The MaNGA Integral Field Unit Fiber Feed System for the Sloan 2.5 m Telescope

Abstract: We describe the design, manufacture, and performance of bare-fiber integral field units (IFUs) for the SDSS-IV survey MaNGA (Mapping Nearby Galaxies at APO) on the the Sloan 2.5 m telescope at Apache Point Observatory (APO). MaNGA is a luminosity-selected integral-field spectroscopic survey of 10,000 local galaxies covering 360-1030 nm at R ~ 2200. The IFUs have hexagonal dense packing of fibers with packing regularity of 3 um (RMS), and throughput of 96+/-0.5% from 350 nm to 1 um in the lab. Their sizes range from 19 to 127 fibers (3-7 hexagonal layers) using Polymicro FBP 120:132:150 um core:clad:buffer fibers to reach a fill fraction of 56%. High throughput (and low focal-ratio degradation) is achieved by maintaining the fiber cladding and buffer intact, ensuring excellent surface polish, and applying a multi-layer AR coating of the input and output surfaces. In operations on-sky, the IFUs show only an additional 2.3% FRD-related variability in throughput despite repeated mechanical stressing during plate plugging (however other losses are present). The IFUs achieve on-sky throughput 5% above the single-fiber feeds used in SDSS-III/BOSS, attributable to equivalent performance compared to single fibers and additional gains from the AR coating. The manufacturing process is geared toward mass-production of high-multiplex systems. The low-stress process involves a precision ferrule with hexagonal inner shape designed to lead inserted fibers to settle in a dense hexagonal pattern. The ferrule inner diameter is tapered at progressively shallower angles toward its tip and the final 2 mm are straight and only a few um larger than necessary to hold the desired number of fibers. This process scales to accommodate other fiber sizes and to IFUs with substantially larger fiber count. (Abridged)

• The paper demonstrates the novel integration of dense-packed hexagonal IFUs with precision-engineered tapered ferrules achieving sub-micron alignment.
• The research employs Polymicro FBP fibers with a 120 μm core and reduced cladding to enhance spatial resolution and achieve a 56% fill factor.
• In-situ tests show the IFU system reaching 96% laboratory throughput and a 5% improvement over previous single-fiber systems due to advanced AR coatings.

Overview of the MaNGA Integral Field Unit Fiber Feed System for the Sloan 2.5 m Telescope

The research paper presents a comprehensive analysis of the design, manufacturing, and performance of the Integral Field Units (IFUs) implemented for the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, a prominent part of the Sloan Digital Sky Survey IV (SDSS-IV). The principal objective of the MaNGA IFU development project is to conduct an unbiased spectroscopic survey of approximately 10,000 local galaxies. The primary technical innovation is the integration of dense-packing hexagonal IFUs with low focal-ratio degradation and high throughput, designed to operate within the existing SDSS infrastructure at the 2.5 m telescope at Apache Point Observatory (APO).

Design and Manufacturing

The MaNGA IFUs employ Polymicro FBP optical fibers featuring a core size of 120 μm, optimized to balance spatial resolution with target density. This core size allows for efficient capturing of light with minimal under-sampling under typical seeing conditions at APO. The fibers utilize a reduced buffer and cladding thickness to maximize packing density, achieving a fill factor of about 56% and enabling integral spatial coverage of galaxies up to 1.5 to 2.5 half-light radii.

Critically, the IFUs consist of a regular hexagonal arrangement of fibers, partially due to the adoption of a novel tapered ferrule design. This design utilizes electro-magnetic discharge machining (EDM) capable of achieving sub-micron precision, guiding the fibers into a regular hexagonal pattern to better than 3 μm RMS. The ferrule accommodates multiple sizes of IFUs, ranging from 19 to 127 fibers, depending on the apparent target galaxy size, leading to six IFU sizes overall.

Performance and Throughput

The on-sky performance of these fiber systems indicates an enhanced throughput of about 96% between 350 nm and 1 μm in laboratory conditions, with a minor 2.3% additional variability due to focal-ratio degradation-related stress during operations. In-situ operations report a throughput level approximately 5% better than previously used single-fiber systems in SDSS-III/BOSS, largely due to the new anti-reflection (AR) coatings on fiber terminations, which significantly reduce Fresnel losses.

Implications and Future Work

These developments in IFU technology provide compelling pathways for future adaptation, potentially accommodating larger IFUs or enhancing fill factors through further buffer and cladding reduction or lenslet integration. The work also underscores the potential to fine-tune manufacturing processes for mass production, emphasizing the viability of precision techniques like EDM in large-scale astronomical instrumentation.

Additionally, the research highlights the importance of automated testing for quality control, enabling systematic characterization of throughput and metrology for application in ongoing and future wide-field spectroscopic surveys. These advances reinforce the capability of the MaNGA survey to yield comprehensive data on the kinematics and composition of nearby galaxies, serving as a benchmark for similar projects.

In summarizing the exponential gains in fiber array precision and the resultant data fidelity, this paper delivers an account of significant progress in the utilization and deployment of IFUs in astronomical research, paving the way for more robust observational capabilities in the study of galaxy evolution.