Instruments
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GMACS
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
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
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.
