How Do You Make a Spectrograph?

There are two very different ways to measure spectra.

In the first class of instruments, scientists use the wave-like nature of electromagnetic radiation. A prism bends the incoming light (refraction), and how much it bends depends on the wavelength. Use a slit to block out any unwanted light, place a prism behind the slit, and record the image (using a photographic plate, for example, or an electronic camera). The position along one direction of the image corresponds to the wavelength. The combination of a dispersive element (something that divides light into its component wavelength, the prism in this case) and an imaging detector makes a spectrograph. Instead of a prism, you can use a grating (a grating makes use of diffraction, another property of waves) as the dispersive element. In fact, this is more common in modern spectrographs.

In the second class of instrument, scientists use the particle-like nature of electromagnetic radiation. Each photon (a packet of radiation) carries a certain amount of energy. Most detectors used in X-ray and gamma-ray astronomy are capable of measuring the energy of each incoming photon --- for specific detector types, see the X-ray Detectors section. This is very different from optical astronomy, where most detectors cannot do this --- this is because X-ray and gamma-ray photons are generally fewer in number but carry more energy per particle. It's much easier to measure the energy of an X-ray photon than that of an optical photon.

Different types of instruments have different strengths and weaknesses. For example, dispersive spectrographs (the first type) are generally better at distinguishing neighboring wavelengths than the second (non-dispersive) types. On the other hand, with the latter, you get imaging and spectroscopic informations at the same time, which you cannot do with a dispersive instrument.