ESO's La Silla Observatory

Chile has one of the darkest night skies on the planet and this location, just at the edge of the Atacama desert, also provides consistent nightly observation opportunities due to the lack of humidity and moisture and hence those pesky clouds that are an astronomers, whether amateur or professional, natural enemy. Nicely protected between the cold Humboldt stream and in the rain shadow of the Andes, this sliver of Chilean land is home to some of the biggest telescopes in the world.

Source: www.lightpollutionmap.info

After a 2-hour drive on ROute 4 you get to the ESO' La Silla exit and after a few minutes rides through the desolate landscape, you are rewarded with a view of Cerro La Silla ("The Saddle" in Spanish) in the distance.

A breathtaking view (not just because of the altitude) welcomes you on your drive to ESO's La Silla Observatory in Chile. A few minutes from this vantage point above you are arriving at the security gate where you have to hand over your visitor's form. You can request visits to La Silla, which are run every Saturday afternoon starting at 2:00pm, on ESO's website. Once all the visitors are assembled, there were 10 cars with various adults/kids from Canada, Germany, and mostly Chile, you begin your 15-minute drive up a winding access road while ascending about 1,000 meters in altitude to make it to the peak. You are greeted by the guides, which are local astronomy graduate students. Our English speaking guide is attending school in La Serena. All tours are managed by Astronomy Chile. The tour began in earnest with a 20-minute presentation by the main guide (all in Spanish) about ESO, how telescopes work and what type of telescopes are on the observatory. The image below shows the impressive collection of telescopes on La Silla. The ones currently in operation are denoted in green and their science objectives will briefly be explained later in the post.

We separated into the Spanish and English group and our first stop was the NTT (New Technology Telescope), a 3.58 meter primary mirror telescope, which saw first light in 1989. It is a Ritchey-Chretien telescope, with an adjustable a hyperbolic primary mirror and a hyperbolic secondary mirror. While writing this post I I found out that the primary mirror of the NTT was polished to the wrong shape, like Hubble, but the active optics system was able to correct this error. The whole telescope building is an azimuth mount, meaning it rotates to move the telescope in the proper position. You might notice the shape of the telescope building is not the typical dome-structure, but a relatively small octagonal building, ventilated by a system of flaps to allow for smooth air flow across the primary mirror. Usually, telescope buildings are ventilated to match the outside temperature to decrease temperature differences, which can lead to deformations of the mirror, something you do not want to happen.

Inside the NTT building, you will encounter a magnificent marvel of human engineering. A simple concept, first introduced by Dutch eyeglass maker Hans Lippershey in the 17th century. News of the invention traveled fast, well, for the times, and made it's path to Galileo Galilei, who dramatically improved on the device and started observing the Sun and sunspots, the phases of Venus, the craters on the Moon, the moons of Jupiter, and brought the Catholic Church to its knees. Simply, the telescope with the help of a primary mirror, captures light, reflects it to a secondary mirror, and focuses the image for an observer. The bigger the mirror, the more light is captured and the better the image is resolved. In the image below you can see the primary mirror (3.6 meter diameter), the fans that keep air moving across the mirror and prevent heat from deforming it and the actuators, who adjust the shape of the primary mirror which is deformed due to external factors like temperature and mechanical stress (gravity). Active optics is a technology that was developed in the 1980s. Without this technology, the construction of larger mirrors would not be possible. It is not to be confused with the concept of adaptive optics, which acts on a much smaller timescale and adjusts for atmospheric disturbances.