Monday, January 23. 2012
Out receiver has been cold for a couple of days now, and we're starting to characterize its properties. Right now, we're using a Fourier Transform Spectrometer (FTS, http://en.wikipedia.org/wiki/Fourier_transform_spectroscopy) to measure the frequency response of the detectors. On the left, you can see the large, silver rectangle of the FTS mounted beside Black Cat on our optics cryostat. The cage on the upper right of the FTS, above Black Cat, takes the light emitted by the FTS and directs it into the cryostat window, where it can shine down on the receiver.
SPTpol has two kinds of detectors, one which is sensitive to light with frequencies near 90 GHz, and one which is sensitive to light with frequencies near 150 GHz. We chose those particular frequencies because 1) the CMB is (relatively) bright in those frequencies of light, 2) the atmosphere at the South Pole is transparent at those frequencies, and 3) the "shadow" which clusters of galaxies cast on the CMB (another kind of science that we're interested in) is most easily detectable at those frequencies. We use various filters to block out light with frequencies that we don't want to detect, and, before we start looking at the sky, we need to measure to be sure we know which light can get in, and which can't.
The FTS measurements are finishing up right now, and, when we're done, the next step will be to actually attach our brand-new receiver to the telescope. CMB, here we come!
Sunday, January 15. 2012
Early yesterday morning, the Black Cat cryostat was sealed up, hopefully for the last time this season. This is the third time that we've sealed up and cooled down. Cooling down the entire receiver from room temperature takes at least five days, so it's something that we can't do very often. We decided to warm up the receiver last time because it was getting too warm (where "too warm" means about 360 milli-Kelvin), which reduces the effectiveness of the sensors. We spent the time we had with the receiver warm doing everything we could to make sure it stays as cold as possible, as well as replacing some of the individual sensor elements which were broken or not as effective as they could be. Now we're done, and Black Cat is once again closed.
As you can see in the photograph below, Black Cat is now connected to the (larger, white) optics cryostat. The optics cryostat contains mirrors, lenses, and filters which transfer the light of the cosmic microwave background from the telescope dish down to Black Cat, where the light will eventually encounter the chilly bolometers which do the work of converting light into an electrical signal.
The first step in the cooldown is to pump all of the air out of the cryostats. We need to make sure that as little heat as possible can flow between the warm outside and the cold receiver inside the cryostat. Removing the air takes away a significant path for heat flow, and makes the cryostat much easier to cool. After the air is gone, we use a pulse-tube cooler (http://en.wikipedia.org/wiki/Pulse_tube_refrigerator) to cool the cryostat down to about 50 Kelvin, then two stages of further cooling which use the evaporation of liquid helium to cool the bolometers down to 300 milli-Kelvin.
If all goes well, the cryostats will remain in this cold vacuum (even colder than deep space) until next summer, when we perform more maintenance and upgrades.
Thursday, January 12. 2012
I realized that, while we posted a photograph of the new guard ring under construction (14 Dec 2011), we haven't yet posted a photograph of the finished product. Here it is.
The major thrust for this season is to install the new, polarization-sensitive receiver, but along with that, we've had to modify the telescope dish itself.
The polarization of the Cosmic Microwave Background is incredibly weak, so we need to be very careful about any Earthly sources which might add polarization to the light from the sky. We also need to prevent the glow from the warm ground from interfering with our view from the sky. Our old ground shield did a great job of this, but it also would have added some small polarization to the light that we measured. Hence, this guard ring.
This is the telescope in the docked position. From here, we can access the equipment which normally lives in the end of the boom, including the receiver itself. Note the blue tip of the new "snout" peeking up from the end of the boom.
Along with the new "snout" around our receiver, it keeps light from leaking around the outside of the dish and into our camera. With these changes, any polarization of the microwaves that we measure will actually be coming from the sky, and not from our telescope!
Monday, January 9. 2012
Today marks the arrival of the SPT's "night shift". Cynthia Chiang and Nicholas Huang completed the long trek to the South Pole, and will remain here throughout the long, cold polar night. No planes can fly in or out during the winter, so after the summer crew leaves next month, it will be up to our brave winterovers to operate and maintain the telescope until next November.
The entire crew is working hard to commission our new camera, so stay tuned for more updates!
