Littrow expansion and its counterpart Littrow compression are optical effects associated with slitless imaging spectrographs. These effects are named after austrian physicist Otto von Littrow.
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 pointlike objects (such as distant stars) this results in a spectrum on the focal plane of the instrument for each imaged object. 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.