Design of a three-lens wide field corrector with aspherical surfaces for the 2.34-m VBT

We are developing a compact three-element Wide Field Corrector (WFC) with spherical and aspherical lenses for the 2.34 m Vainu Bappu Telescope (VBT) to enhance its field coverage for imaging and spectroscopic applications. The design consists of three optical elements, with at least one spherical lens movable to serve as an Atmospheric Dispersion Corrector (ADC), while the aspherical elements remain fixed to maintain optical stability. We are currently testing two design configurations, one with two spherical lenses and one aspherical lens, and another with two aspherical lenses and one spherical lens. The ADC is designed to correct atmospheric dispersion for zenith angles ranging from 0° to 60°. The system is optimized to operate over a wavelength range of 0.4 µm to 0.9 µm, targeting an effective field of view of about 0.5°. Considering the limited mechanical space available at the VBT prime focus, the design emphasizes compactness, ease of alignment, and manufacturability. The system achieves a mean geometrical D80 better than 0.3 ′′ and 0.23′′ for Design 1 and Design 2, respectively, at zenith, and maintains a mean geometrical D80 within 0.57′′ and 0.45′′ up to a zenith angle of 60° after atmospheric dispersion correction. Atmospheric dispersion at higher zenith angles (up to 60°) is corrected using a movable lens element, enabling the system to preserve high image quality across the field.