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Prime Focus Spectrograph (PFS) for the Subaru Telescope: Overview, recent progress, and future perspectives

Naoyuki Tamura, Naruhisa Takato, Atsushi Shimono, Yuki Moritani, Kiyoto Yabe, Yuki Ishizuka, Akitoshi Ueda, Yukiko Kamata, Hrand Aghazarian, Stephane Arnouts, Gabriel Barban, Robert H. Barkhouser, Renato C. Borges, David F. Braun, Michael A. Carr, Pierre-Yves Chabaud, Yin-Chang Chang, Hsin-Yo Chen, Masashi Chiba, Richard C. Y. Chou, You-Hua Chu, Judith G. Cohen, Rodrigo P. de Almeida, Antonio C. de Oliveira, Ligia S. de Oliveira, Richard G. Dekany, Kjetil Dohlen, Jesulino B. dos Santos, Leandro H. dos Santos, Richard S. Ellis, Maximilian Fabricius, Didier Ferrand, Decio Ferreira, Mirek Golebiowski, Jenny E. Greene, Johannes Gross, James E. Gunn, Randolph Hammond, Albert Harding, Murdock Hart, Timothy M. Heckman, Christopher M. Hirata, Paul Ho, Stephen C. Hope, Larry Hovland, Shu-Fu Hsu, Yen-Shan Hu, Ping-Jie Huang, Marc Jaquet, Yipeng Jing, Jennifer Karr, Masahiko Kimura, Matthew E. King, Eiichiro Komatsu, Vincent Le Brun, Olivier Le Fevre, Arnaud Le Fur, David Le Mignant, Hung-Hsu Ling, Craig P. Loomis, Robert H. Lupton, Fabrice Madec, Peter Mao, Lucas S. Marrara, Claudia Mendes de Oliveira, Yosuke Minowa, Chaz N. Morantz, Hitoshi Murayama, Graham J. Murray, Youichi Ohyama, Joseph Orndorff, Sandrine Pascal, Jefferson M. Pereira, Daniel J. Reiley, Martin Reinecke, Andreas Ritter, Mitsuko Roberts, Mark A. Schwochert, Michael D. Seiffert, Stephen A. Smee, Laerte Sodre Jr., David N. Spergel, Aaron J. Steinkraus, Michael A. Strauss, Christian Surace, Yasushi Suto, Nao Suzuki, John Swinbank, Philip J. Tait, Masahiro Takada, Tomonori Tamura, Yoko Tanaka, Laurence Tresse, Orlando Verducci Jr., Didier Vibert, Clement Vidal, Shiang-Yu Wang, Chih-Yi Wen, Chi-Hung Yan, Naoki Yasuda

arXiv:1608.01075v1astro-ph.IMastro-ph.COastro-ph.GA

TL;DR

PFS is designed to support wide-field, massively multiplexed optical and near-infrared spectroscopy on Subaru. The paper describes its instrument design and development, including a 2394-fiber focal plane and broad simultaneous wavelength coverage. At the time reported, subsystem construction and integration were underway, while some performance estimates remained subject to revision.

  • Problem

    The project aims to extend Subaru’s wide-field survey capability across cosmology, galaxy and AGN evolution, and Galactic archaeology.

    Source: §1 · p. 4

  • Method

    PFS uses 2394 reconfigurable fibers in a 1.3-degree field to obtain simultaneous spectra from 380 nm to 1260 nm.

    Source: §1 · p. 3

  • Results

    The article reports PFS design finalization and subsystem-level construction, with integration and validation planned before delivery to the telescope site.

    Source: §5 · p. 15

  • Takeaways & Limitations

    PFS and HSC are intended to enable deep imaging and spectroscopic surveys of the same sky region using the same telescope.

    Source: §1 · p. 3

    Fiber reconfiguration time, throughput, and estimated on-sky sensitivity were still subject to updates as the instrument was built, integrated, and tested.

    Source: §2 · p. 6

Abstract

from arXiv · show

PFS (Prime Focus Spectrograph), a next generation facility instrument on the 8.2-meter Subaru Telescope, is a very wide-field, massively multiplexed, optical and near-infrared spectrograph. Exploiting the Subaru prime focus, 2394 reconfigurable fibers will be distributed over the 1.3 deg field of view. The spectrograph has been designed with 3 arms of blue, red, and near-infrared cameras to simultaneously observe spectra from 380nm to 1260nm in one exposure at a resolution of ~1.6-2.7A. An international collaboration is developing this instrument under the initiative of Kavli IPMU. The project is now going into the construction phase aiming at undertaking system integration in 2017-2018 and subsequently carrying out engineering operations in 2018-2019. This article gives an overview of the instrument, current project status and future paths forward.

