An Integral Field Unit for the Binospec Spectrograph

Binospec is a wide-field optical (360-1000 nm) spectrograph commissioned at the MMT 6.5 m telescope in 2017. In direct imaging mode Binospec addresses twin 8 ′ (wide) by 15 ′ (slit length) fields of view. We describe an optical fiber based integral field unit (IFU) that remaps a 12″ × 16″ contiguous region onto two pseudo slits, one in each Binospec channel. The IFU, commissioned in 2023, fits into the space of a standard slit mask frame and can be deployed as desired in a mixed program of slit masks, long slits, and IFU observations. The IFU fibers are illuminated by a hexagonal lenslet array with a 0 . ″ 6 pitch. A separate bundle of sky fibers consists of close-packed bare fibers arranged within an 11 . ″ 8 circular aperture. The 640 IFU fibers and 80 sky fibers have a core diameter of 150 μm, corresponding to 0 . ″ 90. Three gratings are available, 270 lpm with R ~ 2000, 600 lpm with R ~ 5300, and 1000 lpm with R ~ 6000.