Astronomer on tour

The story of a trip to Chile to observe with the APEX telescope

6 December 2019
What you’ll discover in this blog post:
  • What it’s like to observe using the twelve-metre-wide APEX telescope
  • Why the desert environment surrounding APEX is like an ‘alien landscape’
  • Why APEX was built in such a high and dry environment but must be shut down when it’s too windy or sunny
  • What daily life is like during an ESO observing trip
Measuring a whopping twelve metres across, APEX is a submillimetre-wavelength telescope operating in the southern hemisphere and has a suite of instruments to find out more about the “cold”, “dusty” and “distant” Universe. APEX is operated by ESO on behalf of the Max Planck Institute for Radio Astronomy, the Onsala Space Observatory and ESO itself, meaning that many ESO astronomers get to spend time at the telescope each year. ESO Student Katja Fahrion tells us about her recent experience observing with this special machine.


The first day of my two-week observing trip to the Atacama Pathfinder EXperiment (APEX) began at 4 am on 22 August 2019 in the ESO Guesthouse in Santiago, Chile. After a quick breakfast, a taxi took me to the airport and at 9 am I was in Calama, in the Atacama Desert. A driver picked me up and after about an hour of driving through the desert, I arrived at the APEX basecamp, close to San Pedro de Atacama.

APEX is a submillimetre telescope, observing at millimetre and submillimetre wavelengths — between infrared light and radio waves, from a variety of astrophysical sources. It consists of a single dish with a diameter of twelve metres, located on the Chajnantor Plateau (the same plateau where ALMA resides!) 5100 metres above sea level. Unlike optical telescopes that only operate at night, submillimetre telescopes can also observe when the Sun is up.

So when I arrived at the basecamp at around 11 am, the morning observing shift was still ongoing. For the first time, I entered the control room — the heart of the basecamp. One wall is covered with screens showing the status of the telescope, the output of the live webcam and the weather conditions, and the other walls are lined with desks and even more screens.

Observers at APEX and other ESO telescopes don’t observe their own science targets, but instead carry out the observing programmes that are proposed by scientists from all around the world. At all times, at least one operator and one observer are present in the control room. While the operator is responsible for operating and controlling the telescope, the observer decides what to observe. The latter is my job as an astronomer and in the beginning, it seemed overwhelmingly complex.

Centre of the Milky Way with Jupiter and Saturn, taken by Katja during her APEX observing trip.
Credit: ESO/Katja Fahrion

I moved into my hut that contained a small desk, a bed and a bathroom. Since it gets very cold in the desert at night, each room also has several radiators and the beds are covered with blankets.

Besides the huts and the control room, there are office spaces, a kitchen where breakfast, lunch and dinner are served, a recreational room including a table tennis table and a rowing machine, and a swimming pool. The swimming pool, that I used almost every day, has a beautiful view of the Sairecabur volcano. During the night, this volcano is not visible, but the view is replaced by the beautiful southern night sky.


Although I got a brief introduction on the first day, I spent most of my second day at APEX in the control room learning how to observe with the telescope.

One specific parameter the observers have to keep in mind is the precipitable water vapour (PWV) describing the amount of water vapour in the atmosphere above the telescope. Because water absorbs electromagnetic radiation at the wavelengths we want to observe, it is critical to have low values of PWV, just like you would not want clouds over your optical telescope. A PWV of 0.4 mm is absolutely great, 0.7 mm is still very good, there are some programmes that can work with 1.5-3 mm, but basically above 4, there is not much to be done and above 6 the telescope is shut down.

Besides PWV, the wind speed is also shown in the control room because if it is too windy, the telescope has to be shut down and parked in a safe position. And then there is the Sun. Although APEX can observe during day, it cannot be pointed at or near the Sun because the antenna would focus the light and all the cables and instruments would melt. This is clearly something that we wouldn’t want to happen!

I felt the lack of oxygen as soon as I arrived; getting my backpack from the boot of the car was already exhausting.

I learned that it is essential to keep a record of everything that happens during an observation. We use a webpage where the records for every observing programme can be accessed and updated. This is important for the person that proposed the programme in the first place, but also for the APEX observers working different shifts.


On my third day at APEX, I got the opportunity to go to the telescope site in the morning with another student and two engineers. This meant driving up the hill from 2300 metres to 5100 metres above sea level. Although the drive is through the desert, on the side of the road, I saw cacti, bushes, donkeys, birds and vicunas.

Up at the telescope, the air is thin and has only half the pressure it has at sea level. I felt the lack of oxygen as soon as I arrived; getting my backpack from the boot of the car was already exhausting. I felt a bit weak and dizzy in the first few minutes, so I was happy to enter the control room that is supplied with extra oxygen.

The APEX antenna.
Credit: ESO/Katja Fahrion

While the two engineers worked on the telescope generators, the other student and I spent some time in the control room to acclimatise. But soon the excitement won, and we went out to take pictures of extraordinary sight up on the Chajnantor Plateau. In the distance, I could see the 66 ALMA antennas under a clear blue sky, surrounded by volcanoes.

