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
