Thermal and mechanical design and test of the CCD mount for the WEAVE spectrograph cryostats
Abstract:
WEAVE is the new multi-object spectrograph for the William Herschel Telescope on La Palma. The culmination of prime focus, the large number of fibers and the wide resolution range has required a stringent optical design, which in turn demands a spectrograph with tight positional tolerances and large final focal plane. To capture this focal plane each of the two cryostats has two e2v 6k x 6k CCDs mounted as a mosaic. As well as being cooled to 150K via liquid nitrogen, the positional tolerances for the sensitive areas are flatness 60μm p-v over the entire image area, rotation around X and Y axis ±50 arcmin, translation in X, Y and Z ± 50 micron.
We have used a Stil confocal measuring head mounted on two Thorlab translation stages to create a X,Y mount, controlled by a Raspberry Pi that is capable of recording measurements in Z to better than 1μm accuracy. This is used to measure the flatness and deformation of the image area under vacuum, and when cooled to 150K and the overall tip and tilt of the image plane to ensure they meet specification and are repeatable.
In addition to this measuring system, we use a Thorlabs CMOS camera with a Navitar 50mm lens to ensure each CCDs image area is within specification with regards X and Y translation.
In order to satisfy the above requirements, we designed the CCD mount to be adjustable (on initial setup), correctly constrained, isolated from liquid nitrogen boil-off vibration, and thermally insulating.
VIRUS: status and performance of the massively-replicated fiber integral field spectrograph for the upgraded Hobby-Eberly Telescope
Abstract:
The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of 156 identical spectrographs (arrayed as 78 pairs, each with a pair of spectrographs) fed by 35,000 fibers, each 1.5 arcsec diameter, at the focus of the upgraded 10 m Hobby-Eberly Telescope (HET). VIRUS has a fixed bandpass of 350-550 nm and resolving power R~750. The fibers are grouped into 78 integral field units, each with 448 fibers and 20 m average length. VIRUS is the first example of large-scale replication applied to optical astronomy and is capable of surveying large areas of sky, spectrally. The VIRUS concept offers significant savings of engineering effort and cost when compared to traditional instruments.
The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX‡), using 0.8M Lyman-alpha emitting galaxies as tracers. The VIRUS array has been undergoing staged deployment starting in late 2015. Currently, more than half of the array has been populated and the HETDEX survey started in 2017 December. It will provide a powerful new facility instrument for the HET, well suited to the survey niche of the telescope, and will open up large spectroscopic surveys of the emission line universe for the first time. We will review the current state of production, lessons learned in sustaining volume production, characterization, deployment, and commissioning of this massive instrument.