A spectrograph is an instrument that separates incoming light by its wavelength or frequency and records the resulting spectrum in some kind of multichannel detector, like a photographic plate.

How do spectrographs work?

How Does a Spectrograph Work? A spectrograph passes light coming into the telescope through a tiny hole or slit in a metal plate to isolate light from a single area or object. This light is bounced off a special grating, which splits the light into its different wavelengths (just like a prism makes rainbows).

How do spectrographs help astronomers?

The astronomers use the spectrographs to classify the stars on the basis of the composition of the elements present in them. The element present in them absorbs all the wavelengths of light and reflects back the wavelength which is not absorbed by the element.