Saturday, December 24. 2011
Putting a new camera on the telescope means taking the old one off. The SPTsze camera was on the telescope for 5 years, and with it we were able to make an excellent CMB temperature map of 2500 square degrees of the southern sky. Using that data, we've discovered many galaxy clusters, published power spectrum results, and generally accomplished a lot of science. It is an amazing camera, and as excited as we are about the new polarization camera, we're sad to see it go. Thanks to everyone who worked on SPT and the SPTsze camera for 5 great years of observations. Hopefully the new camera will be just as awesome.
Wednesday, December 21. 2011
It’s been a busy week. The cryostat that holds the new SPTpol camera arrived and everyone was busy installing the focal plane and closing up the cryostat, affectionately named Black Cat. Closing up required attaching nearly 20 readout cables to the back of the focal plane, then strapping each cable down to several different temperature stages so that the side of the cables attached to the camera stays cold while the other side remains warm at room temperature.
On the window side of Black Cat, the side looking at the telescope, we installed a plastic lens to help focus light onto our detectors, then more filtering and shielding from radio frequency light. We also added cones to the window so Black Cat could mate with the much larger Optics cryostat, which holds the telescope’s secondary mirror.
Mating the receiver cryostat to the Optics cryostat is tricky and requires several people moving both cryostats on chain hoists. But it all went smoothly and Black Cat is now under vacuum and cooling. Good luck, Black Cat!
Wednesday, December 14. 2011
There are about twelve people down at pole working on SPT so things are busy 24 hours a day. During the day the receiver and readout teams have been working on putting the focal plane together and testing the readout. During the night another team works on the guard ring.
The guard ring support structure was finished last night and you can see a beautiful picture of it below:
Yesterday evening Jason, Liz and I finished putting the focal plane together. In one of Liz's earlier posts you can see the feedhorns on the front of the detectors, but now they are covered up with filters (they look orange in the pictures). The filters block out most of the light coming from the sky so that our detectors only see the frequency of light they are designed to observe.
The SPTpol focal plane:
Monday, December 12. 2011
The Prime Minister of Norway is visiting the South Pole this week to celebrate the centennial of Amundsen reaching the South Pole on Dec. 14, 1911. Last night the Prime Minister gave a speech in the galley, highlighting how the international science that occurs at the South Pole is an inspiration to the world. He thanked the South Pole community for hosting him, and commented that though a warm welcome and hospitality are always appreciated, it is especially nice at the South Pole.
Today, the Prime Minister and his delegation toured around all the science sectors on cross country skis. They stopped by SPT and toured the Dark Sector Lab, where Brad Benson and Jamie Bock gave tours of SPT and BICEP2.
Friday, December 9. 2011
SPT is getting a new guard ring around the primary mirror to allow a cleaner view of the sky for the new polarization camera. Work has been going on for awhile now, and today the heavy welders started welding the support structure to the telescope. The guard ring will extend the primary mirror so that it measures 12m in diameter, though the illuminated area of the primary will remain on the precision inner part of the primary.
Wednesday, December 7. 2011
This season we are installing a new camera on the south pole telescope. Summer work at SPT has been going on for the last month, with teams deployed for installing a new guard ring around the primary, modifying the radiation shields in the optics cryostat to accept new filters, and setting up our new DfMUX readout system. They've accomplished a lot so far this summer, and hopefully you'll see some posts from them soon. I arrived at the south pole 2 days ago with the rest of the receiver team (Abby, Brad, and Jason), and our job for this summer is to assemble and test the new receiver. The new receiver is called SPTpol, and it is a polarization sensitive camera with 588 150GHz pixels (1176 TES bolometers) and 192 90 GHz pixels (384 TES bolometers).
The first thing we did upon arriving (after a good night's sleep of course) was head out to the telescope and unpack the focal plane parts that we had hand-carried to pole. Abby, Jason, and I each had a pelican case full of delicate detectors that we hadn't let out of our sight for 5 days. It was a relief to finally get them to the receiver lab where they would be safe. Once unpacked, we decided to test-assemble the focal plane parts that we had with us to ensure that everything fit as expected.
Even though all of the parts had been tested individually, and we had a test cooldown in the states with most of the focal plane parts, this was the first time that we had put all of the final focal plane parts together. Luckily, everything fits as designed and the focal plane looks great so far.
We are due to get the rest of the receiver parts through the cargo system in the next few days, at which point we can start assembling the cryostat and put the focal plane inside. In the meantime, the entire SPT crew continues to work hard on the many other tasks required to deploy the new camera.
|