Reflecting on mirrors

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Space mirrors

NASA’s James Webb space telescope set to launch in 2021 will contain the largest mirror ever taken into space. 

According to recent theoretical work, the telescope could potentially help astronomers observe the first stars and black holes, which were formed 200 to 400 million years after the Big Bang. The instrument will feature a primary mirror 6.5 metres in diameter. 

Building a mirror of that size is challenging, even for use on the ground. If the Hubble Space Telescope’s 2.4 metre mirror were scaled to be large enough for Webb, it would be too heavy to launch into orbit. The Webb team had to find new ways to build the mirror so that it would be light enough – one-tenth of the mass of Hubble’s mirror per unit area – yet very strong.

Samara Polytech scientists developed a way to reduce distortion and increase the resolution of space telescopes

The NASA team decided to use beryllium to make the mirror segments, because the material is both strong and light. Each segment weighs approximately 20kg.

The Webb telescope team also decided to build the mirror in segments on a structure that folds up, like the leaves of a drop-leaf table, so that it can fit into a rocket. The mirror would then unfold after launch.

Positioning on the micron scale

Scientists from Russia’s Samara State Technical University (Samara Polytech) have developed and patented a way to reduce distortion and increase the resolution of space telescopes. This invention could also help to improve the performance of remote sensing devices.

‘We set the telescope mirror in any form required to compensate for the aberrations in the entire optical system. Here we talk about the opportunity to move the surface of the mirror by microns, even shares of microns. This allows us to get rid of distortions,’ said Professor Yakob Klebanov, head of the mechanics department of mechanical engineering, metallurgy and transport faculty of Samara Polytech.

Klebanov specifies that the increase in image quality and resolution of telescopes can be reached mainly by increasing the size of the mirror. However, this is not practical.

In the Samara scientists’ invention, the telescope mirror is not being attached to a rigid base, but to a system of portable piezo-actuators that are led by computer signals and automatically deform the mirror by fractions of a micron in the places required, so that the distortions of the image disappear.

The system is able to reconfigure itself in a split second, and can be used not only in space but also on Earth. The team has developed software for automatic control. 

The work was supported by the Samara Progress Rocket Space Centre.

Alluxa – featured product

Alluxa’s Ultra series of thin-film optical filters and coatings are specifically designed to be integrated into the most sophisticated instruments used in biotechnology, remote sensing, telecommunications, chemical engineering, robotics, and a variety of other fields.

The thin-film optical filters and dielectric mirrors are all hard-coated using the company’s Sirrus plasma deposition process on equipment that was designed and built in-house. This permits the reliable and repeatable production of the same high-performance optical thin films over several different coating runs, which translates to consistent performance across all systems.

The Ultra series thin films are all highly durable, thermally stable, and resistant to laser damage. They can all be custom designed to provide precision transmission, absorption, or reflection at any range of wavelengths from the UV to the IR (~250nm to 6.2µm). Whether a system requires extreme flatness, a large AOI tolerance, TWE control, or any other challenging specifications, the Ultra series optical filters and mirrors can be custom designed and manufactured to exceed these requirements and optimise system performance.

Spectrum Scientific, Inc (SSI) – featured product

Although the benefits of freeform mirrors have been well understood for decades, the cost and complexity of manufacturing meant that it wasn’t until fairly recently that they have started to enter main stream usage. This has been enabled by a replication process, meaning that today’s freeform optics offered by companies such as Spectrum Scientific, Inc (SSI) are low cost, high volume, high fidelity, and high performance that in most cases are diffraction limited. This offers many advantages over conventional optics such as: improved performance; redistribution of tolerances in the system design; additional aberration correction; and a reduction in the number of optics, resulting in fewer optics to align and potentially a significant reduction of the optical system volume. 

Typical applications include analytical and spectroscopic instrumentation, telecom, space and defence systems, beam shaping, HUDs and VR/AR.

Using state-of-the-art replication processes, Spectrum Scientific offers a cost-effective method for the volume manufacturing of freeform mirrors. Features include: aluminium or glass substrates; ellipse and paraboloid profiles; size 0.5 inch to 6.0 inch diameter; typical surface figure down to λ/8 (λ/14 possible);  surface roughness down to 3.5Å; and gold or aluminium coating.

