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Prime Focus Optics

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 tex2html_wrap_inline5509 . 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 tex2html_wrap_inline5513 . As with any refractive system there is a radial distortion which takes the form:-


where tex2html_wrap_inline5517 is the distance of an object from the optical axis, as measured on the sky (in arcseconds), and tex2html_wrap_inline5519 is its distance from the optical axis in mm. For the prime focus corrector tex2html_wrap_inline5521 = 17.57 arcsec mm tex2html_wrap_inline5513 ; tex2html_wrap_inline5525 = -4.12 tex2html_wrap_inline5529 10 tex2html_wrap_inline5531 arcsec mm tex2html_wrap_inline5533 . 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

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Wed Sep 17 12:36:20 BST 1997