FEATURE ASTRONOMY
to mount the lasers and operate them turn-key,’ added Bonaccini Calia.
When in-house scientific developments are successful, companies that develop them gain access to a new line of products. Toptica grew from 80 to 200 people over the project’s duration, with support from 4LGSF funding. The quest for a fully engineered laser guide star had existed for years; but until now was ‘too big to tackle. These features have been transferred to our established product range, including non-linear optics systems,’ explained Enderlein. And MPBC’s industrialisation has enabled unprecedented Raman amplifers that work at virtually any IR wavelength, with single mode fibres and polarisation-maintaining output.
Deformable mirrors There’s no need for atmospheric correction in space telescopes. But the deformable mirrors that enable adaptive optics transcend this separation. They allow fine-tuned alignment and phasing of systems designed on Earth that have to operate in space’s harsh conditions. In 2018, NASA’s James Webb Space Telescope (JWST) will launch, operating in the infrared to let scientists look back in time to early galaxy formation. Vital to the entire telescope’s operation is the Near-IR Camera, or NIRCam, which aligns the 18 individual adjustable segments of the deformable 6.5m primary mirror so science operations can commence. ‘The most important thing about NIRCam is its
LSST optical layout. Three mirror system offers a wide field of view and high image quality
exists at 3-4 Kelvin (-270°C), so passively cooling instruments via sun-shield and radiators is not difficult. But it creates challenging building and testing conditions on Earth. Harris Corporation was responsible for building and cryogenic testing the optical components. Motors and electronics behave differently in
Adaptive
optics lets us look from the ground, as if we were in space
refractive design, which is very compact compared to similar instruments that are all reflective,’ said Alison Nordt, programme manager at Lockheed Martin in California, who designed NIRCam for the University of Arizona. NIRCam’s two pick-off mirrors collect and
reflect light into two separate, mirror-image modules. A coronograph allows light from faint objects to be distinguished from nearby bright stars. Collimating lenses send collected light to a dichromic beam splitter, which separates light into short (0.6-2.3µm) and long (2.4-5.0µm) wavelength paths. These beams go through a ‘dual filter wheel’, which separates light to use in different detectors. The shortwave wheel contains elements for wavefront sensing that inform how to adjust the primary mirror segments. After passing through the wheels, both long- and short- wavelength beams are focused by corrector lenses onto extremely sensitive HgCdTe detectors for imaging.
NIRCam operates at cryogenic temperatures, so it can sense even the faintest IR signals. Deep space
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extreme cold. ‘It’s interesting from an engineering perspective; things still tend to work at 100K, but when you get down to the 40K (-233°C) area, things don’t work like they should,’ noted Gary Matthews, director of Universe Exploration for Harris’ Space & Intelligence Systems in New York.
Matthews’ team aligned JWST mirror segments on their
supporting structure, to within ~100 microns. Installation has to account for the gravity shift and temperature change in space. ‘In essence, we are building a precision zero-gravity machine here on Earth,’ added Matthews.
Developing systems that can be made at room temperature and brought into alignment at cryogenic temperatures pushed the boundaries for Harris and Lockheed Martin, with new techniques for both alignment and prediction. For example, the team required cryocoolers for IR instruments. ‘We’ve made a lot of progress,’ commented
Nordt. While some previous designs of crycoolers might be the size of your forearm, now they can fit in the palm of your hand. Materials and techniques from JWST’s optics
offer promise for industry. A new beryllium grade, using pressed spherical powder, was developed for the primary mirror. This allowed the mirror to be polished without pitting, ensuring a high-quality figure. Lenses made from notoriously difficult lithium fluoride, barium fluoride, and zinc selenide required diamond-turning techniques developed at Optical Solutions, pushing the boundary for high-precision lenses.
Big mirrors Not all large telescopes require deformable mirrors. The National Science Foundation/Department of Energy-funded Large Synoptic Survey Telescope (LSST), under construction in Chile, will join the largest Earth-based telescopes in 2019 to conduct a 10-year survey of the southern sky. It does not use adaptive optics or deformable mirrors, because the coherence scale of atmospheric turbulence is very short. Correcting for one point in the field of view would cause harm elsewhere, due to the lack of coherence from one point to another. LSST is a ‘seeing-limited’ telescope, which means it was designed to be limited by atmospheric blurring. Its 8.4m primary mirror is among the single largest pieces of glass manufactured for a telescope, but LSST’s real uniqueness comes from its large aperture combined with extremely wide field of view, 3.5 degrees in diameter. Similar-scale telescopes capture only one degree or less. While most large telescopes have two mirrors (a convex secondary mirror that condenses light from the concave primary mirror), LSST has three. A primary light-collecting mirror combined with a secondary mirror condenses the beam to 3.4m. Next, the beam goes to a five metre tertiary mirror, giving a wide field of view and high image quality. Light from the telescope passes through a three- element refractive corrector to compensate for the
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LSST mirror in its container JUNE 2016 l ELECTRO OPTICS 19
LSST Project
LSST Project
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