Latest products

Edmund Optics offers a large range of mirrors, ranging from those designed for high power, narrow band laser line applications to actively deformable mirrors for adaptive optical systems. 

The firm produces a range of precision parabolic, spherical, and flat mirrors with a number of different substrate materials, which are offered in various metallic and dielectric coatings to suit all application needs. 

Metallic mirror coatings are optimised for different regions of the spectrum. Various metallic coatings are available, for applications using wavelengths ranging from 120nm to beyond 10µm. Edmund’s standard metallic mirror coatings include protected aluminium, enhanced aluminium, UV enhanced aluminium, DUV enhanced aluminium, bare gold, protected gold, and protected silver. 

Protected and enhanced aluminium are used for visible applications. UV and DUV enhanced aluminium can be used for UV and visible applications. Bare or protected gold offers high reflectance for near infrared (NIR) and infrared wavelengths. Protected silver provides the highest reflectance at 500-800nm and excels in NIR and infrared applications.  

Optotune’s dual axis mirror series MR-15-30 is the ideal choice for applications that require large deflections in a compact form factor. With a mirror size of 15mm, the MR-15-30 achieves up to +/- 25° mechanical tilt, which results in up to +/- 50° optical deflection. The mirror includes a position feedback system that allows it to be controlled accurately with a standard PID controller.

The actuator is based on proven technologies. In contrast to galvo mirror systems, the virtual rotation point is very close to the mirror surface. The mirror can be fabricated with various coatings such as gold, protected silver and other coatings on request. 

LBP Optics has been manufacturing metal laser mirrors for over 27 years.  Aluminium and aluminium alloys are increasingly popular for making lightweight scanning mirrors and galvanometer mirrors, or where weight is an issue such as in aerospace, defence and medical laser systems. Its solid aluminium mirrors with electroplated gold coatings are regularly used for scanning applications such as engraving and marking, using both fibre laser and CO2 systems. 

To improve the laser damage resistance and reduce scatter, the firm’s aluminium mirrors have an intermediate layer of electroless nickel deposited. This gives an amorphous, non-crystalline surface of exceptional smoothness. Electroless nickel protects the whole of the mirror from oxidation.

LBP Optics can include mounting holes, dowel pins, O ring grooves, through holes and other alignment features into its aluminium mirrors, eliminating the need for mounts. The plated coatings cover all mirror surfaces including the internal surface of holes, so chemical resistance is maintained. LBP customers find there are many benefits of using aluminium mirrors.

Alongside its E-beam coatings, offered since 1986, Laser Components now has ion assisted deposition (IAD) and ion beam sputtering (IBS) coating capabilities. Being an extremely precise and highly replicable method, IBS coatings are used for the most spectrally demanding applications, such as steep edge dichroic mirrors and very broad band mirrors. 

Unlike other coating technologies, process parameters such as beam energy, layer growth rate and oxidation level, can be individually regulated to a high precision during the coating process. The result is an optical coating with high density and very low thermal drift, thanks to the absence of moisture retention. IBS coatings exhibit low scattering losses and very high reflectivities, greater than 99.99 per cent depending on optimised wavelength. This is a ‘cold’ coating method, and thus is suitable for temperature and moisture sensitive substrates. Laser Components offers custom IBS coating in the wavelength range 248nm to 3,000nm. 

The performance requirements for optical mirrors are extremely rigorous as slight deviations can have significant impacts on the outcome of specific performance. Borofloat glass from Schott meets such demanding requirements and has been used for sophisticated optics around the globe.

Borofloat’s extremely high transparency, visual quality and optical clarity make it the material of choice for many optical applications in research and industry. High transparency in visible and near IR and UV range of wavelengths offers customers a vast wealth of new possibilities. 

Specific light transmittance values are thickness dependent and significantly influenced by Fe2O3 impurity levels. Borofloat specialty glass uses only the purest raw materials resulting in extremely low (~90 ppm) iron impurity levels and hence very high transmission values. 

Low-loss mirrors are a key technology for many different research fields. (Image: Yury Zap/

24 August 2021