Telescopes The observational study of Cosmology is difficult, because it mostly concerns extremely distant and therefore also very faint celestial objects. For this reason, astrophysicists often need to use giant telescopes with large mirrors that can collect sufficient light. Different types of telescopes are used to observe different types of electromagnetic radiation. On the ground, there are radio telescopes and optical/infrared telescopes. Radiation of certain wavelengths, in particular very energetic (short wavelength) radiation, cannot penetrate the Earth's atmosphere, and observations in the gamma-ray, X-ray and ultraviolet spectral regions have to be done from orbiting telescopes. Large distances and very faint objects The more distant an object is, the longer the light has taken to reach us. Light moves with a speed of approx. 300,000 km/sec. We therefore see the Moon - at a distance of about 400,000 km - as it was 1.3 seconds ago. The Sun's light reaches the Earth after having travelled 150 million km during 500 seconds and that from Pluto, some 6 billion km away, arrives after nearly 6 hours. Astronomers use light-years to indicate distances in the Universe - one light-year is the distance travelled by light in one year, or approximately 10,000,000,000,000 km. The nearest bright star, Alpha Centauri, is 4 light-years away and the most distant galaxies seen with modern telescopes are so remote that their light has travelled many billions of years. In fact, we see some of them as they looked less than one billion years after the Big Bang. Observational methods Objects in the early Universe are studied in different ways. For instance, by obtaining photos with different optical filters in front of the camera, it is possible to measure their shapes and colours. When spectra are obtained, a rough chemical analysis can be made, indicating which atoms and molecules are present in such objects. Moreover, their velocities can be measured by means of the Doppler effect in the spectra - since the velocity is proportional to the distance, this gives an estimate of the distance (the Hubble law). New telescopes During the past few years, several large optical/infrared ground-based astronomical telescopes have been constructed. In addition to the ESO VLT, the new telescopes include the Keck, the Subaru and the Gemini telescopes. The orbiting NASA/ESA Hubble Space Telescope (HST) has also provided many important contributions to cosmological studies.

"KUEYEN" is one of the four 8.2-m telescopes that comprise the ESO Very Large Telescope (VLT). It is used, among other things, for cosmological studies of the most distant objects in the Universe.