Interferometry

In principle, the larger a telescope’s mirror, the finer the details it can see. Techniques such as  adaptive optics and active optics, both used by ESO, can unleash the full potential of a big mirror. Adaptive optics helps to counteract the effects of atmospheric turbulence, and active optics is used to steady the main 8.2-metre mirrors of ESO's Very Large Telescope (VLT). These two powerful techniques enable the four Unit Telescopes of the VLT to discern details as fine as 50 milliarcseconds, which is equivalent to the size of a two-euro coin seen from 100 km away. A telescope’s ability to discern details is known as its resolution.

Continuing to increase the size of telescope mirrors is not an easy task, so astronomers have come up with a new technology to see even finer details: interferometry. This observational technique combines the light received by two or more telescopes and allows them to act as a single unit with a mirror diameter equivalent to the distance between the telescopes.