SparsePak: A Formatted Fiber Field Unit for The WIYN Telescope Bench Spectrograph. II. On‐Sky Performance

We present a performance analysis of SparsePak and the WIYN BenchSpectrograph for precision studies of stellar and ionized gas kinematics ofexternal galaxies. We focus on spectrograph configurations with echelle andlow-order gratings yielding spectral resolutions of ~10000 between 500-900nm.These configurations are of general relevance to the spectrograph performance.Benchmarks include spectral resolution, sampling, vignetting, scattered light,and an estimate of the system absolute throughput. Comparisons are made toother, existing, fiber feeds on the WIYN Bench Spectrograph. Vignetting andrelative throughput are found to agree with a geometric model of the opticalsystem. An aperture-correction protocol for spectrophotometric standard-starcalibrations has been established using independent WIYN imaging data and theunique capabilities of the SparsePak fiber array. The WIYNpoint-spread-function is well-fit by a Moffat profile with a constant power-lawouter slope of index -4.4. We use SparsePak commissioning data to debunk along-standing myth concerning sky-subtraction with fibers: By properly treatingthe multi-fiber data as a ``long-slit'' it is possible to achieve precision skysubtraction with a signal-to-noise performance as good or better thanconventional long-slit spectroscopy. No beam-switching is required, and hencethe method is efficient. Finally, we give several examples of sciencemeasurements which SparsePak now makes routine. These include H$\alpha$velocity fields of low surface-brightness disks, gas and stellarvelocity-fields of nearly face-on disks, and stellar absorption-line profilesof galaxy disks at spectral resolutions of ~24,000.