Pupil slicer design for the NASA-NSF extreme precision Doppler spectrograph concept WISDOM

Egan, Mark; Fürész, Gábor; Simcoe, Robert A.

doi:10.1117/12.2234378

Cited by 2 publications

(3 citation statements)

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“…The opto-mechanical realization of the pupil slicer is discussed in [18]. Here we give: a brief description of the optical design; the reasoning behind the choice of a macroscopic pupil slicer rather than a fiber-based or an image slicer; and also discuss some of the design choices and optimization process.…”

Section: Slicer Theorymentioning

confidence: 99%

“…The fibers are mounted on V-grooves machined into the sides of a hexagonal fused silica cone that follows the arrangement of the foci, while the material matches the coefficient of thermal expansion (CTE) of the fibers. Lateral shifting of mirror segments 18 can precisely match the six foci locations to the as-mounted fiber end positions. As shown on Figure 2 the image quality of this purely reflective slicer is excellent and fully achromatic, and even combined with the ADC 3 the image quality is only limited by the telescope and the atmospheric seeing, not the slicer.…”

Section: Slicer Theorymentioning

confidence: 99%

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Fiber link design for the NASA-NSF extreme precision Doppler spectrograph concept "WISDOM"

et al. 2016

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We describe the design of the fiber-optic coupling and light transfer system of the WISDOM (WIYN Spectrograph for DOppler Monitoring) instrument. As a next-generation Precision Radial Velocity (PRV) spectrometer, WISDOM incorporates lessons learned from HARPS about thermal, pressure, and gravity control, but also takes new measures to stabilize the spectrograph illumination, a subject that has been overlooked until recently. While fiber optic links provide more even illumination than a conventional slit, careful engineering of the interface is required to realize their full potential. Conventional round fiber core geometries have been used successfully in conjunction with optical double scramblers, but such systems still retain a memory of the input illumination that is visible in systems seeking sub-m/s PRV precision. Noncircular fibers, along with advanced optical scramblers, and careful optimization of the spectrograph optical system itself are therefore necessary to study Earth-sized planets. For WISDOM, we have developed such a state-of-the-art fiber link concept. Its design is driven primarily by PRV requirements, but it also manages to preserve high overall throughput.Light from the telescope is coupled into a set of six, 32 µm diameter octagonal core fibers, as high resolution is achieved via pupil slicing. The low-OH, step index, fused silica, FBPI-type fibers are custom designed for their numerical aperture that matches the convergence of the feeding beam and thus minimizes focal ratio degradation at the output. Given the demanding environment at the telescope the fiber end tips are mounted in a custom fused silica holder, providing a perfect thermal match. We used a novel process, chemically assisted photo etching, to manufacture this glass fiber holder.A single ball-lens scrambler is inserted into the 25m long fibers. Employing an anti-reflection (AR) coated, high index, cubic-zirconia ball lens the alignment of the scrambler components are straightforward, as the fiber end tips (also AR coated) by design touch the ball lens and thus eliminate spacing tolerances. A clever and simple opto-mechanical design and assembly process assures micron-level self-alignment, yielding a ~87% throughput and a scrambling gain of >20,000.To mitigate modal noise the individual fibers then subsequently combined into a pair of rectangular fibers, providing a much larger modal area thanks to the 34x106 micron diameter. To minimize slit height, and thus better utilize detector area, the octagonal cores are brought very close together in this transition. The two outer fibers are side polished at one side, into a D-shaped cladding, while the central fiber has a dual side polish. These tapered, side-flattening operations are executed with precise alignment to the octagonal core. Thus the cores of the 3 fibers are brought together and aligned within few microns of each other before spliced onto the rectangular fiber.Overall throughput kept high and FRD at bay by careful management of fiber mounting, vacuum feed-throug...

