Observatories

Overview

An observatory is a facility equipped for astronomical observation. Site selection is as consequential as instrument design: altitude, dryness, atmospheric stability and darkness determine what a telescope can achieve before a single optical component is chosen.

• The best optical sites are high, dry, dark and — above all — atmospherically stable.
• Seeing, the blurring caused by turbulence, is the property that most limits ground-based resolution.
• Radio observatories need radio quiet rather than darkness, which drives them to remote inland regions.
• Growing light pollution and satellite constellations are measurable threats to existing sites.

What makes a good site

• Altitude: being above a significant fraction of the atmosphere reduces absorption and turbulence, and is essential for infrared and submillimetre work.
• Dryness: atmospheric water vapour absorbs infrared strongly. The driest high-altitude sites, such as the Atacama, are the only viable locations for some wavelengths.
• Stable airflow: smooth laminar flow gives good seeing. Coastal mountains with a temperature inversion above cold ocean currents — Chile, Hawai'i, the Canary Islands — are favoured for exactly this reason.
• Darkness: distance from artificial lighting, and ideally legal protection of the surrounding sky.
• Clear nights: a high fraction of usable nights per year, which is what makes desert and subtropical high-pressure regions attractive.

The major optical sites

Northern Chile hosts an exceptional concentration of facilities — Paranal, La Silla, Cerro Tololo, Cerro Pachón and the ALMA plateau — because the Atacama combines extreme dryness, high altitude and stable air. Mauna Kea in Hawai'i and Roque de los Muchachos in La Palma are the other principal optical sites, both high volcanic peaks above a marine inversion layer.

Site development is not only a technical question. Mauna Kea in particular is a site of significant cultural importance to Native Hawaiians, and telescope construction there has been the subject of sustained dispute. Recording that is part of describing the facility honestly.

Radio observatories have different requirements

Radio astronomy is unaffected by daylight or cloud, so darkness and clear skies are irrelevant. What matters is freedom from human-generated radio interference, which is why major facilities sit in remote inland areas, often inside legally protected radio quiet zones.

Radio telescopes also gain resolution by interferometry: linking widely separated dishes so that the effective aperture is the separation between them. Very long baseline interferometry links antennas across continents, and the Event Horizon Telescope extended that to an Earth-sized synthetic aperture, which is what made horizon-scale black hole imaging possible.

Threats to existing sites

Light pollution has increased measurably at many established observatories as nearby settlements have grown. It raises sky background and disproportionately harms observations of faint extended objects; some observatories now depend on local lighting ordinances to remain viable.

Large satellite constellations are a newer concern. Bright satellites leave trails across long exposures and are a particular problem for wide-field survey instruments, and their radio emissions can affect radio observatories. Mitigation efforts — darkening treatments, orbit and orientation changes, and software rejection of affected pixels — reduce but do not eliminate the impact, and the scale of the problem grows with the number of satellites launched.

Continue in the data

Catalogues, hubs, and reference pages that hold the underlying records for this topic.

• Observatories catalogueFacilities, telescopes, and sites as entities.
• Observatory frontierNext-generation facilities and detectors.
• InstrumentsThe instruments mounted on these telescopes.
• InstitutionsThe organisations that operate them.

Frequently asked

Why are observatories built on mountains?

To get above part of the atmosphere. Higher sites have less air to look through, less water vapour absorbing infrared, and — where a marine inversion layer produces smooth laminar airflow — better seeing. Altitude also usually means distance from urban lighting.

What is astronomical seeing?

The blurring of images caused by turbulence in Earth's atmosphere, measured as the apparent angular size a point source is smeared into. At an excellent site it is around half an arcsecond or better; typical locations are worse. It is the property that most limits ground-based resolution, and it is the problem adaptive optics exists to solve.

Why are radio telescopes in remote places if radio waves pass through clouds?

Because the threat is interference, not weather. Radio telescopes detect extraordinarily faint natural signals that are easily swamped by transmitters, mobile networks and even electrical noise from vehicles and appliances. Remote inland locations, often inside legally designated radio quiet zones, minimise that contamination.

Are satellite constellations affecting astronomy?

Yes, measurably. Bright satellites leave trails across long exposures, which is especially damaging for wide-field survey telescopes, and their radio transmissions can interfere with radio observatories. Operators have implemented darkening and orientation mitigations that reduce brightness, but the impact scales with the number of satellites in orbit.