Infra-red (IR) lies between the optical and radio parts of the electromagnetic spectrum at wavelengths of microns (millionths of a metre). Anything that is warm emits IR photons - outdoor security lights are often triggered by IR movement sensors.

Because clouds and water vapour in the atmosphere affect IR light, these observatories are usually situated on very high mountains or orbiting the Earth well above the atmosphere.

Famous infra-red observatories include UKIRT on Hawaii, the ISO satellite, the Spitzer satellite, and Gemini in Hawaii and Chile.