Processing the Spectral Image

The image may be that of a slit, onto which an extended object has been focused, or simply that of a star when the spectrograph is used in slitless mode. The former would be used for the Sun (carefully, or off a cloud or other reflective surface), the moon, planets or nebulae. A slit would also be used for testing and calibrating the spectrograph by using an artificial light source such as a Hg or Ne lamp.