A novel zoom-lens spectrograph for a small astronomical telescope

A novel astronomical spectrograph has been constructed using a zoom lens as the focusing element. The linear dispersion is continuously variable from 33 to 1272 Å mm −1 by the use of a grating turret with three gratings and a zoom lens with a focal length range of 50–250 mm. The absorption losses and chromatic aberration are shown to be acceptable. The instrument has been used successfully on a 0.4-m Cassegrain telescope with a cooled CCD detector for a number of student projects, including the determination of the redshift of the brightest quasar, 3C 273.