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Freeform active mirrors could become a basis of extremely large telescopes

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Posted November 18, 2014

Modern optical telescopes face certain physical limitations regarding their light-collecting power and spatial resolution. Most engineers specializing in the field of astronomy agree that most of these limitations could be overcome by increasing dimensions of the new generation of telescopes. But it is not easy to manufacture such finely-processed mechanical giants.

Luckily, it is not a dead end. An international team of scientists from France, The Netherlands, Hungary, and Scotland recently proposed a novel way to develop mirrors for the use in optical systems for astronomical observations.

Two active array prototypes used to control the freeform mirror.

Two active array prototypes used to control the freeform mirror. Image courtesy of the researchers.

According to them, the overall complexity of instruments could be potentially reduced by the use of so-called freeform active mirrors. Such concept of engineering design is not new and is currently used in smaller-scale devices such as portable close-range image projectors and similar devices. There, a freeform optics is used to reduce the size of image-projecting device and reduce the distance from which a full-size image could be displayed.

Reduction in size, mass and volume are not the only benefits of the freeform optics. Scientists note, that this technology also results in increased reliability and operational availability, and can also provide increased throughput of our observatories.

(a) 2-mm thickness mirror flat polished in AISI420b stainless steel. (b) The same mirror hydroformed

(a) 2-mm thickness mirror flat polished in AISI420b stainless steel. (b) The same mirror hydroformed. Image courtesy of the researchers.

According to the most popular definitions of freeform optics, the main difference lies in the extreme deviation from the best-fit sphere, which can be as large as a millimeter. In their paper (available online on arXiv.org), the authors argue that such optical mirror components require substantially different manufacturing technologies compared to the ‘regular’ ones. They also provide the experimental analysis of how a freeform mirror could be manufactured using the hydroforming method and present the preliminary results of the prototyping of the active array which could be used to control the freeform mirror in the telescope.

Freeform active mirror control and driving electronics. Image courtesy of the researchers.

Freeform active mirror control and driving electronics. Image courtesy of the researchers.

In their work, scientists describe the concept called FAME (Freeform Active Mirrors Experiment) and its current status of development. “The goal of FAME is to produce a freeform mirror whose extreme aspherical shape is actively controlled by a dedicated array of actuators”, the authors explain. Based on their idea, the hydroforming of a prepolished thin mirror could be used to manufacture a thin mirror. However, this technique alone is not sufficient to create a perfect freeform mirror. Additionally, active array control has to be used to correct (i.e. compensate) shape errors. In this way it is possible to somewhat lower the requirements on the manufacturing and the optical alignment of the mirror.

Optical design methodology used in the design of freeform optical components. Diagram courtesy of the researchers.

Optical design methodology used in the design of freeform optical components. Diagram courtesy of the researchers.

In the freeform optical components, the shape of the mirror can be adjusted mechanically. Such method of adjustment is the basis of the proposed active control. The team describes the analysis of several possible control array arrangements, where pistons of different shapes could be used to exert a force on the mirror to generate required elastic deformations. In this way (at least during performed practical experiments) the mirror obtains a complex aspherical shape adapted to particular conditions of astronomical observations. Electronic automated control loops can be used to drive such mirror arrays, so no further adjustments are required from the user.

Smaller, more efficient, easier to produce, with different control possibilities – the adaptive freeform mirror developed by FAME project has many promising features. The authors say that in the future such freeform components could be used in most of extremely large telescopes. We certainly hope to see a practical implementation of this technology in real observatories sometime soon.

Written by Alius Noreika

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