spectrohelioscopes

A spectrohelioscope is an astronomical instrument designed to observe the Sun in a narrow spectral band by isolating light from a specific spectral line, such as hydrogen alpha (H-alpha) or calcium II K. This highlights features of the solar chromosphere and upper photosphere that are often faint in broadband images, and it supports both real-time viewing and guided imaging.

Light from the Sun is passed through a dispersing element or a narrow-band filter to isolate the chosen line. An adjustable slit or tunable monochromator selects a small wavelength range, and the re-imaged solar disk forms a monochromatic image on an eyepiece or detector. Some designs scan the image to produce a full-disk view in that line.

Developed in the late 19th to early 20th century, spectrohelioscopes enabled study of solar activity by observing prominences, filaments, and plages in specific lines. They are related to spectroheliographs, which photograph monochromatic solar images rather than display them in real time.

Modern solar instrumentation often uses narrow-band filters, Fabry-Pérot etalons, or tunable spectrometers to achieve similar monochromatic views, with H-alpha remaining a standard diagnostic of chromospheric activity and Ca II K used to study magnetic networks and plages. The term remains a historical marker of an instrument class that advanced solar spectroscopy.