An End to Calibration

 Yesterday was our last day for calibration measurements.  First thing in the morning we went out and unplugged the calibration source.  Everything was packed up onto the back of our snowmobile and we brought it back to SPT.   There will be a second round of calibration measurements made in late January, but we can't leave the source outside for that long.  The source is about 3 km away from the telescope.  To get an idea of the scale, that's me in front of SPT (10 meter dish).  Looking out at the source from the top of the telescope, you can only see a tiny black dot at the end of the road.   Zooming in,  it resolves into a wooden fence (that we use
for shielding) and a big metallic square reflector.  The reflector itself is 25 feet high, and in the middle is actual calibration source.  Once we took the calibration box out, it was an opportune spot to take a picture!

 As soon as we finished calibrations, the telescope was docked to lower the receiver!  Basically, the telescope slews around and down until the boom (the long straight white structure that extends from the dish) is facing the building and is right above it.  There is a sliding door in the roof to the building that opens and the entire boom lowers down onto the hole.  There are two more doors into the telescope boom that open onto the receiver cabin.  

 If you look straight up into the receiver cabin, this is what you'd see.  On the left is all of the readout electronics (all the grey cables and racks).  The large white round cryostat on the right contains all of the secondary optics, and the red and black one in the middle contains the focal plane and detectors.  The optics cryostat is about the size of a smart car.

 In order to take out the detectors for upgrading, the whole thing has to come down into the laboratory space below.  It took about three hours total, during which we lowered both cryostats on the four chain hosts you can see in the picture.  It's a tricky job, because you want to keep it level, and there are places that it can scrape against three of the cabin walls. 

  

Eventually it was down, with no problems!  Now it's just a matter of time until the receiver team can take out the focal plane.  The coldest part detectors inside was around 300 millikelvin when we stopped observations and both cryostats are under vacuum.  We have to wait until it is around room temperature to open the cryostat, or else risk condensing any residual atmospheric water vapor that is inside on the detectors and ruining them.  Sometime on Friday, the focal plane will come out!

Now that the calibration is over, I'm moving on to my last task here for SPT, upgrading part of the readout electronics.  

Hero Shots

We've been continuously taking calibration data now for over a week.  We've been back out to the calibration source a couple times, and I finally took a turn at driving the snow mobile.  It's pretty fun!  We'll be shutting down shortly so that the receiver team can lower the instrument out of the telescope and get to work.   Data taking mode means that I've had more time to take pictures, both at the telescope, the source and around the station.   So I went back out to the pole marker with a couple of fellow SPTers and played in the snow for a bit.  

This first picture is of a fellow SPTer walking through the snow out at the source.

The pole marker is a thing of beauty.  Each year a new one is made for when the pole is moved, and all of the old ones are kept in a case inside the station.  2012 was the 100 year anniversary of Scott reaching the pole, so the marker is in honor of that.  The machine work to make it is exceptional.   In early January this marker will be retired and the new installed.






 

The weather the last few days has been exceptionally clear.  Without blowing snow, you can see out for several kilometers.   Here, you can see me, with the vast nothing in the background. 

After pictures at the geographic pole, we took the quick walk over the ceremonial pole to take a few 'hero shots'.  The ceremonial pole  looks just like something out of a movie, red and white stripped with a shiny metal ball on top.  Several times now I've seen daytrippers (people that fly in and out the same day) run out quickly to the ceremonial pole, take their pictures, look around for about five minutes and then go back inside without going by the geographic pole.   The ceremonial pole is shiny, so I was able to take a picture of myself in it.

Happy Thanksgiving!

 We celebrated Thanksgiving here yesterday.   That meant time for relaxation, an amazing dinner, and a dance party afterwards.  

South Pole Thanksgiving Menu

Starters:

Baked Brie

Assorted Cheese and Crackers

Shrimp 

Main:

Turkey

Mashed Potatoes

Sweet Mashed Potatoes with Candied Pecans

Stuffing

Green Bean Casserole

Cranberry Sauce

Dessert:

Pumpkin Pie

Pecan Pie

Apple Pie

Fresh New Zealand Whipped Cream

2% Relative Humidity

 The receiver team arrived earlier this week!  Within in the next few days we'll shut down our calibration observations so that they can take instrument out of the telescope and do some maintenance and upgrade work on the detectors inside.  As an added bonus, the station celebrates American Thanksgiving this weekend.   That means an extra nice dinner on Saturday and some down time.  There has been talk of going sledding on some of the large snow piles around the station.   We've had some nicer weather this week and could actually see the telescope and the station from calibration source.  It's still cold enough to freeze your breath though!


