A ‘sky lens’ images a strip of sky on the entrance slit of a spectrograph. Then the intensity distribution of each spectral emission from the strip of sky is mapped along the corresponding spectral line. In order to reduce light losses to a minimum a single lens component may be used for this purpose (in place of the usual two components), but one surface should be aspherized. If this surface is made an hyperboloid the lens performs very well over a 20° field.