The Giant Magellan Telescope Multi-object Astronomical and Cosmological Spectrograph (GMACS) is a wide field, multi-object, moderate-resolution, optical spectrograph being designed for the Giant Magellan Telescope (GMT). Our goal is to create an instrument capable of spectroscopically observing the faintest possible targets, which are currently known only from imaging observations. High throughput, simultaneous wide wavelength coverage, accurate and precise sky subtraction, moderate resolution, relatively wide field, and substantial multiplexing are crucial design drivers for the instrument.
GMACS has a long history of design studies and active collaboration with GMT partner institutions, in the form of instrument software development by Korean collaborators (S. Pak), mechanical design planning from partners in Brazil (K. Taylor), optical design studies by Carnegie (S. Shectman and R. Bernstein), and project management provided by C. Froning at UT-Austin.
A GMACS community workshop during 2014 March 18-19 at Texas A&M University helped to define the scientific requirements for the instrument. The workshop included 51 participants, representing all of the GMT partners and including representatives from national and international non-partners. The participants provided expertise in a wide range of astrophysical areas through contributed talks and breakout groups focused on (1) stars, star-formation, and planets; (2) resolved Galaxies (including Dwarf Galaxies) and near-field cosmology; and (3) distant galaxies (including reionization and first light science). Past design studies, requirements from GMTO and input from the scientific community have been condensed into key design requirements and formal instrument development has begun. An advantage of the instrument history is that most technical risks to the project are well known and most have well-considered mitigation strategies (i.e. appropriate detectors, optics blanks, lens fabrication vendors, etc. are identified or commercially available).
Click to read the Conceptual Design Report for the previous iteration of GMACS.
The current concept for GMACS includes ≥20 interchangeable multi-object slit masks which are placed at the focal plane of the GMT. After passing through a field lens and collimator lens group, a dichroic splits the beam into “blue” (reflected) and “red” (transmitted) channels split at around 600nm. Exchangeable VPH gratings disperse the light which is then focused onto each red or blue optimized CCD array (8k x 12k pixels) via camera lens groups, again optimized for each wavelength range. The position of the camera optical assembly is controlled with an active flexure control system. As the telescope tracks an object, this system responds to the changing gravity vector, removing flexure induced image motion. GMACS will also contain an internal wavelength calibration system and cameras for target acquisition and mask alignment.
Key design parameters are summarized in the table below.
|Parameter||Requirement / Goal||Comments|
|Field of View||30 arcmin sq. / 50 arcmin sq.|
|Wavelength coverage||350-950nm / 320-1000nm|
|Spectral Resolution||Blue: 1000-6000 Red: 1000-6000||0.7” slit width, full coverage at lower resolutions, wavelength coverage at higher resolutions is sacrificed|
|Image Quality||0.30 / 0.15 arcsec||80% EE|
|Spectral Stability||0.3 / 0.1 spectral resolution elements/hour|
|Grating Exchange||1 / ≥2 gratings||Multiple wavelength regions|
|Slit Mask Exchange||12 / ≥20||Cassette style mask changer|
How GMACS fits into the GMT
GMACS will be mounted in the Gregorian Instrument Rotator as shown in the figure below. The instrument design will also include handling and test carts to facilitate assembly, integration and verification of the instrument, as well as instrument exchange at the telescope.
For more information on GMACS and the GMT, please visit our Publications page.