An astronomical observatory is any infrastructure which contains telescopes and supporting sets of instruments with which the celestial bodies can be observed. They can be different types depending on the part of the electromagnetic spectrum they are primarily designed to study. The largest number are the optical observatories, i.e. the ones that study mainly the visible (with the human eye) light coming from the sky. Other ground-based observatories are built to study the cosmic radio waves. There are also space observatories, the majority of which have special detectors and instruments for studying the space emission that can not reach the Earth’s surface, such as the high-energy gamma and X-rays and the less energetic infrared emission. Other observatories are built underground or underwater aiming to detect even more exotic types of particles and radiation. In this section we describe the different types of astronomical observatories and attach images with examples. We also offer some practical exercises for students with different ages.

Astronomical observatories are mainly divided into four categories: ground-based, space- based, airborne, and underground/underwater-based.

Ground-based observatories.

The classical ground-based astronomical observatories are stationary scientific installations with well developed infrastructure, equipped with scientific instruments for systematic observations of celestial objects and phenomena. They may have one or more optical or other – mainly radio, but also near-infrared and millimeter telescopes of different sizes.

The majority of the optical ground-based observatories are built far away from the big cities in order to avoid the light pollution. Ideal sites for such modern observatories are high mountain tops with dark skies, dry air, a lot of clear nights and a calm atmosphere with minimal turbulence, which leads to a better quality of the astronomical images obtained. The best sites on Earth are the peaks of the Hawaii and the Canary islands, the high tops of the Atacama desert in Chile, the state of Arizona in the USA, etc. At these sites at present are built the most powerful optical ground-based observatories: ESO (the European Southern Observatory) and Paranal Observatory in the Atacama desert (Chile), Roque de los Muchachos Observatory on the Canary Islands (Spain), and Mauna Kea Observatory on the Hawaii Islands (USA).

At the moment, the newest ESO observatory is under construction on the Cerro Armazones mountain in the Atacama desert (at 3046 m above sea level). It will be the home of the largest optical and near-infrared telescope on Earth – the 39.3 m telescope ELT (Extremely Large Telescope).

Leading optical observatories in the partner countries are:

• SK - Skalnaté Pleso Observatory:

• CH - Ondřejov Observatory:

• BG - National Astronomical Observatory Rozhen: http://nao-

In the observatories exploring the emission of celestial objects in the radio domain, the main instrument is the radio telescope. As an operating spectral range, the radio telescope takes first place in the study of the electromagnetic emission; the infrared, optical, ultraviolet, X- ray and gamma-ray telescopes are operating at higher wavelengths (in that order).

The radio telescopes may have different sizes, configurations and construction. This is due to the fact, that the radio domain of the electromagnetic spectrum is very large. For observations at frequencies 10 – 100 MHz (wavelength of 30 m to 3 m), the radio telescopes are directional antennae similar to the TV antennae. Or they can be big, stationary reflectors made of metal mesh with a movable focus. For higher frequencies, parabolic dishes are used. To obtain better spatial resolution, the sizes of the radio telescopes need to be significantly bigger than, say, optical telescopes. For example, for frequencies of 100 MHz to 1 GHz (3 m - 30 cm wavelength), the dishes have diameters of about 100 m. Often, to obtain even better spatial resolution, radio interferometers are used. These consist of many separate dishes, placed in special arrays, that are so finely tuned that they can combine the signals they observe in a way that simulates a single very large antenna with extremely high spatial resolution.

As a rule, radio telescopes are built at great distances from populated areas in order to avoid or minimise the contamination of the celestial signals by ground-based artificial radio sources such as TV and radio signals, radars, microwave ovens, wi-fi networks, etc. Unlike the optical observatories, radio observatories are situated in valleys, where they are better protected from technological noises.

The largest single-dish radio telescopes are the 500 m Five hundred meter Aperture Spherical Telescope (FAST) in China and the 305 m Arecibo telescope, situated in the crater of the extinguished volcano Arecibo on the island of Puerto Rico. The largest single-dish telescopes with movable dishes are the 100-m Green Bank Telescope in West Virginia, USA and the 100-m Effelsberg telescope near Bonn, Germany. The most famous interferometer is the Very Large Array (VLA) in Soccorro, New Mexico, USA. It combines the signals, received simultaneously from 27 separate parabolic antennae.

Some images of ground-based observatories are shown in Attachment 1.