Optical Analysis and Characterisation of the Climate Research Instrument STEAMR

Licentiate thesis, 2014

A growing interest for making high resolution observations in the upper troposphere and lower stratosphere has motivated the development of the multi-beam satellite-borne instrument Stratosphere-Troposphere Exchange And climate Monitor Radiometer (STEAMR). The measurements made by STEAMR will be used to monitor trace-gases and link the occurence of these to the Earth's climate by accurate modeling of the atmosphere. To interpret the measured data require a firm understanding of the coupling between the radiation emitted from the atmosphere to the signal being received by the instrument's optical system. Thus, precise knowledge of the instrument's optical performance while in orbit is crucial. This thesis is dedicated to the study of optical analysis and measurements of the highly advanced optical system of the STEAMR instrument. A mechanical tolerance analysis of the complete optical system consisting of two polarisation separated focal plane arrays, a four reflector anastigmatic relay optics chain and an off-axis Ritchey-Chretien telescope was performed. Using Monte-Carlo simulations in combination with ray-tracing and physical optics/physical theory of diffraction, it could be concluded that the overall reflector alignment accuracy from manufacturing should be better than 100 µm. Surface distortion analyses of the 1.6 m × 0.8 m primary reflector highlighted the need for an optical system with high stability in orbit (<30 µm). Furthermore, it was shown that the manufacturing techniques were compliant with the requirements dictated by the tolerance analysis. The design, manufacturing and characterisation of a 340 GHz smooth-walled spline feed horn antenna for STEAMR is presented also presented. Planar measurements of the amplitude and phase were performed at 329 and 338 GHz, where the corresponding coupling to the fundamental Gaussian beam mode was 97.7 and 97.8%, which is in excellent agreement with simulations.