Circular blacked metal screen, 75 mm dia with 12.5 mm central
aperture. A screw controlled, spring loaded precision slit
is mounted on one side of the disc, the adjustment drive being
arranged so that the slit cannot be over tightened. The slit
has bevelled jaws with brass, dovetail section slides, and
provides a maximum slit opening of 4 mm approx., with a useful
length of 10 to 12 mm. Mounted on a standard 6 mm dia brass
rod, the optical centre height is 80 mm approx.