To develop technologies for improving surface roughness and shape accuracies of large (300mm) and strongly aspherical aluminium mirrors applying cost-effective polishing processes. The objective is to demonstrate the technology by manufacturing and testing a breadboard of a strongly aspherical mirror with diameter up to 300mm, shape accuracy better than 30nm (rms), and surface roughness lower than 2nm (rms).

 

 

The activity was able to develop and improve technologies and processes (such as polishing) that will enable manufacture and production of lightweight, highly aspherical, metallic optical mirrors with the accurate surface shape and low roughness needed for high performance optical systems, such as telescopes and high-resolution spectrometers. 

These designs have been increasingly investigated in the build of compact, lightweight and high performance space-based instruments. But the effort and time needed to polish these mirrors to meet the requirements for highly-detailed space instruments has meant their reliability has been hard to predict. This has led to uncertainty in lead time and cost. Now, advances in melt-spinning technology, the development of new aluminum alloys and changes to manufacturing methods for metallic mirrors manufacturing has opened new possibilities for fully-metallic instruments with challenging designs

These developments mean instruments such as compact spectrometers, large field-of-view telescopes or relay systems, could be developed using only metallic materials, making them better able to withstand cryogenic temperatures.

The activity studied different materials and ways of polishing them to understand how best to manufacture these mirrors. Eventually, lightweight mirrors of 10cm square and 20cm by 10cm were made using an aluminium alloy, AlSi40, plated with NickelPhosphorous, and their structure and longetivity evaluated through a test campaign.

The largest mirrors survived the thermal and vacuum cycling tests before the performance of the mirror in cryogenic conditions was evaluated, using real-time monitoring of the surface error as the mirror was subjected to temperatures ranging from 295 to 200 Kelvin.The activity was able to successfully develop two mirrors that met the strict requirements of optical instruments.

Single Point Diamond Turning has proven to be a very cost-effective method to produce strongly aspherical mirrors. A limitation of this technology is the achievable surface roughness limited to 4 nm (rms) and the poor shape accuracy for mirrors with diameter larger than 250mm.Aspherical mirrors of large diameter can be manufactured in other substrates like Zerodur and SiC using conventional polshing methods, however strong asphericy can only be achieved at the expenses of a long proceses time and high cost. Deterministic polishing, now available thanks to recent developments in manufacturing tools, has not yet been proven on lightweight aluminium optical mirror. This technology, if successful, will provide the process for developing strongly aspherical and lightweight aluminium mirrors with diameters up to 300mm. These mirrors are instrumental to reduce mass, envelop and cost of optical systems, both for Earth Observation instruments and for compact high- performance optical instruments for planetary probes and scientific payloads. Furthermore, the capability to manufacture low roughness strongly aspherical mirrors opens the possibility to innovative optical design of both panchromatic cameras and spectrally resolved instruments.