There is one obvious question about SPT that I haven't answered yet: Why the South Pole?  There are definitely easier places to build and work at a telescope.  SPT observes the light from the CMB at millimeter wavelengths.   At these wavelengths, any water in the atmosphere will absorb light, preventing much of it from reaching a telescope.  Places at higher altitudes (like the South Pole) have less atmosphere and therefore more of the CMB light reaches the ground.  Also, because it is so cold outside all of the water vapor in the atmosphere is frozen into ice crystals, which don't absorb millimeter light.  On a sunny day, the air here actually sparkles!  Combining the altitude and the cold, the South

Pole is one of the premier observing sites in the world for cosmologists!

Polarization, Calibration and Chocolate Cream Pie

I've been spending the last week or so (along with the rest of the calibration team)  working on setting up and making measurements with the SPT polarization calibration source.   The source is located a couple kilometers away from the station and telescope, so on a day with bad visibility you become very isolated.    We've had a couple of those, where all you can see is the snowy ground around you and then nothing in the background (just a wall of snowy wind).  When riding back in, eventually SPT and then the station start to come into focus, like looming giants in the distance.  The last time we went out, the weather was fairly nice, and we were able to see SPT in the distance (it's the dark circle right above the fence in the picture).   The temperature has steadily increased since we first arrived.  The average temperature is about -35 degrees Celsius, but the wind has also increased.  We were getting gusts of about 20 knots the other day, which takes the windchill back down to -50 C.  It's amazing how quickly being outside in the cold can sap your energy.  It's very easy when you're out at the calibration source to imagine how isolated Scott and Admunsen must have felt when trekking to and from the South Pole.  Personally, I'm happy to hop on the snowmobile and drive to a heated station for a good meal and clean socks.

The calibration measurements we're taking will be used to measure the polarization of the Cosmic Microwave Background.   Light has a total intensity as well as a directionality (called the polarization).  Polarization occurs when the electromagnetic (EM) waves from a source are aligned in a given direction.   One way to visualize it is with two people holding the ends of a long piece of string.  If one person starts moving their hand up and down, there will be a vertical wave in the string.  But if the same person instead moves their hand from side to side, the wave will be horizontal.  The polarization of light waves is similar, the waves can be vertically or horizontally polarized (or any angle in between).  This is referred to as 'linear polarization'.   Light can also be unpolarized (each EM wave is polarized at an angle between 0-360 degrees so when you sum them up you can't tell the difference), or circularly polarized (but that's a whole other story).  We expect the CMB to have a slight linear polarization, but from two very different sources.  

In my previous post, I said that after neutral hydrogen forms, the light from the CMB can travel freely through space.  As time continues this neutral hydrogen begins to form structures and eventually evolves in to the people, planets, stars, galaxies and galaxies clusters we see today (this is a bit simplified, but I'm trying to summarize billions of years into a couple sentences).   The CMB photons interact with these structures through gravitational lensing.  The gravitational force of the matter distorts the path through space that the light travels (just like a glass lens would, hence the name gravitational lensing).   The gravitational lensing of the CMB polarizes the light on small scales (size of galaxy clusters).  

The second source is far more exotic.  The current theory of cosmology, where the universe started with a Big Bang and rapidly expanded is not the full story.   In order to explain current observations, cosmologists and created the theory of inflation.  The idea is for a short period of time after the Big Bang, the universe was expanding exponentiall.  After it was over, the infant universe would be 10^26 times bigger in size (that's 100000000000000000000000000 times larger)! 

Inflation is still just a theory, but there is a measurement that could prove it!  Inflation is predicted to leave a very specific pattern polarization in the CMB light.  There are no other ways (that anyone has theorized) that would create this pattern, so a measurement of it would point directly towards inflation.  It's an extremely challenging measurement, because the signal is so faint.   Which brings me back to the calibration measurements we're taking right now.   In order to measure such a faint signal, we have to understand how much and what angle of polarization each of our detectors measures.  The calibration has to be extremely precise, or the faint CMB polarization signal will be washed out by contaminating signals from the instrument itself. 

Overall, this past week living in the station has been a bit surreal.  One minute I'm working on a piece of analysis software in our communal lab office and it's basically just like being at home (with different faces and furniture).  Then I walk down the hallway to dinner and look out the window onto the Antarctic snow, complete with telescopes, the summer camp housing,  giant pallets of cargo waiting to be loaded on a plane, and snow mobiles zooming around.  It's a jolt in consciousness to remember where I am are.  The little details of life are different from home as well, but walking up and down the same corridor everyday and traveling out the telescope quickly became very normal.  It's very easy to be comfortable here, as everyone is friendly (there is a real sense of community) and the food is excellent.   Who doesn't love a meal of steak, potatoes and chocolate cream pie after a long day of work? 

Cosmology in an Snowy Desert

Since we arrived and settled in here at the pole, things have gotten busy fast.   Originally, we had about 12 days planned to take calibration measurements before the next team of SPT people arrived.  That was before travel delays in McMurdo, and so our schedule has been tight.   The receiver team is supposed to arrive this evening, so we only have a couple days left.   Their job is to remove the instrument from the inside of the telescope and upgrade the detectors inside, so we have to finish before they can begin.  We've been working on the calibration source and trying to understand the preliminary data we've taken with it.

