We measured the influence functions of a 9-actuator warping harness of an ELT primary mirror segment prototype using a fiber interferometer. The compact setup consists of a stiff, lightweighted aluminium plate with a similar diameter as the segment (1.25 m), holding 24 fiber-fed collimators arranged in three concentric rings. We measure simultaneously the 24 absolute distances to the mirror surface with a nominal precision of 0.5 ppm. The recorded noise level in a quiet environment was below 50 nm rms. We found good agreement between the measured influence functions and the finiteelement model by the segment support manufacturer. These measurements were performed at ESO Garching in June 2017, with the “Absolute Multiline” metrology system.
Following the scientific requirements developed by a ESO Working Group on Multi Object Spectroscopy, we present a design concept for a facility suitable for massively-multiplexed optical spectroscopy. We propose a very wide-field Cassegrain telescope optimised for fibre-fed spectroscopy. Our design provides an optical and ADC-corrected field of 2.5 degree diameter for an 11.4m primary, with a three-element corrector and ADC. A gravity invariant focus for the central 10 arc-minute field can be inserted to host a giant IFU. The telescope has an exceptionally large etendue and provides adequate image quality in the 360- 1300 nm, or in the 1300-1800 nm wavelength range. The telescope is very compact enabling an economic enclosure. We stress the importance of developing simultaneously detailed designs for the telescope and instrumentation highlighting curved detectors as key elements to optimally exploit the telescope’s potential via fast spectroscopic cameras at low cost. With this concept, more than 15000 fibres can be positioned in the focal plane with existing technology enabling a revolution in spectroscopic discovery space and follow-up of panoramic imaging surveys including LSST.
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