Optics for Earth Observation Instruments
Doctoral thesis, 2019
The optical system is an essential part of every remote sensing instrument used for detecting electromagnetic waves. Careful design, fabrication and characterization is therefore crucial, especially for satellite-borne missions where the possibility of post-launch repairs is highly limited. This thesis presents contributions to a new type of reflector optics and horn antennas for three instruments for Earth observation, which operate at wavelengths between the UV and millimeter-wave regions: Mesospheric Airglow/Aerosol Tomography Spectroscopy (MATS), International Submillimetre Airborne Radiometer (ISMAR) and Stratosphere-Troposphere Exchange And climate Monitor Radiometer (STEAMR).
A free-form three-mirror off-axis telescope was developed for the limb instrument on board the MATS satellite. The f/7.3 (D = 35 mm) design achieved diffraction-limited performance (at 270-772 nm) over a wide field (5.67° × 0.91°) by applying a new design method that corrects for linear astigmatism. Single point diamond turning was used to fabricate the free-form mirrors, which resulted in a telescope with a modulation transfer function of 0.45 at 20 lp/mm. Simulations and measurements were used to assess stray light rejection of the limb instrument. Measurements of a breadboard front baffle with a new type of extremely black coating showed a point source transmittance down to 10-6, which was in excellent agreement with simulations. Detailed modeling predicted a stray light rejection of 10-10-10-4 in the most critical region below the nominal field of view.
Two 874 GHz Schottky mixer receivers with integrated low noise amplifiers, spline horns and low-loss dielectric lenses were developed for ISMAR, which exhibited record-low receiver noise temperatures of 2260-2770 K. Radiation patterns were measured between 868.7-880.0 GHz in a setup capable of resolving side lobes down to 25-30 dB below the main peak. The main beam full-width-half-maximum was in good agreement with simulations and well below the required 5°.
Spline horn antennas at frequencies 120-340 GHz were also developed. An efficient optimization algorithm based on mode matching in circular waveguides was used for all designs, which exhibited Gaussicity values of 98% over bandwidths up to 19%. Far field radiation patterns were measured using a setup for spherical and planar scanning geometries.
A mechanical tolerance analysis was performed for the optical system of STEAMR, which consists of two polarization-separated focal plane arrays, a four-reflector anastigmatic relay optics chain and an off-axis Ritchey-Chretien telescope. Using Monte-Carlo simulations based on ray-tracing and physical optics, an overall reflector alignment accuracy requirement of 100 μm was obtained. Surface distortion analyses of the 1.6 m × 0.8 m primary reflector highlighted the need for an optical system with small mechanical variations in orbit (<30 μm). A relay optics demonstrator showed that alignment accuracies down to 50 μm could be obtained.
In conclusion, the methods for design, manufacturing and characterization presented in this thesis can be used to develop new instruments for Earth observation and related fields.