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.