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24 September 2018 Near infrared throughput and stray light measurements of diffraction gratings for ELT-HARMONI
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Abstract
The High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph (HARMONI) will be one of the instruments installed on ESO's 39-meter Extremely Large Telescope (ELT) at first light. The instrument will operate from 0.47 - 2.45 μm with Δλ/λ = 3,000 - 17,000. On-sky spatial pixels (spaxels) are divided between four spectrographs, each equipped with 11 transmission diffraction gratings to cover the ranges of wavelengths and spectral resolutions. These spectrographs will be cooled to ~140 K to decrease thermal radiation at longer wavelengths.

In all configurations, the diffraction grating will lose a greater fraction of scientific light than any other single optic in the instrument. Additionally, manufacturers are often unable to measure the fraction of transmitted light at HARMONI's longest wavelengths. For these reasons, we have developed a setup to measure the efficiencies of transmission diffraction gratings across HARMONI's bandpass. The setup uses modulated signals, a single detector, and a lock-in amplifier to minimize sources of systematic errors. A modified version of this setup may be used to measure stray light. These setups and initial results are presented.
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John I. Capone, Myriam Rodrigues, Fraser Clarke, Andrew Earle, Thomas Foster, Andrea Hidalgo Valadez, Ian Lewis, James Lynn, Kieran O'Brien, Matthias Tecza, Niranjan Thatte, and Ian Tosh "Near infrared throughput and stray light measurements of diffraction gratings for ELT-HARMONI", Proc. SPIE 10706, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation III, 1070644 (24 September 2018); https://doi.org/10.1117/12.2310086
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