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On top of the world: An observing trip to La Palma

Popular Astronomy magazine, Jan/Feb 2016

I must confess that as a professional astronomer I rarely take time to admire the night sky or look through a telescope anymore. Instead I spend most of my time in front of a computer looking at the spectra of stars. However, one of the perks of the job is that occasionally I run out of spectra and need to go and get some more. After being awarded some telescope time on the Nordic Optical Telescope (NOT) at the Observatorio del Roque de los Muchachos on the island of La Palma, I set out on an observing trip at the end of September 2015.

While I had been observing on La Palma in 2013 as part of the NEON Observing School, this was the first time I've ever had to acquire my own data; normally it just appears in my inbox courtesy of another astronomer.

Even though La Palma is part of the popular Canary Islands, it is not easy to get to! Direct flights to the island from the UK are difficult to come by, so an overnight stay in Tenerife was necessary before catching my next flight. However, the most difficult part of the journey is getting up the mountain itself. It stretches over two kilometres above sea level, and the only way up is via a sickeningly winding road with sharp drops off the edge. I held my breath every time the driver took the "racing line" around the corners, but thankfully got distracted by the amazing scenery. The pine forests give way to a harsh, brown landscape, adorned with some spectacular rock formations. The entire island is one large volcano, and as someone with a strong interest in geology, this made my new playground even more enthralling.

I first headed up to the telescope in the afternoon, where I was greeted by the sign "Caution. This building may rotate without warning." Not what you would expect to see on a building, but the NOT was designed so that the entire building rotates with the telescope, including the astronomer observing in the room below. After learning about the telescope, and more importantly, where to make coffee, it was time to head back for dinner before the night's observations began.

The first thing to be done when setting up for a night on the NOT is to open the sideports, which are four sets of two doors along the side of the dome that have to be opened manually. They increase the airflow in the dome, and hence the stability of the observations. Another bonus is that there is an incredible view at sunset of some of the other telescopes on the brink of the caldera. Then the upper and lower hatches are opened via computer commands, and finally the mirror is unveiled.

Once darkness enveloped the mountain, it was time for observations to begin. However, these weren't what I had planned because a telescope override request had come through. A gamma ray burst had just erupted and it was crucial to snap a picture of it as quickly as possible. This was done using the instrument ALFOSC and by comparing the new image to previous ones it was possible to pick out the gamma ray burst.

After this, it was finally time for me to begin getting my own data. My research involves looking at the spectra of stars that are similar to our Sun and trying to tease information out of them. A stellar spectrum is comprised of "lines" caused by different elements absorbing light from the stellar atmosphere. The shape of these lines depends on the temperature and pressure of the star, how many "metals" it contains (that is everything other than hydrogen and helium), the rotation speed of the star, and the convection of material in the atmosphere. All of these effects mean that spectral lines aren't really lines at all and are actually "u" or "v" shaped. I'm currently trying to understand the exact shape produced by the combination of the stellar rotation and convection, but to do this I need spectra of an exceptionally high quality. This is where the NOT, and its high resolution spectrograph FIES come in.

My target stars are in the Kepler field, which means I could only observe for the first half of the night. After around 01:00 UT, the Kepler field gets too low and the light from the stars has to travel through more of the Earth's atmosphere than it does when the stars were higher up, which makes observations too difficult.

On the first night, the humidity hovered around 90%. This is dangerously high, and if it stays above 90% for more than a few minutes, the mirror covers will automatically close to protect the mirror. Fortunately for me, it never went above 90% for very long, although the threat was constantly there. On the second night, I wasn't as lucky, and couldn't observe at all.

At this time of year, the Canary Islands fall victim to the Calima, a weather phenomenon which blows dust in from the Sahara desert. This makes everything very hazy, but fortunately most of my stars are very bright so I could still observe. There is another risk however, in that I have to close the telescope if the wind speed gets above 20 metres per second. In the case of dust, this limit falls to only 12 metres per second. Even though the wind kept threatening to end my observations, I still managed to get my data on the third and fourth nights.

My final two nights were perfect. No Moon, no dust, no clouds, and the seeing was great. After setting my observations running, I went outside to appreciate the incredible view of the night sky from the mountain top. The Milky Way was instantly visible upon stepping out of the dome, and it amazed me how prominent it was, like an artist had added strokes of shimmering paint to the night sky. The familiar constellations became washed out as ordinarily invisible stars poked through the veil of the sky and I was overwhelmed by the sheer number of them. The starlight was even bright enough to see by - although to be on the safe side I didn't go too far without using a torch. The usual orange glow of a nearby town or city was noticeably absent and all that blocked my view of the pristine sky was the telescope dome. Added to the spectacle was the bright green laser used by the 4.2 metre William Herschel Telescope for its adaptive optics, which sliced through the night sky like a knife.

Such an incredible view made me remember what sparked my interest in astronomy in the first place. I grew up in the Irish countryside with very little light pollution, and while those skies can't rival those of La Palma, they always made me want to learn more about the stars. Perhaps now I will remember to occasionally shut down the computer, and go outside to look up.