SparsePak: A Formatted Fiber Field Unit for the WIYN Telescope Bench Spectrograph. I. Design, Construction, and Calibration

We describe the design and construction of a formatted fiber field unit, SparsePak, and characterize its optical and astrometric performance. This array is optimized for spectroscopy of low surface brightness extended sources in the visible and near-infrared. SparsePak contains 82, 4.7" fibers subtending an area of 72''×71'' in the telescope focal plane and feeds the WIYN Bench Spectrograph. Together, these instruments are capable of achieving spectral resolutions of λ/Δλ~20,000 and an area-solid angle product of ~140 arcsec2 m2 per fiber. Laboratory measurements of SparsePak lead to several important conclusions on the design of fiber termination and cable curvature to minimize focal ratio degradation. SparsePak itself has throughput above 80% redward of 5200 Å and 90%-92% in the red. Fed at f/6.3, the cable delivers an output of 90% encircled energy at nearly f/5.2. This has implications for performance gains if the WIYN Bench Spectrograph were to have a faster collimator. Our approach to integral-field spectroscopy yields an instrument that is simple and inexpensive to build, yet yields the highest area-solid angle product per spectrum of any system in existence. An Appendix details the fabrication process in sufficient detail for others to repeat. SparsePak was funded by the National Science Foundation and the University of Wisconsin-Madison Graduate School, and is now publicly available on the WIYN Telescope through the National Optical Astronomical Observatories.