Raw images obtained simultaneously by the three detectors of the X-Shooter instrument on the VLT

The light of a celestial object, in this case a quasar, is dispersed according to its wavelength, or colour to form a very long, very high resolution spectrum covering the full wavelength range from ultraviolet to infrared. The optical elements in the spectrograph arrange this long spectrum in a series of consecutive, shorter spectra, called orders. In this illustration, the wavelength increases from bottom to top of each spectrum, and from left to right. The spectrum of the quasar, recording its intensity as a function of the wavelength, is the thin bright lines visible in each of the orders. The short bright nearly horizontal lines correspond to colours where the atmosphere shines brightly.