SCHOTT solutions no. 1/2016 > Optical filters

Optical filters
Photo: Cefca

70 Filters for Producing a 3-D Map of the Universe

The Javalambre Telescopes in Teruel, Spain, will make it possible for the first time ever to track the positions of hundreds of millions of galaxies and how they develop. SCHOTT has provided several sets of steep-edge and narrow-band bandpass filters that will allow for the narrow wavelength ranges of starlight to be analyzed.

Bernhard Gerl

Only a dusty dirt road leads through the Sierra de Javalambre to the two new telescopes on top of the 1,956-meter high Pico del Buitre in the province of Teruel located in the structurally weak Spanish region of Aragón. The high-tech observatory also contributes to economic development here. But for the researchers at CEFCA (Centro de Estudios de Física del Cosmos de Aragón), this represents a scientific milestone that will perhaps bring decisive progress in searching for dark energy, the mysterious substance that bears nearly 70 percent of the mass of the universe and whose properties we know virtually nothing about. Cosmologists suspect that tiny variations in the distribution of dark energy shortly after the Big Bang can still be seen today in how the galaxies are distributed across the universe. Precise 3-D mapping of all of the galaxies has not been done yet, however. Hopefully, the Javalambre Telescopes and the J-PAS (Javalambre Physics of the Accelerating Universe Astrophysical Survey) project will make a significant contribution to achieving this goal.

”The innovative design of the J-PAS camera and the filter system, for the first time, will make it possible to determine the positions of hundreds of millions of galaxies in the sky, which will provide us with the first complete 3-D map of the universe,” explains Dr. Antonio Marín-Franch, staff researcher and head of the instrumentation group at CEFCA. How far away astronomical objects actually are can be determined by measuring the redshift of the light that they emit. Together with the scientists at CEFCA, SCHOTT has developed sets of filters specifically for the Javalambre Telescopes that make this measurement possible, and that can do more than just that.
Optical filters
Processing and quality control of filter glasses (above) took place at the SCHOTT competence center for high precision optical components and coatings in the Swiss town of Yverdon. These components are used in a filter wheel (below). This allows many parameters that are of importance to the development of galaxies to be collected. Photo oben: SCHOTT/A. Sell, Photo below: SCHOTT/Cefca
”With this filter system, we will be able to measure many parameters that are of importance to the formation of galaxies, for example the temperatures of the stars, their mass, the distribution of their ages and their metal content,” explains Dr. Antonio Marín-Franch. To perform these tasks, 12 different astronomical filters were manufactured for the smaller of the two telescopes,
the JAST/T80, which features a ZERODUR® glass-ceramic mirror substrate 80 centimeters in diameter. The larger JAST/T250 has a ZERODUR® glass-ceramic mirror substrate that is 250 cm in diameter. In fact, the experts even developed 70 different astronomical filters for this telescope. Only two filters, known for their excellent optical and mechanical properties, were manufactured in each of these designs.

Metrology developed specifically for optical filters

What is so unique about these steep-edge and narrow-band filters is that they allow incident light to pass through, but only in a 10- to 20-nanometer wide wavelength range with a variation in transmittance of only 0.06  %, and yet block all higher and lower wavelengths from the near IR to UV (T <10–5). This was achieved by using a combination of a color filter glass for blocking all wavelengths above a certain threshold and an interference filter.

SCHOTT has been manufacturing interference filters since the 1940s and does so today mainly at its plant in Yverdon (Switzerland), where the know-how and an extensive range of instrumentation, for example for magnetron and ion beam sputtering, is available. The laborious calculation of the layer sequences is done using the computer. Then the experts’ knowledge is needed to determine which of the various layer structures that are possible can be converted into technically feasible solutions.
12 different optical filter glasses are used in the JAST/T80 Telescope (above). 70 different steep-edge and narrow-band filters are even used in the larger JAST/T250. Due to the very demanding specifications, SCHOTT developed a special measurement technology for this astronomy project. Photo: Cefca
SCHOTT even had to develop its own metrology to be able to guarantee and verify the very demanding specifications that the CEFCA scientists had set. For example, the wavefront aberration across the entire filter surface of 106 times 106 millimeters had to be no greater than λ/2 (better than λ/4 was even achieved). To verify this, a special broadband Shack-Hartmann wavefront sensor was developed for SCHOTT because standard wavefront sensors measure at 633 nanometers, i. e. where the astro filters block the light.

Because the eight-millimeter thick filters are located directly in the optical path of the telescope, they also affect its imaging properties. For this reason, the researchers at CEFCA had to know the refractive index of the glass filters. ”But our filters were usually opaque at the wavelengths in which the refractive index of glass is usually measured,” explains Dr.-Ing. Ralf Biertümpfel, Product Manager at SCHOTT Advanced Optics. ”This is why we needed a measuring device that would allow us to determine the reflective properties of the glass.” SCHOTT also developed this measuring apparatus on its own.

Through this project, the company has achieved a technological advantage that will open up new markets by combining materials, material characterization, manufacturing, coating expertise and available measurement technology. After all, narrow-band bandpass filters can used for other purposes besides astronomical telescopes. Cemented elements in which multiple bandpass filters can be mounted onto a single substrate are used in satellites to observe the earth in narrow spectral ranges. Nevertheless, due to the experience gained, these filters can also be used in spectroscopy and can now be custom developed and manufactured very quickly for our customers. <
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