Wednesday, April 25, 2012

Astronomy: Observing at APEX

Last week, I went to San Pedro de Atacama in Northern Chile to participate in service observations at APEX Observatory. APEX stands for the Atacama Pathfinder Experiment and is a modified ALMA prototype 12-meter antenna located at the Chajnantor Plateau at a 5100-meter (16,000-feet) altitude. Like ALMA, APEX observes at submillimeter wavelengths and so requires such a high and dry site to observe.
The 12-meter diameter APEX radiotelescope

We actually had to take some medical exams to show that we could work at altitudes of 5000 meters. I'm happy to say that I passed those tests and had no problems with the altitude. That doesn't mean it was comfortable, though: the high site is pretty inhospitable. You have freezing temperatures during the day and it gets colder at night, though we spend the night down at the Sequitor base near San Pedro de Atacama at 2400-meter altitude. At 5000-meters, the atmospheric pressure is only about half of that at sea level, which means you have less oxygen to work with. Even walking outside for a short while will fatigue you. The control room inside was oxygenated so we could function properly. The thin atmosphere also means more ultraviolet light can pass through. That means we had to go out with sunscreen, sunglasses, hats, etc to prevent sunburn.
Panoramic shot at Chajnantor. APEX is near the center, ALMA is off to the left.

I mentioned APEX was a prototype ALMA antenna. I previously talked about ALMA in this article about the Fomalhaut disk. Well, both ALMA and APEX are located at Chajnantor. We saw ALMA every day when going up. Here's a photo of ALMA as seen from APEX:
The ALMA array, from APEX

In general, this place is extremely dry. However, the recent Bolivian winter has brought a lot of rain and so things were surprisingly green. That's good for nature, but not for submillimeter astronomy. Water is very good at absorbing (sub)millimeter light. It's actually how microwave ovens work, albeit those frequencies are somewhat different from what we observe here at APEX. Here is a plot of the atmospheric transmission at Chajnantor for various cases of precipitable water vapor (measured in millimeters):

Notice that for larger precipitable water vapor (pwv), the transmission is lower. That is: all that extra water is absorbing the sub-millimeter light. Typical conditions here have pwv of ~1 mm. During our run, however, we had some excellent weather the first 2-3 nights with pwv down to 0.2 mm. I had a program requesting 9 hours of time to reach a particular sensitivity, but we reached (and exceeded) the goal in just 5 hours.

What sort of science can you do at submillimeter wavelengths? It turns out a lot; with the Age of ALMA just around the corner we'll start learning a ton about the submillimeter universe.
Some of my research interests are actually tied to submillimeter astronomy: the study of circumstellar gas and dust disks. I've talked about gas-poor debris disks before (here), but there are also cases where gas-rich disks surround young stars. These are protoplanetary disks as they form the sites of planet formation and evolution. I'll leave talking about all the great science involved in protoplanetary disks for a future post and just mention that my project was targeting some of them.

Observing at APEX was a great experience, despite the frequent technical problems we ran into. Chile gets 10% of the observing time for any telescope in the country and APEX is no exception. That means Chile has about 12 days (removing engineering time) per semester for its science projects. This is split typically in 3 weeks throughout the semester and this was the first such week. Those who have accepted APEX proposals (I had two) are asked to assist in these observing runs if possible.
Sunset, as seen from the APEX telescope

The Sun gives off very little sub-millimeter radiation. That's an important fact because the Sun is the main reason why we can't see stars during the day at optical or infrared wavelengths and thus why most astronomers observe at night. No such limitation exists for sub-millimeter and radio observations and so we can carry out observations continuously for 24-hours. Radio and sub-millimeter facilities are very similar, but the extremely dry conditions required for sub-millimeter astronomy are not present for longer wavelength (like centimeter) astronomy.

There were 4 of us from Universidad de Chile attending the run; one was the supervisor who always has to go and schedule the observations. The rest of us got take shifts and go up (or stay down at night) to help carry out the observations. Most of it was actually fairly simple given the existence of reliable computer scripts to carry out tasks. It's only when we had technical problems that things got tricky and in that case all of us where not prepared (we're astronomers not engineers) and had to sit back and let the experts do their thing.

I did get a chance to help refill one of the instruments- LABOCA. It uses both liquid nitrogen and liquid helium to keep it cold enough to detect astronomical signals. I've refilled nitrogen dewars before (at Lick Observatory), but helium was a first for me. Did you know liquid helium is just a few degrees above absolute zero? Yeah, we had to take some precautions for handling that (see the video at the end of this post).
Inside the APEX telescope. LABOCA is the instrument on the left. SHIFI, what I used, was in the adjacent room.

After an exhausting few days of going up and down the mountain (a 1-hour trip each way) and working at high altitude, I figured it was time to take a break and check out the nearby town of San Pedro de Atacama. This was the first time I've been to northern Chile, so I wanted to visit before heading down to central Chile. This was my ride:

It's been nearly 20 years since I'd ridden a bike, but fortunately I still remembered how. I wore long sleeves, a hat, and plenty of sunscreen to avoid the burning Sun- a wise choice considering it took me ~30 minutes to reach town, far longer than I expected.
On the way to San Pedro de Atacama, we can see Licancabur volcano (the big one left of center). The Chajnantor plateau is between the two snow capped peaks in the middle right of the image.

My main stop at San Pedro de Atacama was the museum, pictured below. It had lots of relics and good description of the people living in the region for the past 10,000 years. After that it was a brief exploration of town. There really isn't much to see there, it's mostly a hub from where you can take tours and excursions to the more interesting places outside town, like the Moon Valley or the Tatio Geysers. I felt I didn't have a lot of time, so I didn't take any and just head back after a few hours. Maybe next time...
The archeological museum in San Pedro de Atacama

If you happen to have red/blue glasses, or know how to cross your eyes, you may want to check out my Picasa album of 3D pictures of APEX here.
One of several anaglyphs created from my APEX pictures.


2 comments:

  1. Very interesting post! Can't wait for the upcoming one on gas-poor disks :) Looks like you're having fun and doing excellent science in Chile!

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  2. Cool! Thanks for writing this up.

    - Julia

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