The South Pole Telescope Blog - SPTpol deployment

First Light!

nospam@example.com (Elizabeth George) — Thu, 26 Jan 2012 10:23:35 -0600

We started hoisting the optics/receiver cryostats into the telescope cabin at 9am on Jan 25, 2012. A mere 39 hours later at 11pm on Jan 26th we achieved first light with SPTpol! Below are detector timestreams as we scan across RCW38, one of the many calibration sources we use. In the next few weeks, we'll be working hard to characterize our instrument. We will make maps of calibration sources, characterize detector response and time constants, map our beams, get the telescope in really good focus for the winter, and measure our detector polarization angles. There is a lot to do to get ready for the observing season, but we're all really excited that we can see astrophysical objects with our new instrument! Congratulations and thanks to everyone who worked so hard to make this instrument possible. Stay tuned for updates!

First light with SPTpol! The dips in the timestreams are where we are scanning across RCW38, one of the calibration sources we use.

Sealed Up! (Last Time?)

nospam@example.com (Stephen Hoover) — Sun, 15 Jan 2012 19:27:41 -0600

The inside of Black Cat, just before the start of the third cooldown.

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.

This is the Black Cat connected to the (large, white) optics cryostat.

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.

A Square Dish!

nospam@example.com (Stephen Hoover) — Thu, 12 Jan 2012 16:34:42 -0600

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.

Staring into the dish of the South Pole Telescope. Now with even more guard ring!

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!

The telescope in its observing position.

Thanks for 5 great years!

nospam@example.com (Elizabeth George) — Sat, 24 Dec 2011 21:49:19 -0600

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.

Martin Lueker, who was a graduate student at Berkeley who worked on the SZ camera, hugs the receiver goodbye. Martin is now a postdoc on the Keck/SPUD experiment, which is also deploying at the south pole this year.

Guard ring welding continues

nospam@example.com (Elizabeth George) — Fri, 09 Dec 2011 20:16:24 -0600

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.

SPT with part of the guard ring support structure installed.

Welders working on installing the new guard ring support structure.

Receiver team arrives at pole

nospam@example.com (Elizabeth George) — Wed, 07 Dec 2011 21:21:59 -0600

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.

Jason Henning putting the finishing touches on the assembly of the 150 GHz modules.

Abby Crites touching up solder joints on the PCB board connecting to the 90 GHz pixels.

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.

The backside of our focal plane partially assembled.

Liz George holding the partially assembled focal plane. The 150 GHz horns are open, while the feedhorns on the 90 GHz pixels are covered in aluminum tape.

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.