Abstract
Mirrors are a subset of optical components essential for the success of current and future space missions. Most of the telescopes for space programs ranging from earth observation to astrophysics and covering the whole electromagnetic spectrum from x-rays to far-infrared are based on reflective optics. Mirrors operate in diverse and harsh environments that range from low-earth orbit to interplanetary orbits and deep space. The operational life of space observatories spans from minutes (sounding rockets) to decades (large observatories), and the performance of the mirrors within the mission lifetime is susceptible to degrading, resulting in a drop in the instrument throughput, which in turn affects the scientific return. Therefore, the knowledge of potential degradation mechanisms, how they affect mirror performance, and how to prevent them is of paramount importance to ensure the long-term success of space telescopes. In this review, we report an overview of current mirror technology for space missions with a focus on the importance of the degradation and radiation resistance of coating materials. Special attention is given to degradation effects on mirrors for far and extreme UV, as in these ranges the degradation is enhanced by the strong absorption of most contaminants.
Highlights
GOLD—Instituto de Optica—Consejo Superior de Investigaciones Científicas, Serrano 144, Cixi Institute of Biomedical Engineering, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Zhongguan West Road 1219, Ningbo 315201, China
For Silicon carbide (SiC), a severe reflectance decrease was observed when the coating was oriented in the ram direction, and it was measured that the presence of silicon oxide on the surface was three times larger than for the witness sample kept in the lab, which was attributed to the direct exposure to atomic oxygen (AO)
In view of the sensitivity of bare Al reacting with AO, some procedure to significantly reduce the rate of impingement of oxidizing species must be developed, either based on the selection of high orbits [66] or through the use of some scheme that shields the mirrors from ambient oxygen [67,68]
Summary
The trend for the future space missions is the use of high-resolution, large bandwidth telescopes [1,2,3]. Examples are mission concepts such as Large Ultraviolet Optical Infrared Surveyor (LUVOIR), HabEx, the Galaxy Evolution Probe, and x-ray observatories [4,5,6,7] These and many other present and future space concepts [8] introduce new challenges in mirror technologies, from the optical design to the substrate and the coatings. With the introduction of EUV lithography [22]—using 13.5 nm photons—in the semiconductor industry, the understanding and development of such multilayer structures and the overall and long-term performance of such optical systems have received a boost over the last few decades Inside these lithographic machines, optical multilayer components are exposed to high fluxes of EUV radiation, and to a peculiar type of plasma which is induced by the photo-ionization of the low-pressure background gas inside these machines [23,24]. The extreme environment where they must operate implies severe issues in terms of stability and resistance
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