Going up to the telescope was not the only exciting event on this day. Every Saturday, the Asado takes place. Everyone gathers at the kitchen and even the observers and operators bring their laptops to observe remotely. There are drinks and many different foods such as deep-fried cheese empanadas, ceviche and small sandwiches. There is also a barbeque with lots of beef and sausages. Music plays and after dinner the party carries on in the kitchen or around the fireplace.


On the fourth day of my stay at APEX, I carried out observations during the evening shift for the first time on my own. During the previous days, I had become accustomed to the different observing programmes and roughly knew the weather constraints and priority of observing targets on the sky. Due to Earth’s rotation, the targets move in the sky and can only be observed when they are high enough above the horizon. So it is important to know which programme can be observed at any time of the day. This has to be balanced against the weather conditions and the priority of the programme, but after a few days of watching other observers making decisions, I was able to continue with ongoing projects.

Two flamingos having a drink at Laguna Chaxa.
Credit: ESO/Katja Fahrion


At the beginning of my stay, there were at least four observers at any time, so shifts lasted six hours instead of the typical eight hours. This meant that we had a lot of free time, especially as I was not yet on the official schedule. So on 26 August, another student and I drove to the nearby Laguna Chaxa. An hour’s drive from the basecamp, this Laguna is known for its beauty and an impressive flock of flamingos.


On 27 August, I began my (almost) regular shift schedule of 5 pm to 11 pm. On this day and the next, I had quiet shifts because the weather was not great. We observed a very time-intensive programme with the instrument PI230 that can be used even when there is a lot of water vapour in the air. We created maps of a molecular gas cloud in our own galaxy, the Milky Way. Because molecules such as carbon monoxide form at very low temperatures, they are not visible with optical telescopes. With submillimetre telescopes like APEX, however, we can observe bright spectral lines at a very specific wavelength and can thus observe the source. With telescopes such as APEX it is possible to either observe a single spectrum or to create a small map of a region in the sky that shows the structure of a source emitting at a certain wavelength. In both modes, it is also important to observe a reference position in the sky to remove unwanted background emission from Earth’s atmosphere. Sometimes the reference position is contaminated by other astronomical light and this is one of many reasons why the observer has to look at the data while they are being taken.

Large and Small Magellanic Clouds above the antennas that are used for communication between basecamp and the APEX telescope.
Credit: ESO/Katja Fahrion


My first and only night shift was from 10 pm to 4 am. During this night, the weather conditions were very good at first, so we used the ArTeMiS instrument that requires the best conditions to create beautiful maps of astronomical sources. Later, we switched to SEPIA. Switching the instrument requires some time, so it’s best not do it too often. After my night shift, I was very tired, but I took the opportunity to take some pictures of the night sky.


After my night shift I slept in. The weather was not great again, so during my shift in the evening, we made more maps with PI230. It was a relaxing shift that gave me time to work on my own projects.


On the second Saturday of my stay, I had the opportunity to go back up to the telescope. Even the second time, the visit was exciting. On the way, we saw llamas and several Vicunas that were very close to the road. My shift was during the Asado, but I could still spend some time with the others in the kitchen, enjoying empanadas and the barbecue.

I would get up, have breakfast, work on my PhD project and go swimming. In the evening, from 5 to 11 pm, I was in the control room doing my shift.
Valle de la Luna
Credit: ESO/Katja Fahrion


Only a few days of my shift at APEX were left and by then I was used to the routine. I would get up, have breakfast, work on my PhD project and go swimming. In the evening, from 5 to 11 pm, I was in the control room doing my shift. The weather was at first very good for observing with the most demanding instruments but then it got worse and we even had to close the telescope for an hour one night due to strong wind. The sunsets during these last days were beautiful because for the first time, there were clouds in the sky.

On my last full day, 4 September, another observer from ESO and I visited the nearby Valle de la Luna. We were rewarded with astonishing views of an alien-looking landscape — similar to the surface of Mars or the Moon!


After 13 nights at the APEX basecamp, it was time to leave. I had finished my last shift the day before, and after lunch, the driver brought me to the bus terminal in Calama. From there I took a four-hour bus ride to Antofagasta, 300 kilometres southwest of San Pedro. The next day, an official ESO bus took me to my two-night stay at the Very Large Telescope. Not to work, but just to visit.

Katja Fahrion

Numbers in this article

5 Time in the afternoon that Katja’s observations started.
6 Limiting PWV value for APEX to be shut down.
11 Time in the evening that Katja’s observations finished.
12 Diameter of the APEX dish.
13 Number of nights Katja spent at the APEX basecamp.
66 Number of antennas that make up ALMA.
300 Number of kilometres between Antofagasta and San Pedro.
2300 Height above sea level of the APEX basecamp (in metres).
5100 Height above sea level of the Chajnantor Plateau (in metres).

Biography Katja Fahrion

After receiving her bachelor’s and master’s degrees from the University of Heidelberg, Katja Fahrion moved to ESO Headquarters near Munich to carry out her PhD. At work, Katja focuses on star clusters and their role in galaxy evolution, and her hobbies include swimming, diving, hiking and cooking.