Monday, November 19, 2007

Holding my breath

Bear with me - this is a long and technical update. But if you've been wondering what's been keeping me busy or why I'm here, here's more information than you ever wanted!

Normally, the work that I do for the SPT involves lots of programming. My main job is to put together the software tools that we need to look at the raw data from the telescope and process it into images that we can use to learn something about the evolution of the universe. A huge part of analyzing this sort of data is trying to understand as much as possible about the behavior of the telescope and the characteristics of the detectors that we use to observe light from the early universe. The measurements we are trying to make are exceptionally sensitive, and if the telescope isn't pointed exactly where we think it is, or if we don't fully
understand how the detectors respond to the radiation that they absorb, we can't interpret the raw signals that are recorded while we're observing the sky.

To me, the challenge of fully characterizing an instrument and all of the possible sources of uncertainty in a measurement is the most interesting part of experimental physics. Even though I work with this sort of stuff every day, I still think it seems crazy that we can claim to make valid, robust statements about something as grandiose or remote as the dawn of the universe or the evolution of its major components. What I find most inspiring and profound about experimental work is the careful and detailed arguments that experimentalists must make to justify and defend every statement that they make, in order to build up robust arguments backing up each claim. I personally want to be able to stand behind our measurements, to defend the correctness of our inferences, and to feel that I could explain every cross-check and every step in the argument that takes us from measurements of voltages in our detector readout to saying something about the entire universe. That's a tall order! But to me, it's the most interesting challenge and what motivates me.

Both because this whole facet of experimental work fascinates me, and also because I was available to spend the whole season at the pole this year, I got involved in some calibration measurements that we need to perform in order to understand our optics and response to light of different frequencies. Before we can interpret the signals that we recorded during our last season of observing, we need to know how sensitive the whole system is to microwave light of different wavelengths or frequencies. Like our eyes, the telescope is sensitive to a range of different frequencies of light. For our eyes, different frequencies manifest as different colors. Our eyes are most sensitive to colors that are yellow, and fall off in sensitivity at the low frequency end of the visible spectrum (the infrared) as well as the high frequency end (the ultraviolet). Similarly, we want to know which microwave frequencies our detectors see best, accounting for their own response as well as how the different parts of the optical system affect the light that they see.

To make this measurement, we take a radiating source (basically just a really hot object that emits a lot of light across a broad spectrum), and we send the light from that source through an optical device called a Fourier Transform Spectrometer (FTS). The FTS bounces the light through two paths and recombines it, then sends it through the window of our "optics cryostat", where it bounces off a big mirror and finally hits the detectors, which are held at a temperature just barely above absolute zero inside the "receiver cryostat". When we look at the signal that the detectors receive as we vary the path of the light through the FTS, we are able to probe how the whole system responds to light of different frequencies.

The FTS that we are using consists of an assembly of mirrors and fine wire grids mounted in a big metal box along with the radiation source. We had very little time to build this device before we came down, so we contracted with a company to build one for us. However, there were some shipping delays and I never got to see the thing before flying down here to meet it. We didn't even know for sure that all of the parts would fit together or that we would be able to mount it onto the optics cryostat where it could send light in towards the detectors, much less perform meaningful measurements with it.

So, when it finally arrived here, you can imagine how nervous I was to unpack the thing and see how it worked. Professor Steve Meyer back at Chicago designed many of the parts of the whole system, and he did a great job. It took a bit of problem solving to figure out how to put the thing together and get it mounted, but we were able to do so in just a couple of very busy days, with no major snags. Below is a picture of me mounting one of the mirrors into the box (which has its walls removed to make this easier). The picture was taken by Jeff McMahon.

Altogether, the FTS weighs a couple hundred pounds. So once we had the major pieces together, five of us VERY carefully lifted it and bolted it to the optics cryostat. The optics cryostat is the big white thing in the picture below. At the top of it, you can see part of a white circular window that is how microwave radiation enters the cryostat (the foam of the window is transparent to microwaves, but not to visible light). The red box that you see is the receiver cryostat. Inside that are all of the detectors. During normal operation, the two cryostats are hoisted up into the boom of the telescope and aimed so that the window faces the 10-meter dish of the telescope. In this configuration, we will mount some lenses to take the output of the FTS box and direct it into the window instead of pointing the window at the telescope mirror and out to the sky.

Below is another picture of the two cryostats, from the other side, so you can get a better sense of the size of these things. They're quite large, and the space in our control room is quite tight. A huge part of our efforts is keeping everything inside these cryostats very cold. Everything inside the big white cryostat is held at around ten degrees above absolute zero, while the detectors inside the red cryostat are held at a quarter of a degree above absolute zero. Only when we achieve these tiny temperatures can we actually use the detectors to make any kind of measurements.

The last step in the whole process of getting this thing ready, once we had the FTS mounted and the detectors cold, was to remove the metal plates protecting the fine wire mesh grids that are part of the optics of the FTS. These wire grids are made of tiny, fine wires stretched between two rings. The wires are so fine you can barely see them. They are so delicate you don't want to even breath on them, so the operation of getting in there with a wrench and removing those plates without damaging them was nothing short of petrifying. I think I held my breath the whole time, and I was sweating and shaking by the time I had all six plates removed. It's hard to photograph something this shiny, but you can see the wire grids in the photograph below - they look slightly gold-colored.

The last big question was whether we could turn on the radiation source and run the FTS and see it with the detectors. Two nights ago, we had a tiny window of time to try this, and graduate student Ryan Keisler and I stayed late out at the telescope to give it a first shot. With two laptops, we sat down on the floor and pulled up some plots showing us the data streaming live from one of the detectors. We moved the FTS around until the light from it was pointing right at that detector, then we ran the FTS. Amazingly, it produced a beautiful series of wiggles that told us immediately that everything was working. I just spent the last day looking at this data, and it looks great. I think we'll be able to get the calibrations that we need over the next week, and then move on to the next tasks for the season.

Right after we saw the first data from the FTS, Ryan and I shut down the laptops and raced back to the station to catch the rest of a science lecture being given by John Carlstrom, the head of the SPT project. The audience filled the galley and included all sorts of people from the station - janitors, cooks, carpenters, technicians... It was such an incredible feeling to walk in, breathless from the cold, and see this diverse and unique audience glued to John's descriptions of the telescope project and the science we are trying to do.

I didn't realize until we saw those beautiful little wiggles race across the computer screen that night that I had, in some sense, been holding my breath since arriving, not sure whether we would be able to pull this off. It's rare that you put together a brand new instrument and everything works the first time. So, now, I'm taking a huge deep breath and feeling a lot of well as excitement for the chance to get a better understanding of this incredible telescope.


Blogger D said...

Wow, what an awesome and exciting post. Thanks for explaining what is happening and letting us experience the excitement of cutting edge science.
You obviously have such a key role in making this happen. You are awesome, too. D

8:29 PM  
Blogger glenn said...

Hey Kathryn. Glad you guys made it back and the products of you labor are positive. I insulated the legs on SPT last summer. What a great job! I hope you love that beautiful place as much as I do.
Wish there was some way we could watch or listen to the science lectures from the states.
Thanks for the blog.

11:35 AM  
Blogger admin said...

Chair lift for stairs

1:45 PM  

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