Prime Focus Spectrograph (PFS) for Subaru Telescope

PFS portal site

   PFS is a fiber fed spectrograph system with a fiber positioner system designed to be mounted at the prime focus of the Subaru telescope on Mauna Kea, and feeding 4 fixed spectrographs mounted near the Nasmyth focus. With 2400 fibers & fiber positioners, PFS allows simultaneous spectral observations of up to 2400 astronomical targets. The left figure is a schematic for the PFS instrument system, showing the subsystem components allocated on Subaru telescope, i.e. the wide-field corrector (WFC), the prime focus instrument (PFI), the fiber cables with fiber connectors, the spectrographs system, and the metrology camera system. 

   PFI is a PFS dedicated instrument which includes system elements that are required to be at the prime focus of the telescope. PFI will be housed within a prime focus unit which then interfaces to the telescope prime focus spider as well as the WFC. The fiber cable, which conducts the 2400 fibers, will be segmented in 3 parts with 2 sets of multi-fiber connectors along the way in the telescope structure, to relay light incident at the prime focus focal plane to the 4 spectrographs that will be located in a spectrograph room adjacent to the telescope, above the Nasmyth focus. The metrology camera system serves to verify the configuration positions of the 2400 scientific fibers on the prime focus focal plane, and will be located at the Cassegrain focus as a Cassegrain instrument for Subaru.

    The technical design of the PFS system is conducted via an elaborative collaboration led by IPMU, with the labor on subsystem components divided to all the institution collaborators, i.e. USP/LNA in Brazil, Caltech/JPL, Princeton & JHU in USA, LAM in France, ASIAA in Taiwan, NAOJ/Subaru, IPMU in Japan. The right figure is a block diagram which summarizes the interaction or interface relation between each components, for better reference/understanding to the following descriptions of the PFS concept. Note that different colors for blocks represent the division of responsibility to the corresponding institutions as shown.
   Besides the new design, the PFS instrumentation utilizes several Subaru provided elements that are already developed for the HyperSuprime Cam (HSC) instrument, including the wide-field corrector (WFC), the POpt2 prime focus unit, the hexapod, and the instrument rotator. The newly designed prime focus instrument (PFI) will be housed in the POpt2 and interfaced via the positioner frame structure to the instrument rotator. The PFS Field Element, located close to the telescope prime focus, is to substitute the filter and dewar window used with HSC, serving to compensate the spherical aberration correction. A microlens is attached to each fiber entrance edge in order to transform the F-ratio to a fainter (or slower) one, which ensures an efficient acceptance of light coming through WFC and the field element and also eases the spectrograph design. Each Science Fiber is fiber-tip-controllable in-plane by a piezo-electric Fiber Positioner. Each fiber tip can be positioned within its 9.5 mm diameter circular patrol region. The patrol regions are in a hexagonal close-packed pattern, with 8 mm separation, and fill a hexagonally shaped 1.3 degree field of view. The overlap between adjacent patrol regions enables 100% sky coverage of this hexagonal field. The hexagonal shape of the field of view allows efficient tiling of the sky for large area surveys. The fibers can be completely reconfigured for a new field in 40 seconds with each fiber tip placed to an accuracy of 5 microns (corresponding to less than 0.054 arc seconds on the sky), which provides good observation efficiency.

   The fibers are about 55 meters in length and are divided into four groups providing 600 inputs to each spectrograph. A key component of the fiber system is Fiber Connectors, which provide flexibility in instrument exchange, as well as testing, and retain the possibility of using the fiber system to feed other instruments. The Spectrographs provide the capability to simultaneously measure 2400 spectra. Each Spectrograph uses an optical system composed of a Schmidt collimator, grating, and vacuum Schmidt camera optics. Each Spectrograph has three independent channels separated by dichroics in the parallel beam and covers a wide wavelength range from 0.38 um to 1.3 um. At the image plane of each optical (blue/red) spectrograph is a Detector System consisting of a pair of close butted 2K x 4K pixel CCDs inside a dewar and associated cooling system. The 600 spectra are dispersed along the columns (4K pixels), so the gap falls between spectra. Each near-infrared arm uses a Detector System consisting of a single 4K x 4K pixel near-infrared detector inside a dewar and cooling system. Both the CCDs and the near-infrared detectors have exceptionally high quantum efficiency, which improves overall instrument efficiency.

  The Acquisition & Guide System is composed of six cameras located on the instrument optics bench in the PFI. Once calibrated to the fiber optic focal plane the A&G System will provide feedback to update the telescope’s pointing and field rotation by tracking on guide stars within the cameras’ field of view. The Metrology Camera is mounted in a Cassegrain container "directly" looking up at back-illuminated fiber tips, viewing through WFC. The metrology camera determines the location of the fiber tips, allowing accurate positioning of the fibers on the selected science targets. Calibration of the image distortion for metrology camera from the overall optics (mainly caused by the WFC) is achieved by having several back-illuminated fixed Fiducial Fibers whose positions are known accurately. The centroid of each fiber image is calculated to determine each science fiber position with respect to the Fiducial fiber locations. This coordinate system is accurately referenced to the Acquisition & Guide camera allowing accurate placement of astronomical targets on the science fibers. The science fibers are back-illuminated by sources located inside the four spectrographs.

   The PFS OBCP is a dedicated system software which provides the capability to manage an observation’s preparation and execution. It will be compatible to and commanded under the current Subaru observation management software (Gen2). Preparation tasks include parsing large surveys into a series of individual observations, matching fibers to designated targets and selecting appropriate guide stars, predicting observing times, and describing observations in a Subaru-format file (OPE file). Execution of an observation begins with moving the telescope to the desired field center. While the telescope is slewing, the Fiber Positioning System executes commands from the Control System to simultaneously move each of the 2400 fibers to their required positions, with the real positions verified by the Metrology System. With several iterations, the position error of each Science Fiber can be effectively minimized. In parallel with this process, the Acquisition & Guide System refines the pointing of the telescope and angle of instrument rotator by providing feedback to the telescope Control S/W (software) during the observation. The hexapod is commanded to the appropriate position to maintain the wide-field corrector alignment in the presence of flexure in the POpt2 structure. Once positioning is complete, the Detector System collects data. A preliminary Data Reduction System produces quick-look information on the completed observation so that data quality and survey progress can be monitored. The collected raw data will be transferred to and archived in the STARS data archive system, which then provides data retrieval for further scientific Data Reduction and analysis.
   Spectral Calibration is a crucial process for a multifiber spectrograph, which will be an especially demanding task for PFS since the sky is much brighter than most of the PFS interested objects (click here for more information on PFS instrument performance). The requirements are driven primarily by the difficulties associated with sky subtraction and the necessity to do sky subtraction with exquisite accuracy. For PFS the required accuracy in sky subtraction will be 0.5 % in intensity and 0.01 pixel in wavelength. With such demands, the general beam-switching method would not work since exposures of order 1000 sec are necessary to achieve photon-noise limited signals, but the sky in the red and IR changes dramatically on timescales like this. The strategy for PFS sky subtraction will be to reserve some fraction of the 2400 fibers for sky fibers, to get an averaged high-quality sky spectrum for all the PFS object spectra. Since the focal ratio degradation in each fiber, for each fiber positioning configuration in each telescope elevation angle, is unpredictable but believed to be crucial for PFS, a Spectral Calibration Source which can be quickly deployed between each observation exposure is required. In preliminary design phase, such Spectral Calibration Source is designed to be a flat fielding screen beneath the primary mirror cover, which can be quickly deployed  to cover the primary mirror for illumination.

Information about each component: