Feb 12 2014
A spectrograph is a device that separates the incident light from an object into a frequency spectrum and measures the amount of radiation at each wavelength. The device operates by dispersing or splitting the light into its component wavelength such that it can be stored and analyzed.
Working Principle
A typical spectrograph consists of an entrance slit, a collimator, a prism or grating, a camera, and a detector. The incident light enters the slit which is located at the focal point of the telescope and diverges as the light beam exits the slit. The light is collimated by the collimator, and the collimated light hits the grating. The grating is placed in a tilt position relative to the path of the incoming light. The reflected light is then focused on to the detector by a camera.
Detectors used in modern spectrographs are usually charged coupled devices having finite pixels. These detectors provide a discretized sampling of the dispersed light. The result is the formation of spectral lines of known wavelength from which the spectrum of the distant source can be calibrated.
Echelle Spectrograph
The Echelle spectrograph is a special device that employs two diffraction gratings that can be rotated 90° corresponding to each other. It also uses a two-dimensional charged coupled device chip for recording the spectrum. The gratings have a wide spacing, and they are blazed such that a number of higher orders in addition to first-order are made visible to achieve a fine spectrum. With the utilization of a small chip, the collimating optics of the spectrograph need not be optimized.
Sources and Further Reading