1.5mRC Shutter-Newall masks

Accurate timing of exposures is obtained by means of a fast computer controlled shutter. A pair of Newall masks are available for quantitative focusing of the spectrograph. Successive exposures of a comparison source are taken through each mask. At perfect focus, the positions of the arc lines in these exposures coincide exactly. Increasing departures from focus lead to greater displacement of the arc lines between the images. The focusing procedure using the Newall masks is normally performed by observer support personnel (see §15).