Design of a custom wideband camera for MISTRAL imager-spectrograph

MISTRAL is a visible and near infrared imager and spectrograph working with the 1.93m telescope at L’Observatoire de Haute-Provence. The goal of the present project is to design and build one custom lens covering the entire working band 370 − 1000nm with an enhanced throughput and resolution. The proposed design has the focal length of 100 mm with f /# = 2 and consists of 5 lenses with 2 aspheres. It is capable to work in spectroscopy or direct imaging mode with the spectral resolving power up to R590 − 1675 or energy concentration of 84% within ±1pix. The throughput varies from 79to98% in the main band of 400-1000 nm with a commercial AR coating and could be yet improved with a custom one. We also demonstrate that with this image quality can be maintained in a ≤ 10% margin with practically reachable tolerances.