Located at the Nasmyth Platforms A and B, the Prefocal Stations of the Extremely Large Telescope (ELT) are the last opto-mechanical components before the light from the giant telescope comes to a focus. The mission of the Prefocal Stations are threefold. Firstly, these high-precision opto-mechanical and optical sensing devices propagate the light collected on the telescope into science instruments and other test equipment. Very high optical quality, stability, and low vibration are key characteristics of the deployable M6N and M6C mirrors, that provide the optical propagation function. Secondly, by means of three Sensor Arms, they pick and adapt the light from up to three guide stars for its use in the Acquisition, Guiding and Wavefront Sensing to support the telescope active and adaptive optics. The active optics stabilize the images delivered to the science instruments, despite the constantly changing effects of wind and other disturbances on the telescope, and periodically realign the telescope to keep the adaptive optics working in their operating range. The adaptive optics compensate for the wavefront distortion caused by the atmospheric turbulence by acting on the deformable mirror (M4). Thirdly, the Prefocal Stations provide optical sensing to support phasing of the ELT primary mirrors, diagnostics, and maintenance of the optics. These tasks are performed by the Phasing and Diagnostic Station, which is located on the Coudé path. The functions provided by the Prefocal Stations are critical for the commissioning and operation of the ELT telescope. Here we report on the final design of the Prefocal Stations, with an emphasis on the Prefocal Station Main System.
WEAVE is a new wide-field multi-object spectroscopy (MOS) facility proposed for the prime focus of the 4.2m William Herschel Telescope. The facility comprises a new 2-degree field-of-view Prime Focus Corrector (PFC) with a 1000-multiplex fibre positioner, a small number of individually deployable integral field units, and a large single integral field unit (IFU). The IFUs and the MOS fibres can be used to feed a dual-beam spectrograph that will provide full coverage of the majority of the visible spectrum in a single exposure at a spectral resolution of ~5000 or modest wavelength coverage in both arms at a resolution ~20000. In order to compensate the field rotation, the Prime Focus Rotator (PFR) is assembled in between the WEAVE Fiber Positioner (system that positions the fibers in the focal plane) and with the Central Can (contains the Prime Focus corrector optics) on the William Herschel Telescope (WHT). The Prime Focus Rotator must provide a rotation degree of freedom for the Fibre Positioner with a high bending stiffness (causing a deflection smaller than 0.008° between interface flanges) adding the minimum mass possible to the system (less than 700kg). This is easily identified as the main design driver to be considered. The Prime Focus Rotator positions the Fibre Positioner to an accuracy of 5 arcsec when tracking and guides all the fibres and other power and control lines through a cable wrap, for which the available space is limited. IDOM proposal to comply with these coupled requirements consists of an optimized structural system with a slightly preloaded cross roller bearing providing the highest possible stiffness to weight ratio. The rotation is driven by means of a direct drive motor powered by a servo drive. For the Cable Wrap, a compact design based on a concept previously developed by IDOM for the Folded Cassegrain Sets the GTC was proposed.
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