In the fifth century BC, Sophocles condemned astronomy as ‘impossible to understand and madness to investigate’. But thanks to advances in optics and photonics, investigating astronomy is no longer the madness it was thought to be in ancient times.
Space exploration and instrumentation developments are helping raise the profile of optics and photonics technology, both in terms of how important it is and what it can do. Novel telescopes and detectors extend the limits to which we can investigate the universe. In turn, these instruments drive technologies that benefit the optics industry.
Adaptive optics
Image distortions plague ground-based telescopes, because incoming light is blurred by the atmosphere. To compensate, historical astronomers determined how much aberration the atmosphere caused and how to correct for it, based on light from a reference star with well-known properties. But as telescopes looked further into space, finding a neighbouring bright star became impossible.
The modern solution is to create a reference ‘star’ – or a laser guide star – at a known distance and brightness using a high-powered laser. At the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile, an adaptive optics facility, known as the 4 Laser Guide Star Facility (4LGSF) has been used to create a high-power guide star – the ‘SodiumStar’ – whereby lasers excite sodium atoms at the mesosphere’s edge, 80-100km above Earth’s surface; these fluoresce in a point-like fashion to create the star.
‘It’s like when you have a headlamp; that’s a bright source that lets you sample the atmosphere,’ said physicist and systems engineer Domenico Bonaccini Calia. ‘Adaptive optics lets us look from the ground, as if we were in space.’
In the telescope, light from the guide star is separated from astronomical data and analysed with sensors by measuring aberrations of the wavefront. Because the light travelled the same path as did light from real stars, wavefront aberrations are a direct measurement for atmospheric blurring. A real-time computer adjusts the deformable mirror in the telescope’s optical train to compensate for measured distortions.
‘Amazingly, the whole control loop allows running up to roughly one kilohertz,’ said Martin Enderlein of Germany-based Toptica Photonics, who along with MPB Communications (MPBC) developed the system.
The required wavelength is very difficult to lase. ESO patented a new narrow-band amplification principle, based on inelastic ‘Raman’ scattering of photons, to generate a 40W, 1,178nm, single-mode fibre source, which is then converted into a free-space 589nm source at 22W. The system is turn-key, insensitive to telescope environmental and gravity changes, and able to function reliably without supervision in Chile’s Atacama desert. Toptica and MPCB spent four years developing the product, under contract with ESO and with partial funding from Keck and the European Union.
MPBC produced the fibre pump and narrow-band, high-power Raman amplifiers, while Toptica integrated the components and has been the main ESO contractor.
‘Until now, running a laser for adaptive optics required specialised personnel on site and on shift, attending the laser and adjusting it continuously – a very special operation. Now it’s not a problem anymore. Telescopes will be able 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 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 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 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 vacuum cryostat’s thick window, behind which sits the 3.2 billion pixel detector system.
Few places in the world can manufacture an eight metre glass mirror, but the LSST’s combined primary and tertiary mirrors were uniquely fabricated out of a single piece of glass. The University of Arizona’s Stuart Observatory Mirror Lab developed a technique by which molten glass is spun within an oven to achieve the rough shape; this is then cooled, solidified, and subsequently finished to its precise final figure.
‘The interesting thing for a telescope building project is that the primary-tertiary mirror pair was finished well ahead of schedule and is now being stored in a box. That’s never happened before,’ said Chuck Claver, LSST systems scientist with the LSST Project.
Optical components for projects like the LSST are competitively bid by industry. ‘Some optics drive innovation at various companies; for example, the secondary mirror being built by Harris Corporation can’t be tested all at once,’ Claver remarked. Harris demonstrated that they could stitch the metrology together to validate surface quality. It behoves them to apply that ability to other projects.
Further, the 1.6m-diameter front lens of LSST’s camera is ‘huge for a refractive optic’, according to Claver. ‘It will be the largest refractive optic ever put into service for astronomy. Achieving high performance surfaces over large optics opens a new parameter space for the photonics industry.’
Small and sweet
Smaller scopes lend themselves to optics developments, too.
The two-metre Liverpool Telescope, a joint venture between the UK’s Liverpool John Moores University and Royal Greenwich Observatory, became operational in January 2004 on the Canary Island of La Palma, Spain. Apart from being fully robotic, so that it requires no human operator, it is set apart from most other telescopes by its instrumentation and enclosure.
A rotating fold mirror, which reflects the converging beam near the telescope’s focus, allows seven science instruments to be simultaneously mounted and permanently available. It takes under a minute to switch between instruments. Further, the clamshell enclosure provides all-sky access, enabling rapid response to targets of opportunity. In contrast, conventional domes often limit reaction speeds.
The telescope is entirely responsible for opening and closing itself, scheduling science operations, and safely shutting down in the event of weather or technical issues. The critical scheduler system chooses observations entered into the queue based on science priority and conditions. ‘Some systems are easier to roboticise and operate reliably than others. Instrument cryogenic cooling can be particularly troublesome,’ explained Chris Copperwheat, astronomer in charge.
Plans for a second four-metre successor, comprising a fast design using novel materials to reduce structure mass while maintaining stiffness are under way. ‘Fast’ means a focal ratio of ~f/1.5; which enables a small primary-secondary mirror separation. The mirror will also be segmented to reduce mass, and the instrument suite replaced with a single, prime focus imaging camera.
‘Major telescopes are ideal testbeds for technologies that can later be commercialised,’ commented Copperwheat. Liverpool JMU has partnerships with optical and engineering experts in the Merseyside area and beyond. Additionally, these projects raise the profile of the optical science, with students developing skills that they take outside of academia.
Testing and metrology
Huge projects like these lead to the commercial availability of better-performing products that otherwise may not have been invented. For example, astronomy has driven new CCD types, now used in medical imaging and consumer cameras. The commercial remote-sensing market has also been helped by the development of imaging equipment for high-resolution commercial satellites.
Component suppliers take their cues from the latest trends. For Edmund Optics that means adaptive optics. While Edmund does not manufacture products specifically for astronomy, their mirrors and filters are purchased for use in adaptive optics systems.
Customers use Edmund products on test benches, for designing systems before building versions for a telescope. ‘We get a lot of requests for filters and eyepieces from amateurs who want to customise their own telescopes,’ noted Daniel Adams, applications engineer.
Metrology systems designers also benefit. McPherson, manufacturer of photometers and spectrometers, recently sent a new ultraviolet spectrophotometer system for optical coating metrology to NASA Goddard. They optimised performance in the ‘deep’ ultraviolet region (90-169nm), designed to work with wavelengths beyond those of conventional deuterium lamps.
The spectrophotometer system will most likely help to develop, inspect and qualify optical materials and coatings used for very high altitude and extraterrestrial space flight missions. ‘NASA’s newest heliophysics satellite, the Ionospheric Connection Explorer, uses the same type of 1m monochromator that McPherson just shipped to NASA,’ said McPherson’s Erik Schoeffel.
