Littrow expansion

These effects are named after Austrian physicist Otto von Littrow.

[1] In a slitless imaging spectrograph, light is focused with a conventional optical system, which includes a transmission or reflection grating as in a conventional spectrograph.

This disperses the light, according to wavelength, in one direction; but no slit is interposed into the beam.

For distributed objects with emission-line spectra (such as the Sun in extreme ultraviolet), it results in an image of the object at each wavelength of interest, overlapping on the focal plane, as in a spectroheliograph.

The Littrow expansion/compression effect is an anamorphic distortion of single-wavelength image on the focal plane of the instrument, due to a geometric effect surrounding reflection or transmission at the grating.

from a flat mirror, measured from the direction normal to the mirror, have the relation which implies so that an image encoded in the angle of collimated light is reversed but not distorted by the reflection.

Because the sine function (and its inverse) are nonlinear, this in general means that for most values of

, yielding anamorphic distortion of the spectral image at each wavelength.

This configuration preserves the image aspect ratio in the reflected beam.