Entries by Stephen Hoover

Monday, January 23. 2012

Our Fourier Transform Spectrometer, cuddling up to the Black Cat.

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!

The FTS mounted on the optics cryostat.

Posted by Stephen Hoover at 19:00 | Comments (0) | Trackbacks (0)

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Sunday, January 15. 2012

Sealed Up! (Last Time?)

If all goes well, we won't see this again until next year!

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.

Posted by Stephen Hoover in SPTpol deployment at 19:27 | Comments (0) | Trackbacks (0)

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Thursday, January 12. 2012

A Square Dish!

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.

It's a beautiful telescope, isn't it? ... Even if it is square.

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.

Posted by Stephen Hoover in SPTpol deployment at 16:34 | Comments (0) | Trackbacks (0)

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Monday, January 9. 2012

Winterovers Arrive

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!