At Prime Focus a four-element field corrector and a two-element atmospheric
dispersion compensator are provided, the layout of these elements is shown
in Figure . The field corrector provides good images over an unvignetted
field 40 arcmin in diameter, and
45% vignetting at 1 degree diameter. The
atmospheric dispersion corrector is specified to compensate for atmospheric
dispersion over a wavelength range of 3300 to 11000Å, at zenith distances
up to 73.