In my last post, I promised to write a bit about the what and why of SPT,  so here is the beginning! 

Objects in the universe (such as stars, galaxies, gas, and dust) emit and absorb light at different wavelengths.  For example, we see the sun at 'visible' wavelengths but other hotter stars are seen instead in the ultra-violet.  Things that are hot emit shorter wavelength light and things that are cool emit longer wavelengths.  The South Pole Telescope observes millimeter wavelength light (think microwaves).   We're looking for relic radiation (light) from the very early universe that is now only a few degrees above absolute zero (2.73 Kelvin to be exact).   Measuring the intensity of this light across the sky, we can determine what the early universe was made of, how much of each part there was, and how it evolved into today's universe.  

The current leading theory of cosmology (the study of the evolution of the universe) suggests that the universe started with a Big Bang.  This very young universe was so hot and dense that all of the protons, electrons, and photons were constantly interacting with each other and no atoms or molecules could form.   But immediately after the Big Bang, the universe began to expand.  As it expanded, it cooled down, eventually to the point where atomic hydrogen could form (about 380,000 years after the Big Bang).   Suddenly, without electrons everywhere to scatter off of,  the photons were able to travel freely through space.  Cosmologists refer to this as the time of recombination or the surface of last scattering.   These photons, known as the Cosmic Microwave Background or CMB, continued to travel through space interacting very little with the universe around them.  Therefore, the spatial distribution of the photons is still basically the same as it was then when we look at it now.  Because the photons, protons and electrons were coupled together before recombination, the CMB is a fingerprint for the size and location of structures in the early universe.   For now, the CMB is the oldest light in the entire universe that astrophysicists can actually observe and its everywhere in space.  If you have an old TV with an antenna, about 1% of the fuzzy noise you get in between channels is from the CMB!

SPT has been observing the sky for several years now, and has made beautiful measurements of the total intensity of the  CMB.   The current challenge is to measure the polarization of the light.  I'll write more about this next time.  In the meantime, here are two pictures of me investigating the marker of the geographic South Pole.  They make a new one every year and install it in the correct spot taking into account the few meters that the ice sheet moves every year.   The station where we live is in the background.

Welcome to South Pole Station

 After nearly a full week of traveling, we're here at the Admunsen/Scott South Pole Station.  There are a few minor differences from McMurdo.  The first is the weather.  As you can see from the weather info, it's significantly colder.   As long as I keep the majority of my face covered, the big red and overalls keep me pretty warm.  Everyone outside walks around with ice crystals frozen on the front of their fleece neck gaiters where their breath has frozen. 

 The other big difference is the access to the internet.  The South Pole only sees satellites for a fraction of their orbit, so there are big gaps in the internet availability.  We've been pretty lucky this year, there are two satellites that are up and cover most of the 'daytime' hours.  I put quotes around daytime because both here and at McMurdo the sun only sets once a year.  Right now, in summer, the sun just circles around the horizon everyday.  24 hours of daylight is pretty strange,

 but I'm getting used to it.  The window in my room is completely sealed off, so that when I go to bed it feels like night.  But last night I was working in the science lab and the sun was coming right in the windows.  It was about 11 pm, but my body thought it was about 3-4 pm.
One last thing about the South Pole, it's at an altitude of about 9306 ft.  But the atmosphere is more dense here at the South Pole (i.e. the pressure drops faster the higher up you are compared to being at the equator), so  the physiological altitude is about 1000 ft higher right now.    The first couple days, going up stairs in my ECW gear definitely left me a little winded, but I'm starting to acclimatize now.

Now that we're here, we've started working with the telescope.  The team of four of us that is here right now (plus our two intrepid winterovers) are working on making precise calibration measurements of our detectors.  Part of this includes going out and working at a site about 3km away from the station where we live, so we get to take a snow mobile.  We use it haul both equipment and people right now. 

At first glance, the South Pole telescope is an engineering masterpiece.  The primary dish (the circular part in the photo) is 10 meters across.  The entire telescope can spin around 360 degrees on its mount to access any portion of the sky. 

I'll try and write some about the telescope, the receiver, and our science in my next post.  I'm not the first person to take this trip, so if you'd like to find out more now, check out the blog of Jason Henning (an SPT grad student) at:  http://destination90south.blogspot.com/ .  He has some awesome pictures of the instrument guts.  The SPT collaboration also has a blog at: https://pole.uchicago.edu/blog/.

There is also an excellent video online that was made last year by the American Museum of Natural History featuring the South Pole telescope.  http://www.youtube.com/watch?feature=player_embedded&v=3pu84BQVxK4