Outside of the very near infrared and visible portions of the electromagnetic (EM) spectrum, and parts of the radio, the atmosphere is opaque to other wavelengths, which must be observed using instruments on spacecraft, orbiting above the obscuring atmosphere. Visible wavelength space observatories also have the advantage of being above the effects of the atmosphere's scintillation (twinkling), making for sharper images of celestial objects.
Links to selected recent, operating or in-development space telescopes are listed at left. All these instruments are sensitive to EM radiation. A common unit of energy for measuring EM radiation is the electron-volt (eV), the change in energy of one electron moving across a one-volt difference. The prefixes are T (Tera = 1012), G (Giga = 109), M (Mega = 106), k (kilo = 103), m (milli = 10–3), μ (micro = 10–6), and n (nano = 10–9). Thus, the EM spectrum spans many decades in energy, with the vast majority of it lying outside of the visible, which spans only a factor of two in energy: 1.6 eV (red) to 3.4 eV (violet).
For reference, 1 eV (energy) ↔ 1.24 μm (micrometers, wavelength). Energies and wavelengths of light are inversely related — the formula is:
Energy (in eV) = 1.24 ÷ [Wavelength (in μm)] ;
whereas energies and frequencies of light are directly proportional:
Frequency (in Tera Hz) = 242 × Energy (in eV) .
The spacecraft ordering is from highest to lowest energy (values are rounded): gamma-ray (100's of GeV to 100 keV); X-ray (100 keV to 100 eV); ultraviolet (100 eV to 4 eV); the visible; infrared (1 eV to 1 meV); and microwave (1 meV to 1 μeV). Several of these spacecraft have multiple experiments, spanning more than one portion of the EM spectrum. Note that TeV gamma rays are observed using ground instruments.