European Southern Observatory

Astronomy On-Line

from the Atacama Desert

Welcome to La Silla!

The La Silla observatory is run by the European Southern Observatory (ESO). The observatory is located atop a 2400-m mountain in the southern extremity of the Atacama desert in Chile. La Silla has its own website where you might either want to read more about La Silla or even view the La Silla Picture Gallery.

Why La Silla?

La Silla was chosen as a site for an observatory in the southern hemisphere on account of its superb ``seeing'', its clear weather, and its dark skies.

Good seeing

The term ``seeing'' is used by astronomers to describe the affect the atmosphere has on the light passing through it. The atmospheric conditions at La Silla (steady, non-turbulent winds) give rise to sharp images, thus allowing better observations to be made. To check what ``seeing'' you can expect during your observations, go to the La Silla Seeing Statistics page.

Great Weather

During typical years, about 70 percent of the night-time is useful for astronomical observations. In winter, an occasional snow storm may disrupt observations for a few nights, while during the rest of the year nights can be lost to strong wind, high humidity or cloud cover. Both statistics of La Silla's weather and a current satellite map are available.

Dark Skies

The brightness of the night sky, in part, determines how faint an object can be observed with a telescope. The brighter the night sky, the more ``noise'' there is in an image, and the harder it is to distinguish an astronomical object from this noise. After sunset at La Silla, heavy curtains are drawn in all rooms and cars can only be used with their side lights on. This helps contribute to the darkness of the La Silla sight. One of La Silla's big advantages is its remoteness from civilization and the inevitable light pollution which accompanies it (those of you interested in learning more about light pollution may want to contact the International Dark-Sky Association ). The nearest sizeable town to La Silla is La Serena, at a distance of around 150 km, which on a moonless night can be seen as a faint orange glow on the horizon. On the nights of the Astronomy On-Line observations (November 19/20 and 20/21), the Moon is around 75% illuminated (see ephemerides ) meaning that the La Silla night sky will be moderately bright.

General Information

Geographical data:
European Southern Observatory
Alonso de Cordova 3107
Casilla 19001
Santiago 19 Chile, S.A.
Phone: (+56) 02 228-5006
Latitude: 29o 15' South
Longitude:70o 44' West
Altitude: 2400 metres

The Dutch 91cm Telescope

Astronomy On-Line will have the nights of November 19/20 and 20/21 to make astronomical observations at the Dutch 91cm telescope. A general description of the telescope is available, and we suggest you read this first. There is also a Users Manual which contains much information unnecessary for Astronomy On-Line observers, however, some observers may find it of use and/or of interest.

Preparing a Proposal

To help you in the preparation of your observing proposal, we have made a few notes on the capabilities of the Dutch 91cm telescope, as well as other practical matters.


The observations will be carried out by members of ESO's 2p2 Team (the team responsible for the operation of the small and mid-sized telescopes at La Silla). You can reach the 2p2 Team by sending email to The images will become available about 5 to 10 minutes after the completion of each exposure. They may then be downloaded from this directory. The image format will be FITS and they will be compressed with gzip (which can be downloaded here for PC's (winzip for windows). gzip as source code and precompiled for different Unix platforms is available from

Please send any comments about this page directly to the Astronomy On-Line Webmaster for La Silla - James Brewer (