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Section: Slicer Theorymentioning

confidence: 99%

Section: Slicer Theorymentioning

confidence: 99%

Fiber link design for the NASA-NSF extreme precision Doppler spectrograph concept "WISDOM"

et al. 2016

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“…Project costs scale strongly with beam size and such a design would be difficult to fit within the space constraints of the designated spectrograph room at WIYN. More on the implementation of the pupil slicer can be found in [7] and [10], here we just point out that with a 6-way slicing of the pupil we managed to reach the R=110k target resolution for the 1.2″ fiber size with a 125mm beam. (HARPS has a 200mm beam for similar R and Dtel, while for a smaller aperture of φ=1.0″.)…”

Section: Pupil Slicermentioning

confidence: 99%

WISDOM: the WIYN spectrograph for Doppler monitoring: a NASA-NSF concept for an extreme precision radial velocity instrument in support of TESS

Fürész

Simcoe

Barnes³

et al. 2016

SPIE Proceedings

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The Kepler mission highlighted that precision radial velocity (PRV) follow-up is a real bottleneck in supporting transiting exoplanet surveys. The limited availability of PRV instruments, and the desire to break the "1 m/s" precision barrier, prompted the formation of a NASA-NSF collaboration 'NN-EXPLORE' to call for proposals designing a new Extreme Precision Doppler Spectrograph (EPDS). By securing a significant fraction of telescope time on the 3.5m WIYN at Kitt Peak, and aiming for unprecedented long-term precision, the EPDS instrument will provide a unique tool for U.S. astronomers in characterizing exoplanet candidates identified by TESS. One of the two funded instrument concept studies is led by the Massachusetts Institute of Technology, in consortium with Lincoln Laboratories, Harvard-Smithsonian Center for Astrophysics and the Carnegie Observatories. This paper describes the instrument concept WISDOM (WIYN Spectrograph for DOppler Monitoring) prepared by this team.WISDOM is a fiber fed, environmentally controlled, high resolution (R=110k), asymmetric white-pupil echelle spectrograph, covering a wide 380-1300nm wavelength region. Its R4 and R6 echelle gratings provide the main dispersion, symmetrically mounted on either side of a vertically aligned, vacuum-enclosed carbon fiber optical bench. Each grating feeds two cameras and thus the resulting wavelength range per camera is narrow enough that the VPHG cross-dispersers and employed anti-reflection coatings are highly efficient. The instrument operates near room temperature, and so thermal background for the near-infrared arm is mitigated by thermal blocking filters and a short (1.7µm) cutoff HgCdTe detector. To achieve high resolution while maintaining small overall instrument size (100/125mm beam diameter), imposed by the limited available space within the observatory building, we chose to slice the telescope pupil 6 ways before coupling light into fibers. An atmospheric dispersion corrector and fast tip-tilt system assures maximal light gathering within the 1.2″ entrance aperture. The six octagonal fibers corresponding to each slice of the pupil employ ball-lens double scramblers to stabilize the near-and far-fields. Three apiece are coupled into each of two rectangular fibers, to mitigate modal nose and present a rectilinear illumination pattern at the spectrograph's slit plane. Wavelength solutions are derived from ThAr lamps and an extremely wide coverage dual-channel laser frequency comb. Data is reduced on the fly for evaluation by a custom pipeline, while daily archives and extended scope data reduction products are stored on NExScI servers, also managing archives and access privileges for GTO and GO programs.Note: individual papers, submitted along this main paper, describe the details of subsystems such as the optical design (Barnes et al.,, the fiber link design (Fűrész et al.,, and the pupil slicer (Egan et al.,.

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Pupil slicer design for the NASA-NSF extreme precision Doppler spectrograph concept WISDOM

Cited by 2 publications

References 3 publications

Fiber link design for the NASA-NSF extreme precision Doppler spectrograph concept "WISDOM"

Fiber link design for the NASA-NSF extreme precision Doppler spectrograph concept "WISDOM"

WISDOM: the WIYN spectrograph for Doppler monitoring: a NASA-NSF concept for an extreme precision radial velocity instrument in support of TESS

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