A raw spectrum obtained with EFOSC2

A long, narrow slit isolates a small strip of sky. On this image, the slit is horizontal. The spectrograph then splits the light from the slit into its individual colours, with each point along the slit forming a vertical rainbow. In this spectrum, the light from NGC 5775, a galaxy, forms faint vertical bands. The bright vertical line is the spectrum of a star. The horizontal lines correspond to colours at which the sky itself emits light.