helioscope

A helioscope is a device designed to observe the Sun. In solar astronomy the term is rarely used, with observers more often referring to solar telescopes, spectrographs, or radiometric instruments used to study solar light, spectra, and phenomena such as sunspots, flares, and the solar corona. Some older or specialized equipment used to image the Sun has been described as helioscopes, but the standard vocabulary tends to favor solar telescope or solar observatory.

A different sense of the term arises in particle physics. An axion helioscope is an apparatus built to search for axions produced in the solar core by converting them into X‑ray photons in a strong laboratory magnetic field. The concept, proposed by Pierre Sikivie in 1983, relies on the inverse Primakoff effect: solar axions entering the magnetic field region can convert to photons, which are then detected by X‑ray detectors at the ends of the magnet.

Designs for axion helioscopes typically feature a long, high‑field magnet that can track the Sun, a cryogenic or thermal system to maintain the magnet, and sensitive X‑ray detectors with low background. A buffer gas may be used to adjust the effective photon mass, extending sensitivity to higher axion masses.

Notable experiments include the CERN Axion Solar Telescope (CAST), which used a decommissioned LHC magnet with X‑ray detectors to search for solar axions and set limits on the axion–photon coupling. The proposed International Axion Observatory (IAXO) aims to improve sensitivity further with a larger magnet and optimized detectors.

See also: axion, helioscope (particle physics), solar telescope.