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Register a new userVery fast transmissive spectrograph designs for highly multiplexed fiber spectroscopy
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Authors
Will Saunders
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Very fast (f/1.2 and f/1.35) transmissive spectrograph designs are presented for Hector and MSE. The designs have 61mm x 61mm detectors, 4 or 5 camera lenses of aperture less than 228mm, with just 6 air/glass surfaces, and rely on extreme aspheres for their imaging performance. The throughput is excellent, because of the i-line glasses used, the small number of air/glass surfaces.
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We present two novel designs for a telescope suitable for massively-multiplexed spectroscopy. The first is a very wide field Cassegrain telescope optimised for fibre feeding. It provides a Field Of View (FOV) of 2.5 degrees diameter with a 10m primary mirror. It is telecentric and works at F/3, optimal for fibre injection. As an option, a gravity invariant focus for the central 10 arc-minutes can be added, to host, for instance, a giant integral field unit (IFU). It has acceptable performance in the 360-1300 nm wavelength range. The second concept is an innovative five mirror telescope design based on a Three Mirror Anastigmatic (TMA) concept. The design provides a large FOV in a convenient, gravity- invariant focal plane, and is scalable to a range of telescope diameters. As specific example, we present a 10m telescope with a 1.5 degree diameter FOV and a relay system that allows simultaneous spectroscopy with 10,000 mini-IFUs over a square degree, or, alternatively a 17.5 square arcminutes giant IFU, by using 240 MUSE-type spectrographs. We stress the importance of developing the telescope and instrument designs for both cases.
The Prime Focus Spectrograph (PFS) is an optical/near-infrared multifiber spectrograph with 2394 science fibers distributed across a 1.3-deg diameter field of view at the Subaru 8.2-m telescope. The wide wavelength coverage from 0.38 {mu}m to 1.26 {mu}m, with a resolving power of 3000, simultaneously strengthens its ability to target three main survey programs: cosmology, galactic archaeology and galaxy/AGN evolution. A medium resolution mode with a resolving power of 5000 for 0.71 {mu}m to 0.89 {mu}m will also be available by simply exchanging dispersers. We highlight some of the technological aspects of the design. To transform the telescope focal ratio, a broad-band coated microlens is glued to each fiber tip. A higher transmission fiber is selected for the longest part of the cable system, optimizing overall throughput; a fiber with low focal ratio degradation is selected for the fiber-positioner and fiber-slit components, minimizing the effects of fiber movements and fiber bending. Fiber positioning will be performed by a positioner consisting of two stages of piezo-electric rotary motors. The positions of these motors are measured by taking an image of artificially back-illuminated fibers with the metrology camera located in the Cassegrain container; the fibers are placed in the proper location by iteratively measuring and then adjusting the positions of the motors. Target light reaches one of the four identical fast-Schmidt spectrograph modules, each with three arms. The PFS project has passed several project-wide design reviews and is now in the construction phase.
The Prime Focus Spectrograph (PFS) is an optical/near-infrared multi-fiber spectrograph with 2394 science fibers, which are distributed in 1.3 degree diameter field of view at Subaru 8.2-meter telescope. The simultaneous wide wavelength coverage from 0.38 um to 1.26 um, with the resolving power of 3000, strengthens its ability to target three main survey programs: cosmology, Galactic archaeology, and galaxy/AGN evolution. A medium resolution mode with resolving power of 5000 for 0.71 um to 0.89 um also will be available by simply exchanging dispersers. PFS takes the role for the spectroscopic part of the Subaru Measurement of Images and Redshifts project, while Hyper Suprime-Cam works on the imaging part. To transform the telescope plus WFC focal ratio, a 3-mm thick broad-band coated glass-molded microlens is glued to each fiber tip. A higher transmission fiber is selected for the longest part of cable system, while one with a better FRD performance is selected for the fiber-positioner and fiber-slit components, given the more frequent fiber movements and tightly curved structure. Each Fiber positioner consists of two stages of piezo-electric rotary motors. Its engineering model has been produced and tested. Fiber positioning will be performed iteratively by taking an image of artificially back-illuminated fibers with the Metrology camera located in the Cassegrain container. The camera is carefully designed so that fiber position measurements are unaffected by small amounts of high special-frequency inaccuracies in WFC lens surface shapes. Target light carried through the fiber system reaches one of four identical fast-Schmidt spectrograph modules, each with three arms. Prototype VPH gratings have been optically tested. CCD production is complete, with standard fully-depleted CCDs for red arms and more-challenging thinner fully-depleted CCDs with blue-optimized coating for blue arms.
In this paper we present the Australian Astronomical Observatorys concept design for Sphinx - a fiber positioned with 4332 spines on a 7.77mm pitch for CFHTs Mauna Kea Spectroscopic Explorer (MSE) Telescope. Based on the Echidna technology used with FMOS (on Subaru) and 4MOST (on VISTA), the next evolution of the tilting spine design delivers improved performance and superior allocation efficiency. Several prototypes have been constructed that demonstrate the suitability of the new design for MSE. Results of prototype testing are presented, along with an analysis of the impact of tilting spines on the overall survey efficiency. The Sphinx fiber positioned utilizes a novel metrology system for spine position feedback. The metrology design and the careful considerations required to achieve reliable, high accuracy measurements of all fibres in a realistic telescope environment are also presented.
The baseline energy-resolution performance for the current generation of large-mass, low-temperature calorimeters (utilizing TES and NTD sensor technologies) is $>2$ orders of magnitude worse than theoretical predictions. A detailed study of several calorimetric detectors suggests that a mismatch between the sensor and signal bandwidths is the primary reason for suppressed sensitivity. With this understanding, we propose a detector design in which a thin-film Au pad is directly deposited onto a massive absorber that is then thermally linked to a separately fabricated TES chip via an Au wirebond, providing large electron-phonon coupling (i.e. high signal bandwidth), ease of fabrication, and cosmogenic background suppression. Interestingly, this design strategy is fully compatible with the use of hygroscopic crystals (NaI) as absorbers. An 80-mm diameter Si light detector based upon these design principles, with potential use in both dark matter and neutrinoless double beta decay, has an estimated baseline energy resolution of 0.35 eV, 20$times$ better than currently achievable. A 1.75 kg ZnMoO$_{4}$ large-mass calorimeter would have a 3.5 eV baseline resolution, 1000$times$ better than currently achieved with NTDs with an estimated position dependence $frac{Delta E}{E}$ of 6$times$10$^{-4}$. Such minimal position dependence is made possible by forcing the sensor bandwidth to be much smaller than the signal bandwidth. Further, intrinsic event timing resolution is estimated to be $sim$170 $mu$s for 3 MeV recoils in the phonon detector, satisfying the event-rate requirements of large $Q_{beta beta}$ next-generation neutrinoless double beta decay experiments. Quiescent bias power for both of these designs is found to be significantly larger than parasitic power loads achieved in the SPICA/SAFARI infrared bolometers.
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