GMACS

The Giant Magellan Telescope Multi-object Astronomical and Cosmological Spectrograph (GMACS) is a wide field, multi-object, moderate-resolution, optical spectrograph designed for the Giant Magellan Telescope (GMT). Currently it is being developed by the Harvard and Smithsonian Center for Astrophysics and at the Steiner Institute in Sao Paulo Brazil. Conceptual design was done here at the Texas A&M University Munnerlyn Astronomical Instrumentation Laboratory.

When designing GMACS, our goal was 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.

Design

split-labeled — Our GMACS concept. After the field lens, the light is spit into red and blue channels. The VPH gratings and camera assemblies can rotate independently of each other (up to 90° camera-collimator angle). Slit masks or the MANIFEST fiber interface are exchanged using a cassette system at the top of the instrument (see Fig. 2).

Our 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, a dichroic splits the beam into “blue” (reflected) and “red” (transmitted) channels split at 558nm. Each channel has a collimating lens group followed by exchangeable VPH gratings. The light is then focused onto each red or blue optimized CCD array (8k x 12k pixels) via f/2.2 cameras. The positions of the optical assemblies are controlled with an active flexure compensation system. As the telescope tracks an object, this system responds to the changing gravity vector, removing flexure induced image motion. GMACS will also contain a wavelength calibration system and cameras for target acquisition and mask alignment.

smem-labeled — Concept for the slit mask exchange mechanism. Masks are loaded into a magazine and can be inserted into the focal plane via the slit mask elevator. Surrounding the focal plane are multiple guide, acquisition, and focus cameras that are used to align target objecs with their respective slits as well as provide focus offsets to the GMT Acquisition, Guiding, and Wavefront Sensing system (AGWS). On the opposite side of the focal plane, the MANIFEST fiber positioner interface is shown. It can be inserted into the focal plane using the same rail system as the slit masks.

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, tied to MANIFEST fiber diameter
Spectral Stability	0.1/0.1	spectral resolution elements per 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 is 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.

gmacs_in_telescope_v2 — GMACS mounted in the Gregorian Instrument Rotator.

History

Due to a generous contribution from George P. Mitchell ‘40 in 2004, Texas A&M was named a founding partner in the Giant Magellan Telescope. The GMT, to be constructed in northern Chile, will be the largest telescope on Earth and produce images up to ten times clearer than the Hubble Space Telescope.

In 2011, Mr. Mitchell and the Mitchell Foundation committed a landmark gift of $25 million to the project, $12.5 million of which is pledged on behalf of Texas A&M. This gift brings Mitchell’s total commitment to the GMT project on behalf of the university to over $21 million, furthering his goal to keep TAMU Physics and Astronomy on the forefront of scientific innovation and discovery.

In 2015, the Texas A&M University System Board of Regents continued Mitchell’s legacy by reaffirming Texas A&M University’s support for GMT and pledging to fund Texas A&M’s participation in the project as a significant partner.

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.

Publications

GMACS: a wide-field, moderate-resolution spectrograph for the Giant Magellan Telescope
DePoy, Darren L.; Schmidt, Luke M.; Ribeiro, Rafael; Taylor, Keith; Jones, Damien; Prochaska, Travis; Marshall, Jennifer L.; Cook, Erika; Froning, Cynthia; Ji, Tae-Geun; Lee, Hye-In; Faes, D. M.; Souza, Aline; Bortoletto, D.; Mendes de Oliveira, Claudia; Pak, Soojong; and Papovich, Casey. In Proc. SPIE, 10702–69, 2018.
(PDF)

The optical design for the Giant Magellan Telescope Multi-object Astronomical and Cosmological Spectrograph (GMACS)
Ribeiro, Rafael A. S.; Schmidt, Luke M.; Jones, Damien; Taylor, Keith; Prochaska, Travis; Cook, Erika; DePoy, Darren L.; Faes, Daniel M.; Froning, Cynthia; Ji, Tae-Geun; Lee, Hye-In; Marshall, Jennifer L.; Mendes de Oliveira, Claudia; Pak, Soojong; Papovich, Casey; and Souza, Aline. In Proc. SPIE, 10702–340, 2018.
(PDF) (Poster)

The optomechanical design of the Giant Magellan Telescope Multi-object Astronomical and Cosmological Spectrograph (GMACS)
Prochaska, Travis; Albert, Carson; Beck, James; Cook, Erika; DePoy, Darren L.; Faes, Daniel M.; Froning, Cynthia; Grant, Walter; Gutierrez, Isaac; Ji, Tae-Geun; Jones, Damien; Lee, Hye-In; Marshall, Jennifer L.; Mendes de Oliveira, Claudia; Pak, Soojong; Papovich, Casey; Ribeiro, Rafael; Sauseda, Marcus; Schmidt, Luke M.; Souza, Aline; Taylor, Keith; and Williams, Darius. In Proc. SPIE, 10702–364, 2018.
(PDF) (Poster)