1. INTRODUCTION

PFS is a wide-field, massively multiplexed spectrograph designed to pair with HSC for deep imaging and spectroscopy of the same sky regions. Its broad wavelength coverage and fiber density support surveys spanning cosmology, galaxy and AGN evolution, and Galactic archaeology.

1. INTRODUCTION

  • Figure 1 presents PFS science cases through cosmology comparisons, redshift access to cosmic star formation history, and a Sculptor dwarf galaxy pointing plan.The caption associates the pointing plan’s efficiency with PFS’s wide field and high multiplicity.Source: Figure 1 · p. 3
  • Subaru’s earlier FMOS survey FastSound mapped about 3,000 galaxies around z ~1.4 and detected redshift-space distortions.FMOS provided near-infrared spectra, while Suprime-Cam delivered wide-field imaging.Source: §1 · pp. 2–3
  • PFS places 2394 reconfigurable fibers across a 1.3-degree field and simultaneously covers 380–1260 nm in one exposure.HSC and PFS can survey the same sky region with imaging and spectroscopy on the same 8.2-meter telescope.Source: §1 · p. 3
  • The planned survey targets cosmology, galaxy and AGN evolution, and Galactic archaeology, with their joint implications aimed at understanding the universe’s dark sector.The wide field, high multiplicity, and high fiber density inform the survey design across these three science cases.Source: §1 · p. 4
  • An international collaboration led at the initiative of Kavli IPMU was constructing and integrating PFS, with science operations targeted from mid-to-late 2019.Instrument modeling was also underway to characterize on-sky capabilities and refine the survey design.Source: §1 · p. 4

2. THE INSTRUMENT

PFS routes light from prime-focus fibers through a fiber cable to spectrographs, where it is dispersed into spectral images on detectors. Its four subsystems include the prime-focus instrument, metrology camera, spectrograph system, and fiber system; several performance parameters remain subject to revision as testing proceeds.

2. THE INSTRUMENT

  • PFS feeds light from astronomical objects and sky through prime-focus fibers and a cable to the spectrograph system, which delivers spectral images to detectors.The instrument comprises four subsystems, with software and survey operations described separately.Source: §2 · p. 4
  • The 42-module fiber-positioner system uses 57 Cobra actuators per module, and prototype testing showed satisfactory target convergence in each fiber’s patrol field.The fiber tips carry planoconcave microlenses to increase the input beam’s focal ratio to 2.8.Source: §2 · p. 6
  • The metrology camera images back-lit science and fiducial fibers to measure their positions, enabling closed-loop operation of positioners that have no encoders.It can image all such fibers in one exposure.Source: §2 · p. 6
  • The spectrograph splits collimated fiber beams into blue, red, and near-infrared channels, disperses them with a VPH grating, and forms spectral images on detectors.Four identically designed modules each deliver about 600 spectral images; a grating exchange mechanism permits medium-resolution red spectra.Source: §2 · p. 6
  • FOCCoS comprises two short-fiber systems and an approximately 65-meter cable routed between the prime-focus instrument and spectrograph.Connectors allow POpt2 detachment and facilitate spectrograph delivery, integration, operation, and maintenance.Source: §2 · p. 6
  • Fiber reconfiguration time, throughput, and estimated on-sky sensitivity are provisional and may be updated as PFS is built, integrated, and tested.The basic instrument parameters are fixed.Source: §2 · p. 6

3. OPERATION CONCEPT

PFS operations coordinate target and fiber configuration, telescope control, data reduction, calibration, and object-level quality assessment. The design seeks efficient observing while addressing sky subtraction and calibration challenges without routine beam-switching.

