Look to the skies

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LLNL optical engineers Justin Wolfe (left) and Simon Cohen are seen through the 'r' filter (credit: Garry McLeod)

A telescope, described as one of the world’s newest, has been completed following a project undertaken by researchers to develop some very specialist optical filters 

The new Vera C. Rubin Observatory facility, situated in northern Chile, is complete with a Legacy Survey of Space and Time Camera (LSSTCam). With work now completed, the telescope is due to begin imaging the southern sky in 2024. 

Construction on the observatory started in 2014 on El Peñon, a peak 8,800ft high along the Cerro Pachón ridge in the Andes Mountains, located 220 miles north of Santiago, Chile. The observatory was funded by the National Science Foundation (NSF), the US Department of Energy’s Office of Science and private funding Garry McLeod raised by the LSST Corporation. 

The 8.4m telescope at Rubin Observatory will take digital images of the entire visible southern sky every few nights. During a 10-year time frame, Rubin Observatory will detect about 20 billion galaxies and create a time-lapse ‘movie of the sky’. It is hoped the data will help researchers better understand dark matter and dark energy, which make up 95 per cent of the universe, but whose makeup remains unknown, as well as study the formation of galaxies, track potentially hazardous asteroids and observe exploding stars. 

The telescope’s camera, which is the size of a small car and weighs more than three tonnes, will capture full-sky images at such high resolution that it would take 1,500 high-definition television screens to display just one picture. 

Delivered in full 

Because of the size of the telescope, the optical filters are so specialised that researchers at Lawrence Livermore National Laboratory (LLNL) have been working on them for almost 10 years, while their industrial partners fabricated the components. The last of six of these optical filters, used for the telescope’s camera, have been delivered to the SLAC National Accelerator Laboratory in Menlo Park, in California, completing the research project. They will be shipped from SLAC to the telescope site in Chile next summer. 

The six optical filters were fabricated using glass raw material from New York State that was shipped to Thales Space SESO in Aix-enProvence in southern France. Thales Space SESO shaped and polished the glass into the optical filters between 2016 and 2020. They were then transported to Westford, Massachusetts, for coating. Here, a special chamber was used for coating the LSSTCam filters, as well as a custom metrology station to measure the filter coatings. 

LLNL optical engineer Justin Wolfe inspects the alignment of the optic and lift fixture for one of six optical filters for the Rubin Observatory (credit: Garry McLeod)

Justin Wolfe, optical engineer at LLNL and the LSSTCam optics subsystems manager, revealed two factors that made coating the LSSTCam optical filters particularly challenging: the first was their size, and the second, a need for stringent uniformity within about two per cent. ‘It’s one thing to have uniformity over two inches; it’s another to have it over 30,’ he said. ‘It is a complex process to get a substrate ready for coating when you have three different vendors, 18 months of preparation and an irreversible coating process if mistakes are made.’ 

Each filter is about 76cm in diameter and weighs about 90lbs, ranging in thickness from about 13 to 26mm. The filters are labelled u, g, r, i, z and y; each transmits light from a segment of the electromagnetic spectrum, progressing throughout the entire visible range and moving from the near-ultraviolet (u) to the near-infrared (y). 

Pass the test 

The r-band filter, which covers a spectral band near the centre of this range, is likely to be used most often for the LSSTCam. It will be particularly useful for testing the camera because standard optical techniques can be used to verify the optical performance of the camera when this filter is in place. Wolfe continued: ‘As optics for large telescope systems are always a challenge, it is very gratifying to complete the work on these filters and all of the camera optical assemblies. The team did a great job and we are all very proud to have played a role in delivering these first-of-akind optical assemblies. Given unanticipated delays in the last couple of years due to the pandemic and other factors, it is a great relief to know the optics have all been delivered in good shape in time to meet the needs of the project.’ 

Once coated, the filters were shipped to LLNL to be placed into lab-designed mechanical mounts in the National Ignition Facility (NIF) optical assembly building. Vincent Riot, LLNL engineer and Rubin LSSTCam manager, added: ‘We have benefited from NIF’s state-of-the-art assembly building, which offers a top-notch clean room and skilled technicians who are used to handling large optics.’ 

A key feature of the LSSTCam’s optical assemblies will be its three lenses, one of which is the world’s largest high-performance optical lens ever fabricated, at 1.57m in diameter. It was listed in the latest edition of the Guinness World Records. This and its two companion lenses (at 1.2m and 72cm in diameter) were shipped from LLNL to SLAC for integration into the LSSTCam in 2019. 

SLAC is managing the overall design and fabrication, as well as the subcomponent integration and final assembly of the Rubin Observatory’s $168m, 3,200-megapixel digital camera. In addition to SLAC and LLNL, the team building the camera includes an international collaboration of universities and labs, including the Paris-based Centre National de la Recherche Scientifique and Brookhaven National Laboratory in New York state. 

Commercial products 

Vendors that offer optical filters include Alluxa, which recently developed 15-band optical filters for use in the Exoplanet Transmission Spectroscopy Imager (ETSI) instrument at Texas A&M University, Mitchell Institute for Fundamental Physics and Astronomy, and the Department of Physics & Astronomy. 

Edmund Optics offers a variety of optical filters, including bandpass interference, notch, edge, dichroic, colour substrate or neutral density. Ocean Insight’s filters absorb light energy in certain regions of the spectra for a variety of applications, including fluorescence and Raman experiments. MKS Newport’s optical filters range from fluorescence filters for DNA analysers to notch filters for laser safety eyewear. Materion was the company that coated the LLNL filters for the Rubin Observatory. It offers bespoke designs for markets, including space, science and astronomy, defence, process control, gas sensing and flame detection, thermal imaging, display, automotive sensing and medical instrumentation. 

CeNing Optics provides a variety of optical filters, including coloured glass, narrowband interference and bandpass. Spectrogon designs and manufactures optical interference filters for several applications, such as medical, process industry, thermal imaging, power distribution and surveillance and defence. 

Laser Components offers different designs, including beam splitters and bandpass, band-stop, shortpass and long-pass filters, with wavelength ranges from the UV to the FIR. Thorlabs’ optical filters include dielectric-coated filters, coloured glass filters, neutral density filters, spatial filters, and a tuneable optical filter based on liquid crystal technology. UltraFast Innovations (UFI) offers spectral filters for ultrashort pulse applications – for example, for preserving the pulse duration of reflected or transmitted pulses. Artifex Engineering offers custom absorption filters and dielectric filters, including bandpass, long-pass, shortpass or neutral density. Iridian Spectral Technologies’ optical filters and coatings cover the spectral range from UV 300nm to LWIR 10μm. Products include narrow bandpass filters, steep-edge long- or short-pass filters, dichroic-edge filter, notch filter, multi-zone filter and other special coating filters.

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Featured product: Spectrogon

Spectrogon designs and manufactures optical interference filters for several applications such as Medical, Process industry, Thermal imaging, Power distribution and Surveillance & Defense. Spectrogon offers optical filters and windows in the range 400-14, 000nm and has production capability for wafer sizes Ø6 to Ø200mm. The wafers can be diced to required size and delivered on tape frames. Bandpass, Narrow Bandpass, LongwavePass, Shortwave-Pass, Broad-Bandpass and Neutral Density filters can all be produced after customer’s required specifications. Each filter is designed with advanced state of the art technology with production in Sweden. Spectrogon also manufactures Antireflection, High reflective Coatings and Holographic Gratings. Spectrogon has several different coating machines for volume productions. Further information www.spectrogon.com