Preliminary optical design for the WEAVE two-degree prime focus corrector

We present the preliminary optical design for a new two-degree refractive prime focus corrector for the 4.2m William Herschel Telescope optimised for the wide-field multi-object spectrograph, WEAVE (WHT Enhanced Area Velocity Explorer). From the two conceptual designs described previously [1], the counter-rotating atmospheric dispersion corrector approach was selected and further optimized to meet the flat image surface requirement. The preliminary design provides good polychromatic image quality. The PSF does not exceed 0.6 arcsec (80% encircled energy) over a wavelength range from 370 to 1000nm covering a two degree FOV for zenith angles up to 65 degrees. We describe the corrector's performance and the trade-off between telecentricity and the requirement for a flat image surface. We present the results of the tolerance and thermal analyses, ghost and scattered light calculations and the finite element analysis that are necessary to establish the PSF error budget for the corrector.