Radio window

[1][2] It plays an important role in astronomy; up until the 1940s, astronomers could only use the visible and near infrared spectra for their measurements and observations.

[3] The lower and upper limits of the radio window's range of frequencies are not fixed; they depend on a variety of factors.

The upper limit is affected by the vibrational transitions of atmospheric molecules such as oxygen (O2), carbon dioxide (CO2), and water (H2O), whose energies are comparable to the energies of mid-infrared photons: these molecules largely absorb the mid-infrared radiation that heads towards Earth.

[7] The lower limit is proportional to the density of the ionosphere's free electrons and coincides with the plasma frequency:

[8] When performing observations, radio astronomers try to extend the upper limit of the radio window towards the 1 THz optimum, since the astronomical objects give spectral lines of greater intensity in the higher frequency range.

[10] To tackle the issue of water vapour, many observatories are built at high altitudes where the climate is more dry.

Opacity of the Earth's atmosphere: the radio window spans larger wavelengths.
The Atacama Large Millimeter Array , an astronomical interferometer of 66 radio telescopes constructed on the 5,000 m (16,000 ft) elevation Chajnantor plateau in Chile .