Electronics prototypes for the Giant Magellan Telescope Multi-object Astronomical and Cosmological Spectrograph (GMACS)
Cook, Erika; Prochaska, Travis; Lee, Hye-In; Ji, Tae-Geun; Pak, Soojong; DePoy, Darren L.; Marshall, Jennifer L.; Schmidt, Luke M.; Faes, Daniel M.; Froning, Cynthia; Jones, Damien; Mendes de Oliveira, Claudia; Papovich, Casey; Ribeiro, Rafael A. S.; Souza, Aline; and Taylor, Keith. In Proc. SPIE, 10702–365, 2018.
(PDF) (Poster)

Systems engineering applied to ELT instrumentation: The GMACS case
Faes, Daniel M.; Souza, Aline; Froning, Cynthia; Schmidt, Luke M.; Bortoletto, D.; Cook, Erika; DePoy, Darren L., Ji, Tae-Geun; Jones, Damien; Lee, Hye-In; Marshall, Jennifer L.; Mendes de Oliveira, Claudia; Pak, Soojong; Papovich, Casey; Prochaska, Travis; Ribeiro, Rafael A. S.; and Taylor, Keith. In ArXiv e-prints, arXiv:1806.06390 2018.
(PDF) (Poster)

Optical design for the Giant Magellan Telescope Multi-object Astronomical and Cosmological Spectrograph (GMACS): design methodology, issues, and trade-offs
Ribeiro, Rafael A.S.; Jones, Damien; Schmidt, Luke M.; Taylor, Keith; Cook, Erika; DePoy, Darren L.; Faes, Daniel; Froning, Cynthia; Ji, Tae-Geun; Lee, Hye-In; Marshall, Jennifer L.; Mendes de Oliveira, Claudia; Pak, Soojong; Papovich, Casey; Prochaska, Travis; and Souza, Aline. In Proc.SPIE, 10690–14, 2018.
(PDF) (Poster)

Optomechanical Design Concept for the Giant Magellan Telescope Multi-object Astronomical and Cosmological Spectrograph (GMACS)
Travis Prochaska, Marcus Sauseda, James Beck, Luke Schmidt, Erika Cook, Darren L. DePoy, Jennifer L. Marshall, Rafael Ribeiro, Keith Taylor, Damien Jones, Cynthia Froning, Soojong Pak, Claudia Mendes de Oliveira, Casey Papovich, Tae-Geun Ji, Hye-In Lee 2016, Proc. SPIE, Vol. 9908, 375 doi:10.1117/12.2233530
(PDF) (Online) (Poster)

Optical design concept for the Giant Magellan Telescope Multi-object Astronomical and Cosmological Spectrograph (GMACS)
Schmidt, L., Ribeiro, R., Taylor, K., Jones, D., Prochaska, T., DePoy, D.L., Marshall, J.L., Cook, E., Froning, Ji, T., Lee, H., C., Mendes de Oliveira, C., Pak, S., Papovich, C. 2016, Proc. SPIE, Vol. 9908, 376 doi:10.1117/12.2233543
(PDF) (Online) (Poster)

GMACS: A Wide File, Multi-Object, Moderate-Resolution, Optical Spectograph for the Giant Magellan Telescope
DePoy, D.L., Marshall, J.L., Papovich, C., Li, T. 2014, Proc. SPIE, Vol. 9147, 72
(PDF)

GMACS: a Wide Field, Multi-Object, Moderate-Resolution, Optical Spectrograph for the Giant Magellan Telescope
D. L. DePoy, R. Allen, R. Barkhouser, E. Boster, D. Carona, A. Harding, R. Hammond, J. L. Marshall, J. Orndorff, C. Papovich, K. Prochaska, T. Prochaska, J. P. Rheault, S. Smee, S. Shectman, S. Villanueva, Jr.; 2012 SPIE.8446E..1ND
(PDF) (Online)

Optomechanical design concept for GMACS, a wide-field, multiobject, moderate resolution optical spectrograph for the Giant Magellan Telescope (GMT)
Stephen A. Smee, Travis Prochaska, Stephen A. Shectman, Randolph P. Hammond, Robert H. Barkhouser, D. L. DePoy, J. L. Marshall; 2012SPIE.8446E..7NS
(PDF) (Online)

Spectrally-quantified chemical reactivity of optical fluids and materials in the GMACS spectrograph for GMT
Tyler W. Behm, J. P. Rheault, J. L. Marshall, D. L. DePoy; 2012SPIE.8450E..42B
(PDF) (Online) (Poster)

The GMACS Spectrograph for the GMT
J.L. Marshall, D.L. DePoy, S.A. Shectman, C. Papovich, S.A. Smee, R.H. Barkhouser, T.M. Prochaska, R.P. Hammond, 2012 AAS
(Poster)

The GMACS Spectrograph for the GMT
Marshall, Jennifer L.; DePoy, D. L.; Shectman, S. A.; Smee, S. A.; Barkhouser, R. H.; Prochaska, T. M.; Hill, G. J. 2011 AAS 21743312
(Poster)