Cross dispersion and an integral field unit for Hectochelle

Abstract

We describe our plans to add cross-dispersion and an integral field unit to the Hectochelle spectrograph, a multiobject, fiber-fed echelle spectrograph for the converted MMT. Hectochelle was originally designed without cross-dispersion to be used in a single order or overlapping orders selected by interference filters. The addition of cross-dispersion allows us to trae off multiplex advantage for spectral coverage. Our cross-disperser uses an unusual segmented, zero-deviation prism that is very compact, allowing it to fit into the existing instrument without modification. The planned integral field unit can be used with either Hectochelle or the moderate-dispersion Hectospec bench spectrograph. Both spectrographs were originally designed to be fiber-fed with a robotic fiber positioner as a front end, so adding an integral field capability is a natural enhancement. The integral field unit will use smaller diameter fibers than the robotic fiber positioner (subtending 0".6 vs. 1".5), so that both spectrographs will achieve higher spectral resolution in integral field mode. With the integral field unit Hectochelle will reach a two pixel resolution, R approximately 70,000, and Hectospec will reach R approximately 2000 with its 270 line mm^-1 grating.