ABSTRACT
Although the majority of spectrometric instruments are designed around reflection gratings there are a number of situations where transmission gratings are preferred. One of them arises from the fact that any camera or telescope can be converted into a spectrograph by interposing a transmission grating in front of the objective. Typical applications arise when the source presents itself as a luminous point or line, like falling meteors, lightning, or solar eclipses. Normally these gratings are formed on plane blanks, but they can also be generated on the face of a prism. As grating prisms, or GRISMs, they are especially convenient for telescope prime focus spectrographs, in association with array detectors, where a great advantage is to have the central wavelength with no deviation.
