The 4.2 metre William Herschel Telescope is provided with a four-element field corrector. Two of the elements are doublet prisms, and these can be rotated to provide atmospheric dispersion correction. It is not possible to use the field corrector without the atmospheric dispersion compensator, although the elements of the atmospheric dispersion compensator can be rotated to their neutral position. The field corrector is specified to give an unvignetted field of view 40 arcmin in diameter, and the image quality over the wavelength range 4000 to 10000 Å is illustrated by the spot diagrams shown in Figure 1.
Atmospheric dispersion compensation is provided by two prisms, each consisting of two glass elements, which are rotated by equal and opposite amounts under computer control, to give compensation appropriate to the zenith distance of the field centre. The atmospheric dispersion compensator is specified to compensate for atmospheric dispersion over a wavelength range 3300 to 11000 Å, at zenith distances up to 73 . The mechanical layout of the corrector elements is shown in Figure 2.
The focal length of the corrected WHT prime focus is 11739 mm and the plate scale is 17.57 arcsec mm . As with any refractive system there is a radial distortion which takes the form:-
where is the distance of an object from the optical axis, as measured on the sky (in arcseconds), and is its distance from the optical axis in mm. For the prime focus corrector = 17.57 arcsec mm ; = -4.12 10 arcsec mm . There is no evidence for any non-radial distortion terms, and the distortion does not seem to depend on telescope position.
Figure 2: Layout of the WHT Prime Focus field and Atmospheric Dispersion Correctors