Low line density, high efficiency, volume phase holographic gratings for the Extremely Large Telescope's HARMONI spectrograph

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HARMONI is one of the first-light instruments for the Extremely Large Telescope (ELT). It is an integral field spectrograph (also called hyper-spectral imager) that takes simultaneous spectra at near-infrared wavelengths of every point in a two-dimensional field of view on the sky. Coupled with an adaptive optics system, HARMONI is able to achieve the highest spatial resolution possible with the ELT. The exquisite spatial resolution, coupled with the huge collecting area of the ELT, means that HARMONI can spectroscopically study faint objects in detail, such as distant galaxies at early times in the history of the Universe.We are looking to enhance HARMONI's capability by furthering the state-of-the-art in the manufacture of diffraction gratings. The goal of this proposal is to extend current manufacturing techniques to produce low line density, high throughput, high uniformity, volume phase holographic gratings. Use of these gratings in HARMONI will increase the instantaneous wavelength coverage of the instrument by a factor of two, allowing us to observe many more emission and absorption spectral lines from astronomical targets in a single observation. This increases our observing efficiency, making best use of precious telescope time, but also allows us to study time variable phenomenon that we cannot observe in a sequential fashion. As part of this proposal, we are assembling an experimental set-up to quantify the performance of these newly developed gratings, testing to ensure their uniformity and high throughput at near-infrared wavelengths. The set-up will be available to researchers from other research groups in all disciplines across the U.K., thus enhancing the country's instrumentation capabilities for physics and astronomy.