The Multi-Object Double Spectrographs for the Large Binocular Telescope

The Multi-Object Double Spectrographs
for the Large Binocular Telescope

MODS1 and Team MODS in the Instrument Assembly Lab - 2010 Feb 10

MODS1 and MODS2 are a pair of matched low- to medium-resolution Multi-Object Double CCD Spectrographs/Imagers designed and built by OSU for the twin 8.4-meter diameter mirror Large Binocular Telescope on Mt. Graham in southeastern Arizona. We are building two identical spectrographs, one for each of the direct f/15 Gregorian foci of the LBT. The two MODS will work in tandem to exploit the full 11.8-meter effective aperture of the twin LBT mirrors.

Each MODS is a seeing-limited spectrograph and imager working in the 320-1100nm wavelength range with a 6x6-arcminute field of view. Gratings provide a spectral resolution of R~2000 and double-pass prisms provide a low-resolution (R=500-150) faint-object mode. Multi-object spectroscopy is accomplished using laser-machined focal-plane slit masks fed into the beam from a 24-position mask cassette. A beam selector below the slit carries a dichroic that splits the incoming beam into separate red- and blue-optimized channels at a wavelength of 565nm. Each spectrograph channel has separately optimized collimators, dispersers, cameras, and detector, allowing simultaneous operation across the entire CCD band. The beam selector can also direct light into the red or blue channels alone, providing blue-/red-only modes to extend wavelength coverage into the dichroic cross-over for one channel. The MODS science detetors are 3Kx8K monolithic E2V CCDs; blue-coated standard silicon on the blue channel and extended-red coated 40-micron deep depletion silicon on the red channel.

MODS1 passed a laboratory acceptance review in April 2010 and was shipped to the LBT the following May where it was reassembled and tested in the mountain instrument lab. It was successfully installed at the LBT left direct Gregorian focal station on 2010 August 31. Technical and science commissioning ran from late September 2010 through May 2011. MODS1 will become available for regular observing during the 2011B observing semester starting in September 2011. The MODS2 structure is being assembled and tested at OSU and will follow MODS1 to the telescope in late 2012, with binocular MODS1+2 operations commencing later in 2013.