An Integral Field Unit for the Binospec Spectrograph

Abstract

Binospec is a wide-field optical (360 to 1000 nm) spectrograph commissioned at the MMT 6.5m telescope in 2017. In direct imaging mode Binospec addresses twin 8${}^{\prime }$ (wide) by 15${}^{\prime }$ (slit length) fields of view. We describe an optical fiber based integral field unit (IFU) that remaps a 12${}^{\prime \prime }$ x 16${}^{\prime \prime }$ 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${}^{\prime \prime }$ pitch. A separate bundle of sky fibers consists of close-packed bare fibers arranged within an 11.8${}^{\prime \prime }$ circular aperture. The 640 IFU fibers and 80 sky fibers have a core diameter of 150$\hat{I}¼$m, corresponding to 0.90${}^{\prime \prime }$. Three gratings are available, 270lpm with R$\sim$2000, 600lpm with R$\sim$5300, and 1000 lpm with R$\sim$6000.

Accepted for publication in PASP, 12 pages, 11 figures, 1 table