Design and Performance Analysis of an Off-Axis Three-Mirror Telescope for Remote Sensing of Coastal Water

Abstract

We report the design and performance analysis of an off-axis three-mirror telescope as the fore optics for a new hyperspectral sensor aboard a small unmanned aerial vehicle (UAV), for low-altitude coastal remote sensing. The sensor needs to have at least 4 cm of spatial resolution at an operating altitude of 500 m, 4° field of view (FOV), and a signal to noise ratio (SNR) of 100 at 660 nm. For these performance requirements, the sensor's optical design has an entrance pupil diameter of 70 mm and an F-ratio of 5.0. The fore optics is a three-mirror system, including aspheric primary and secondary mirrors. The optical performance is expected to reach 1/15λ in RMS wavefront error and 0.75 in MTF value at 660 nm. Considering the manufacturing and assembling phase, we determined the alignment compensation due to the tertiary mirror from the sensitivity, and derived the tilt-tolerance range to be 0.17 mrad. The off-axis three-mirror telescope, which has better performance than the fore optics of other hyperspectral sensors and is fitted for a small UAV, will contribute to ocean remote-sensing research.

Development of an off-axis TMA telescope for coastal water remote sensing

“…The off-axis optical system had a tilt of 15° from the optical axis to avoid masking, and the location of aperture stop was set as secondary mirror (M2) for optimum design. 2 …”

“…The results show that if the parameters of the most sensitive M3 factor, such as tilt/yaw misalignments and the detectors' defocus factor, are adjusted during assembly, the required performance will be sufficiently obtained. 2 …”

“…The MTF performance was also 0.9 in Nyquist frequency (19.23 lines/mm), which is nearly the diffraction threshold and thus satisfies the optical MTF requirement of 0.75. 2 A thorough tolerance analysis was performed. The effects of most errors on the assembly/alignment steps were determined.…”

“…This was designed as the fore-optics of a new ocean sensor for unmanned aerial vehicles (UAV) that is currently under joint development by the Korea Ocean Satellite Center (KOSC) and Korea Basic Science Institute (KBSI). 2 Unlike commercial lens-type fore-optics used in existing hyperspectral sensors (AVIRIS, Ocean PHILLS) for oceanographic observations, an off-axis reflective fore-optics having high signal-to-noise ratio in broad bandwidth is designed to meet reduced size and weight. 3 However, this significantly increased the difficulty in fabricating and aligning optical elements that form the overall system.…”