Dr. Steven S. Vogt
MTHR (Moderate-To-High-Resolution Spectrometer) for TMT
MTHR is an instrument concept that emerged from a feasibility study of an optical /near-infrared (NIR, < 1 micron) high-resolution spectrometer (HROS) for the TMT (30-meter telescope). The goal was to design an efficient spectrometer that delivers high "throughput" (at least 40,000 arcsecs) and wide spectral coverage in a single exposure, thereby allowing the TMT to deliver the full advantage of its large aperture at first-light for high-resolution optical spectroscopy.
MTHR is situated at an f/15 Nasmyth focus, below the Nasmyth platform. MTHR's primary high resolution mode incorporates a UVES-style dual-arm (red and blue), dual-white-pupil cross-dispersed echelle configuration, combined with 1.2-m aperture HIRES-style catadioptric cameras. Each camera feeds a notional 8k x 8k mosaic of 15-?m-pixel CCDs. In its high resolution mode, MTHR provides typical spectral resolutions of 50,000 - 100,000 (unsliced) for point sources, or for a small number of fiber feeds or multi-slits; and resolutions up to 500,000 with image-slicers or adaptive optics (AO). The spectral range is from 300 to 1100nm, with at least 450 nm coverage per observation. The high-resolution mode delivers a throughput of 46,000 arcsecs (about 15% higher than the best current spectrometers) and could be further increased by yet another 20% if desired.
MTHR also includes provision for future incorporation of fiber-fed multi-object modes for intermediate resolutions (R = 2,300-11,000) on up to a few hundred objects simultaneously over the full 20 arc-minute TMT field of view and/or from AO-corrected fields. This multi-object mode could be fed from a wide variety of fiber inputs and fiber-based deployable integral field units (d-IFUs). Though the fiber unit is not likely to be funded at first-light (it requires the equivalent of an entirely separate fiber-positioner instrument and full-field ADC to feed it), the spectrometer design is being carried out so as not to preclude adding such an option later. MTHR's moderate-resolution multi-object mode could, in the future, fill in the large resolution gap between TMT's wide field optical spectrometer (WFOS) and HROS instruments.
The present level of design shows that MTHR would be a very powerful workhorse instrument for TMT, enabling the telescope to achieve and even surpass its full "area-of-aperture" advantage for high-resolution 300-1100 nm spectroscopy, without the need for adaptive optics. Because of its strong design heritage (from the very successful instruments UVES and HIRES), low technical risk, ability to work under most any observing conditions without AO, future upgrade possibilities, flexible de-scope options, and strong science drivers, MTHR seems an ideal candidate for a first-light TMT instrument.