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PG&O delivers coated mirror for dark energy experiment

Thin film coating specialist Precision Glass & Optics (PG&O) has delivered specialised optical mirrors to Texas A&M University and the University of Texas at Austin to be used in the first-of-its-kind VIRUS instrument, a key part of the Hobby-Eberly Telescope Dark Energy Experiment, or HETDEX. The $36m international research project will study the force that causes the expansion of the universe to speed up, rather than slow down. This poorly understood force is known as dark energy. Since dark energy makes up more than 70 per cent of the universe, understanding this mysterious force is considered one of the top challenges in physics today, according to HETDEX researchers.

The innovative VIRUS system is comprised of 150 small, individual replicas of a single spectrograph. The optical mirrors provided by PG&O for the VIRUS collimator fold mirror were specified for operational wavelength range of 345-700nm and are optimised in the 350-590nm range, with average reflectivity of greater than 99 per cent, and greater than 95 per cent between 345 and 700nm. The absolute reflectivity is greater than 98 per cent between 350 and 590nm, and greater than 92 per cent in the 345-700nm range. Angle of incidence is at 12.5 ±5°. This coating also has to perform in extreme environmental conditions (-25 to +66°C) and to meet Mil-C-48497 abrasion/adhesion durability standards. The other optical specifications include a surface figure of λ/8 at 632nm RMS surface quality at 40-20 scratch-dig, and surface roughness of 2nm RMS.

PG&O provides high performance optical components and thin film coatings for extremely demanding tasks. To meet the requirements for VIRUS, the reflective coatings on borosilicate glass have an operational lifetime of 20 years.

The astronomical instrument is designed to gather light from distant galaxies and split it into individual spectral wavelengths. The spectrum will reveal an object’s chemical composition, its temperature, and the speed at which the universe is stretching between the detected galaxies and Earth. VIRUS will capture spectra from 33,000 points on the sky simultaneously, using fibre optics to transfer the light from the Hobby-Eberly Telescope (HET) focal plane to the huge replicated array of spectrographs. This will permit HETDEX astronomers to produce a 3D map of a large volume of space, indicating how fast the universe expanded at different periods in history, which will help reveal the role that dark energy has played during different eras. It will search for any evidence that the strength of dark energy changes over time and also will provide the most precise measurement of the geometry of the universe, which is related to the physics of the universe at the moment of Big Bang.

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