GMACS is a wide field, multi-object, moderate-resolution, optical spectrograph being designed for the Giant Magellan Telescope (GMT) by partners at Texas A & M, Johns Hopkins University, and Carnegie Observatories. GMACS will take advantage of the large aperture of the GMT to observe the faintest possible targets, those that are substantially fainter than the sky. High throughput, simultaneous wide wavelength coverage, accurate and precise sky subtraction, moderate resolution, and wide field (for an extremely large telescope) are the crucial design drivers for the instrument. The range of science projects enabled by the instrument is huge: from mineralogical studies of distant asteroids and KBOs to stellar population studies of high redshift galaxies.

Figure 1: A 3-D view of all four GMACS arms, the “tent” mirror, and the last element of the GMT corrector

Click to read the Conceptual Design Report

Click to use the GMACS exposure time calculator

The conceptual design for GMACS provides complete, simultaneous spectral coverage over the wavelength range from 0.36 to 1.02 μm for hundreds of objects in an 8 x 18 arcmin field of view. The resolution with a 0.7 arcsec slit is 1400 in the blue and 2700 in the red. GMACS incorporates a multi-slit mask to provide the best possible sky subtraction and instrumental throughput. To create the maximum possible field, the optical design makes use of multiple collimators and cameras that would be deployed across the telescope focal plane in a "fly's-eye" approach. In the complete instrument, four collimators, with fields of 8 x 8 arcmin each, would make efficient use of the corrected field, providing a 16 x 16 arcmin overall field of view.

How GMACS fits into the GMT

GMACS Layout
The optics for GMACS will be mounted in three pods to fit in the new radial instrument mounting system. The two pairs of channels will be housed in side pods which will be rigidly attached to the top of the instrument mount via kinematic mounts. The corrector lens, tent mirrors and slit masks will be housed in the center pod which is mounted to a carriage and stored under a side pod. The carriages are generally used to move Gregorian-mounted GMT instruments from their storage areas into the telescope's optical path. GMACS's center carriage will move the pod into the optical path and will also have a lifting mechanism to reposition the pod in the center of the instrument mounting system.

Click on the image below to see a rotating view of GMACS


Optical Design

The GMACS optical design makes extensive use of refractive elements and has a baseline beam diameter of 300mm. Typical optical element sizes are ~400mm. The collimator design, shown below, is an all-refractive design that includes two fused silica elements followed by a BaK2/CaF2 doublet. Four collimators will separate the focal plane into four channels. The four collimated beams are divided into a blue and a red channel by a dichroic element; each of the eight channels are then dispersed by a VPH grating.


For more information on GMACS and the GMT, please visit our Publications page.
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