MaNGA Fiber Feed
MaNGA is a spectroscopic survey of nearby galaxies that uses the two BOSS spectrographs (also used by eBOSS, TDSS, and SPIDERS in SDSS-IV) in a new and different way. The survey is described in the overview paper by Bundy et al. (2015).
MaNGA groups the individual optical fibers into 29 fiber bundles of different sizes, using custom-designed ferrules (these are metal fiber-optic terminators) to closely pack the bundles of circular fibers to form hexagonally shaped integral field units (or IFUs). The IFUs are used to spectroscopically map galaxies over the wavelength range of 360 nm to 1040 nm at a resolution (λ/δλ) of roughly 2000. The MaNGA fiber feed instrument allows observation of 17 galaxies at a time. Additional fibers and mini-bundles are used for calibration (sky fibers and spectrphotometric calibration on standard stars). The detailed allocation of IFUs and other fibers between galaxy targets and calibration, a brief summary of the BOSS spectrographs, and the fiber cartridges that connect the telescope to the spectrographs, are described below.
Additional information on the MaNGA IFUs, their fabrication and performance can be found in the technical paper by Drory et al. (2015).
The fibers have 120 micron core diameters, which map to 2″ on sky. Including cladding and buffer the surround the optically active core, the close-packed fiber IFUs have a 54% live-core fill factor. In total, MaNGA uses 1423 fibers, an increase from the 1000 fibers used in other surveys with the BOSS spectrograph. MaNGA can afford to pack the fibers more closely together in the spectrograph slit to gain this 42% increase in survey efficiency because adjacent fibers in the slit sample comparable regions on the sky.
Seventeen IFUs, ranging in size from 19 to 127 fibers (diameters between 12 and 32 arcsec on the sky), are designed to be placed on a different galaxy within the same, 3 degree diameter field of view (7 deg2) of the Sloan 2.5m telescope, thus enabling detailed observations of 17 galaxies simultaneously for MaNGA. In detail, this IFU complement contains 2×19; 4×37; 4×61; 2×91; 5×127 IFUs. These IFUs are split to feed the two BOSS spectrographs.
The remaining 12 IFUs are “mini-bundles” of 7 fibers there are placed on standard stars across the field of view for spectrophotometric calibration. Six mini-bundles feed each of two BOSS spectrographs so the spectrographs can be independently calibrated.
Other calibration fibers
In addition to the above IFUs, each bundle has associated with it some number of individual fibers (between 2 and 8) that are placed near the primary target (nominally centered on the IFU) to sample the sky foreground local to the object on the sky. These fibers are inter-mingled with the IFU fibers along the spectrographs’ slits to also sample variations of the spectrograph response. In detail, the fibers are terminated in blocks of between 21 and 39 fibers, with the sky fibers placed at either end of each block. There are a total of 92 sky fibers, split between the two BOSS spectrographs.
MaNGA Fiber Feed Fast Facts
- Buffered fibers with 120 micron (2″) core diameters
- Close-packed hexagonal fibers IFUs, 54% live-core fill factor
- IFU size from 19 to 127 fibers, diameters from 12″ to 32″
- IFU complement per plate: 2×19; 4×37; 4×61; 2×91; 5×127
- 92 IFU-associated sky fibers
- 12 7-fiber “mini-bundles” for spectrophotometric calibration
- Total number of fibers: 1423
The BOSS spectrographs are two identical instruments are rebuilt from the original SDSS spectrographs, which were used for the SDSS Legacy and SEGUE surveys. The spectrographs are mounted onto the telescope and fiber-fed from a cartridge that couples the spectrograph collimator to the telescope focal surface. The collimator is a spherical mirror with a 1264 mm radius of curvature. Each spectrograph has two cameras, one red and one blue, with a dichroic splitting the light at roughly 600 nm and a full wavelength range from 360 to 1040 nm. The light feeding the cameras are dispersed after the dichroic with high-efficiency, prism-immersed volume-phase holographic gratings (VPHg) employed in fixed, and nearly zero-deviation configurations. The blue camera VPHg has 520 l/mm index-modulation density, while the red camera has 400 l/mm. The camera optics are all refractive, each containing 8 elements in three groups. The CCDs have 15 micron pixels. The blue camera CCDs are 4Kx4K e2V devices, while the red camera CCDs are 4Kx4K fully-depleted LBNL devices. The geometric demagnification from the collimator-camera focal-length ratio is 2.67. The main BOSS spectrograph page provides some images of the spectrograph optical bench, the dichroic and the prism-immersed VPH gratings. Additional information on the BOSS spectrographs can be found in the technical paper by Smee et al. 2013.
Performance: With 2 arcsec diameter fibers (120 micron physical size) used in MaNGA and the other SDSS-IV surveys, the spectral resolution (λ/δλ) in the blue channel ranges from 1560-2270, and from 1850-2650 in the red channel, in both cases increasing nearly linearly toward longer wavelengths. Spectrograph throughput, from the top of the atmosphere for point sources in 1 arcsec (FWHM) seeing peaks at 31% and is above 27% between 425-900 nm.
Like the other SDSS surveys, MaNGA couples the fibers from the telescope to the spectrograph via a cartridge that attaches to the telescope at the Cassegrain focus, adjacent to the spectrographs. The main BOSS spectrograph page provides a schematic of the spectrograph and cartridge layout and an image of a cartridge; additional illustrations are found in Figures 1-3 of Smee et al. (2013). Each cartridge contains an aluminum plug plate, bent to match the focal-surface of the telescope, and drilled with holes aligned with targets on sky, one plate for every field or field configuration. The fibers (here IFUs and individual sky fibers) are terminated into their own metal ferrules, and are plugged into these plates. The down-stream end of the fibers are arranged into two long slits comprised of a series of blocks, each feeding one the spectrograph collimators. For MaNGA there are six complete sets of IFUs and calibration fibers, one for each of 6 cartridges. These cartridges are plugged by the SDSS Fibre Optic Technicians during the day, and loaded onto the telescope by SDSS Observers at night as field availability, weather, lunar conditions and observing plans determine.