Bandpass of a Spectrograph

Monochromators vs. Spectrographs

In a monochromator, the design concentrates on the path of light from the input slit, off the grating and to the output slit. Light of other wavelengths is absorbed. At any grating setting, only a very small range of angles around the diffraction angle D, figuratively “one” wavelength, passes through the monochromator.

A spectrograph looks simultaneously at a wide range of angles D, and therefore at the range of wavelengths that satisfy the grating equation for this range of angles D. The optics of Oriel’s spectrographs are designed for this purpose. A slit is replaced at the output of a spectrograph by long open area over which the various wavelengths are spread in a known fashion


With a spectrograph, the term bandpass refers to the entire wavelength range in the long strip output to the detector. Thus the bandpass in a spectrograph is very wide, limited only by the output aperture. However, it is not enough to simply remove the slit from a monochromator and use this larger aperture. Generally, wavelengths away from the center of a monochromator slit are focused slightly inside the instrument. Spectrographs are designed to correct this curvature; they have a flat output field matched to flat CCDs and PDAs.

Very often, the length of the detector limits the bandpass. For example, the Oriel MS257 spectrograph has a 28 mm flat field. If this instrument is used with a CCD that is only 25.4 mm in length, the bandpass definition must keep this limitation in mind.