d tuned it to get us the attenuation we needed, the design would’ve called for annular slots of 0.2mm or less on a 25mm circle. No freakin’ way. In the end there was a clear winner design. So we built a prototype and tested it.

The attenuation was better than we’d dreamed of. In case you’re wondering, those numbers will also be in the 2012 SPIE paper. (This is a really cheesy way of saying, “We’re still collecting data and don’t have an answer for you yet.”) In any case our problems with light leak are more or less done. We found which gauge we need in order to continue pumping our instruments once cold, and we now have a clear path forward for new instrument development. Yay!

See? Science fair projects are fun, even for big people.

– Tom

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

 
%d bloggers like this: