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

Abstract

A proto-model of an off-axis reflective fore-optics in a new hyperspectral sensor onboard a small unmanned aerial vehicle (UAV) is developed for low-altitude coastal remote sensing. In spite of the substantial advantages of off-axis optics over refractive or on-axis reflective optics, it is difficult to fabricate and align the mirrors in off-axis systems because of their asymmetric configurations. The fore-optics is designed by Korea Institute of Ocean Science & Technology (KIOST) as a three-mirror system including aspheric primary and secondary mirrors, with an entrance pupil diameter of 70 mm and F-ratio of 5.0. The off-axis mirror is fabricated by a single point diamond turning machine (SPDTM) with a form accuracy of 89.3 nm (RMS) and surface roughness of 7.2 nm (Ra). Coarse alignment of this three-mirror off-axis optical system is accomplished by using a designated base plate and a coordinate measuring machine. The optical performance is tested using the point spread function of the returned collimated beam of a Fizeau interferometer.

Improvement in surface quality of diamond-turned aluminium substrate by using hydrogen peroxide: a molecular dynamics simulation study

“…Aluminium alloy is frequently used work material for diamond turn machining (DTM) due to its machinability, 1, 2 and low density of this material finds its suitability as metallic mirrors in space-borne applications. 3 This type of mirror needs to maintain form accuracy and surface roughness, 4 and this is why DTM-machined aluminium alloy is capable to cater the requirements of space-borne applications. As far as optical quality of such mirror is concerned, surface finish and reflectance play a major role.…”

Al6061 surface roughness and optical reflectance when machined by single point diamond turning at a low feed rate

“…The Al6061 alloy is commonly used in ultra-precision machining because of its good machinability [ 1 – 3 ], and this aluminium is the preferred material for mirrors in spaceborne applications [ 4 ]. Such aluminium mirrors machined by ultra-precision machining technology have more stringent requirements on both the form accuracy and the surface roughness compared to that of optics lighting [ 5 ].…”