IFUM: integral field units for Magellan

IFUM (Integral Field Units for Magellan) consists of three IFUs at the Nasmyth-East focus of the Magellan/Clay telescope. Each IFU consists of a Barlow lens, a lenslet array, a set of fiber assemblies that feed a pair of fiber shoes that allow the fibers to be used with the twin spectrographs of M2FS (the `Michigan/Magellan Fiber System'). The IFUs are optimized for excellent seeing conditions (the `HR' IFU with 864 fibers with 75µm diameter cores), standard seeing conditions (`STD', 552 fibers, 150µm) and extended (low surface brightness) targets (`LSB', 360 fibers, 260µm). The Barlow lenses magnify the field at the lenslet arrays which deliver to each fiber images the telescope entrance pupil (the primary mirror) at f/3.5—as defined by the marginal rays of the pupil image—that becomes an f/3.3 output beam at the spectrographs. The fiber assemblies consist of `sky-end' fiber mounts that are aligned with the lenslet arrays to high precision and `spectrograph-end' fiber mounts that maintain the fibers at the focal surfaces of the spectrographs within the fiber shoes. The fibers can be aligned within the shoes with slits that deliver spectral resolutions ranging from ℛ = 1200 to 40000. Movable focal-plane masks on each IFU enable spectroscopy of faint sources near much brighter point sources (e.g. AGN or young stellar objects). IFUM can be used with a deployable ADC on the Magellan/Clay guider; a separate deployable mirror on the guider allows onboard calibration light sources to illuminate any of the IFUM IFUs. Initial commissioning data are presented.