3. OPERATION CONCEPT

  • PFS software packages are designed to coordinate hardware and software while accommodating faint targets, varied success criteria, and dynamic fiber reallocation.The packages are intended to remain only loosely coupled, and operational planning is coordinated with the observatory.Source: §3 · pp. 6–7

3.1 Observation preparation

  • Exposure Targeting Software prepares target catalogs, pointings, position angles, and fiber allocations, which are recalculated against actual observing time before configuration.Observers also assign fibers to blank sky and flux-calibration stars; optimal numbers and distributions will be determined during on-sky engineering observations.Source: §3.1 · p. 7

3.2 Instrument operation & data acquisition

  • Cobra motor maps relate motor speed to angular position and are updated as data accumulate; all three science areas plan observations without beam-switching to blank sky.The MCS cannot rotate synchronously with PFI, so rotation can elongate back-lit fiber images and could worsen centroiding error.Source: §3.2 · pp. 8–9
  • Differential atmospheric refraction motivates fiber reconfiguration about every 30 minutes, while one configuration is expected to take about 90 seconds across roughly six iterations.The telescope’s rotator is restricted to −60 to +60 degrees; Cobra convergence is assessed using signal-to-noise, which peaks at the fourth iteration in engineering-model tests.Source: §3.2 · pp. 8–9, Figure 4 · p. 9

3.3 Data reduction & spectral calibration strategy

  • The two-stage Data Reduction Pipeline turns raw two-dimensional images into calibrated, sky-subtracted spectra, then measures spectral features such as redshifts and emission-line fluxes.Without beam-switching, sky spectra must be modeled from other fibers; plans include diffuse pupil illumination for calibration, calibration for each nightly fiber configuration if needed, and roughly uniform sky-fiber coverage, whose required number remains to be determined during commissioning.Source: §3.3 · pp. 9–10

3.4 Data quality assessment and assurance for long-term survey processing

  • Survey Planning and Tracking software supports object-by-object quality assessment, allowing fibers to be reassigned when an object meets its success criteria.The team is also discussing rapid on-site reduction and assessment, using sky-fiber analysis to estimate noise and wavelength-dependent limiting fluxes and update subsequent field definitions and configurations.Source: §3.4 · p. 10

4. RECENT DEVELOPMENTS

The international collaboration is advancing PFS through subsystem reviews, production, integration, and instrument characterization. Work includes addressing component and facility challenges while developing commissioning plans and software tools.

4.1 The collaboration

  • A consortium of Chinese institutes joined the PFS collaboration as a full member in December 2015, and the project continued seeking additional partners.The consortium included 11 senior scientists.Source: §4.1 · p. 10

4.2 The instrument development

  • Critical design reviews for most subsystems were completed, and construction, integration, and testing were underway at subsystem level.Reviews cited include the spectrograph system, PFI, fiber positioner system, and metrology camera; Cobra mass production had begun, with about 500 units delivered by June 2016 and life tests progressing after corrective action for a manufacturing flaw.Source: §4.2 · pp. 11, 13
  • PFS spectrographs required IR4 floor restructuring for their weight and distribution, while fiber-system integration and testing proceeded with the telescope cable still in prototype.The existing floor was considered unsuitable for the PFS spectrograph system, and logistics for the restructured floor and clean room were under discussion.Source: §4.2 · p. 13
  • Dichroic products meeting the required sharp transition and high reflectance and transmittance had started being delivered.The target transition is about 20 nm, with reflectance of at least 95% and transmittance of at least 90%.Source: §4.2 · p. 13

4.3 Instrument characterization

  • Commissioning plans divide work between observatory subsystem reintegration and telescope engineering observations, while simulators support sensitivity studies, survey planning, and 2D-DRP development.The team is developing routines to align PFI with the wide-field corrector using acquisition-and-guide camera images, and simulated raw images and spectra are being used to validate the 2D pipeline.Source: §4.3 · pp. 13–14, Figure 10 · p. 15

5. SUMMARY AND FUTURE PERSPECTIVES

PFS combines a wide field with extensive fiber multiplexing to obtain optical and near-infrared spectra simultaneously. The project anticipated science operations and a five-year survey from mid-to-late 2019, with PFS and HSC serving as strategic Subaru survey instruments alongside other facilities.

5. SUMMARY AND FUTURE PERSPECTIVES

  • PFS is designed for 2394 reconfigurable fibers across a 1.3-degree field, simultaneously covering 380–1260 nm in one exposure.The project describes PFS as a very wide-field, massively multiplexed optical and near-infrared spectrograph.Source: §5 · p. 15
  • The project expected science operations and a five-year PFS SSP survey to begin in mid-to-late 2019 after subsystem integration and engineering observations.The stated plan anticipated subsystem delivery to the telescope site in 2017–2018 and about 1.5 years of engineering observations.Source: §5 · p. 15
  • PFS and HSC are described as strategic Subaru instruments through the 2020s and 2030s, with science opportunities through synergies with TMT, LSST, Euclid, and WFIRST.Source: §5 · p. 15
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