SPECTROGRAPHS
The photographic plate and the photoelectric device enhance our ability to detect light from different astronomical sources, but they are not basically analyzing instruments. We can equip an accessory instrument with either of these detectors and attach it to the telescope to analyze light. The two basic types of analyzing instrument are the spectrograph and the photometer.
The. spectrograph disperses the composite light from the source into its component wavelengths so that we can, for example, determine the elements that compose the light source. Spectroscopy, which is the study of the spectra of light sources, is astronomy's fundamental interpretive tool.
A prism or grating spectrograph receives the concentrated light coming from the telescope's objective on an entrance slit. The light diverging past the slit enters a collimator, which delivers a beam of parallel rays to the dispersing device. Then these rays pass through either a prism or reflect off a grating, which separates the light into its constituent wavelengths. The dispersed light is focused by a camera system onto a radiation detector (a photographic plate or a photoelectric device) as individual color images of the entrance slit. Each wavelength forms a distinct image of the slit. The images of the slit in the different wavelengths are arrayed in an orderly progression of colors from red to violet to create the observed spectrum of the composite light falling on the entrance slit.
