A four mirror anastigmat collimator design for optical payload calibration

We present here a four mirror anastigmaticoptical collimatordesign intended for the calibration of an earth observationsatellite instrument. Specifically, the collimatoris to be applied to the ground based calibration of the Sentinel-4/UVN instrument. This imaging spectrometerinstrument itself is expected to be deployed in 2019 in a geostationary orbitand will make spatially resolved spectroscopic measurements of atmospheric contaminants. The collimatoris to be deployed during the ground based calibration only and does not form part of the instrument itself. The purpose of the collimatoris to provide collimated lightwithin the two instrument passbands in the UV-VIS (305 – 500 nm) and the NIR (750 – 775 nm). Moreover, that collimated lightwill be derived from a variety of slit like objects located at the input focal (object) plane of the collimatorwhich is uniformly illuminated by a number of light sources. The collimatormust relay these objects with exceptionally high fidelity. To this end, the wavefront error of the collimatorshould be less than 30 nm rms across the collimatorfield of view. This field is determined by the largest object which is a large rectangular slit, 4.4° x 0.25°. Other important considerations affecting the optical design are the requirements for input telecentricity and the size (85 mm) and location (2500 mm ‘back focal distance’) of the exit pupil. The design of the instrument against these basic requirements is discussed in detail. In addition an analysis of the straylight and tolerancing is presented in detail.