“The innovative design of the J-PAS camera and filter system will make it possible for the first time to determine the positions of hundreds of millions of galaxies, which will provide us with the first complete 3D map of the universe,” explains Dr. Antonio Marín-Franch, staff researcher and head of the instrumentation group at CEFCA (Centro de Estudios de Física del Cosmos de Aragón). His statement reveals the high value of seemingly less spectacular components. But first impressions can be deceptive: the colorful band-pass filters around the frame for the high-resolution astro camera play a key role in a galactic mission – literally.
The Observatory Astrofísico de Javalambre (OAJ), perched 2,000 meters above sea level on the “Pico del Buitre” in the Spanish region of Aragón, aims to photometrically screen several thousand square degrees (Deg²) of sky. CEFCA, the State Research Institute for Astrophysics and Cosmology, operates two telescopes with an exceptionally wide field of view. The JAST/T80 features a ZERODUR® glass-ceramic mirror 83 centimeters in diameter and with a 2-degree field of vision. It conducts initial large-scale sky observations and supports calibration for the astrophysical survey by J-PAS (Javalambre Physics of the Accelerating Universe Astrophysical Survey), intended to contribute to the accurate 3D mapping of all galaxies. The larger JST/T250 telescope, with a mirror 2.55 meters in diameter and a 3-degree field of view, serves this purpose. It is set to screen 800 square degrees in its first five years of operation – one fifth of the entire sky.
More than 80 filters for two telescopes
These observations require special, high-precision astronomical filters that can be used to study narrow wavelength ranges of starlight. This is the only way to determine the distance of astronomical objects – by measuring redshift in the light they emit. “This filter system enables us to measure several parameters necessary for galaxy formation, such as the temperatures of the stars, their mass, their age distribution and their metal content,” explains Dr. Antonio Marín-Franch.
Together with the scientists at CEFCA, SCHOTT’s experts have developed several sets of steep-edge and narrow-band pass filters for just this purpose – twelve for the smaller telescope and approximately 70 for the larger Javalambre. These astronomical filters only allow light to pass through in a 10- to 20-nanometer wavelength range, with a variation in transmittance of less than ±0.25 percent across the filter surface. All higher and lower wavelengths from the near infrared and ultraviolet ranges are blocked (T < 10–5).
This accomplishment comes thanks to a combination of color filter glasses and optical interference filters comprised of up to 200 thin layers. SCHOTT deployed its unique skills to meet the required array of excellent optical and mechanical properties. Its main site in Mainz, Germany, called upon its long years of experience in glass characterization and selection while SCHOTT’s plant in Yverdon, Switzerland, summoned its expertise in the calculation and production of interference filters. “We precisely coordinated the coating design, the glass material and the manufacturing process for batch production,” explains Dr. Ulf Brauneck, principal scientist Coating and Coated Components in Yverdon.
Specifically developed metrology
To guarantee and verify the exacting specifications, SCHOTT developed its own measuring device. It is a customized broadband (white light) Shack-Hartmann wavefront sensor. It enables measuring wavefront aberrations, not only at default values of 633 nanometers where astrofilters block light, but also at any wavelength from UV to NIR. This achieves a wavefront aberration of λ/14 across the entire 106×106-millimeter filter surface – significantly better than the required λ/2.
SCHOTT also developed a measuring device that determines the refractive index of the filter glasses by their reflective properties. As the 8-millimeter-thick filters are located directly in the telescope’s optical path, they also affect its imaging properties. The CEFCA researchers therefore had to determine the exact refractive index. Standard measuring methods, however, were no help. “In the end, we were able to prove to the last detail that we not only produce filters precisely, but that we can also measure them accurately in every respect. Our customers can have complete confidence in our proven ability to fulfill even the most demanding specifications,” concludes Dr.-Ing. Ralf Biertümpfel, Product Manager for Filters at SCHOTT Advanced Optics.