GMACS will be a workhorse instrument for the Giant Magellan Telescope. Broad wavelength coverage (320 nm - 1000 nm), large field (7.4' diameter FoV, 20' diameter FoV accessible when used with MANIFEST), high throughput, and multiple grating choices provide many different observational modes.


Articulating cameras and Volume Phase Holograpic (VPH) gratings are two key features of the GMACS optomechanical design. Each grating requires a specific camera-collimator angle to achieve peak efficiency at the desired central wavelength. As a result, the articulation mechanism is infinitely adjustable between 0° (imaging) and 90° (highest resolution mode).

The grating selections for first light are not yet finalized, so the figure below allows you to choose between four gratings chosen to illustrate the range of potential configurations. Additional gratings can be manufactured for specific science cases, as long as a grating requires a camera-collimator angle between 0° and 90° it is compatible with GMACS.

The default slit width for the GMACS natural seeing mode is 0.7″. When combined with the GMT fiber positioner, MANIFEST, an effective slit width of 0.35″ or 0.25″ may be selected to demonstrate the increase in resolution. The interface with MANIFEST has not been finalized, so these values are representative. A resolution element is sampled by ~12 (0.7″ slit), ~6 (0.35″ fiber), or ~4 (0.25″ fiber) pixels. Future GLAO modes would also allow GMACS to operate with a narrower slit without MANIFEST.

Current Plot:


Wavelength Coverage and Field of View

There is a trade-off between wavelength coverage and the position of a target in the field of view. As resolutions increase, the single shot spectral bandwidth decreases. Especially in the higher resolution modes, wavelength coverage on either side of the chosen central wavelength is most balanced for objects at the center of the field. Objects displaced from the center of the field in the dispersion direction (L-R in the figure to the left) will show a shift in the start and end wavelengths to either redder or bluer wavelengths, depending on the direction moved from the center of the field. Full wavelength coverage is still possible in higher resolution modes by adjusting the camera collimator angle and recording multiple spectra that can then be stitched together.

The GMACS blue channel is optimized for 320-600 nm, the red channel for 500-1000 nm with the dichroic split at 558 nm.

By selecting the grating and desired bandwidth from the drop down menus you can see what fraction of the selected bandpass is lost as a function of the target position in the spectrograph field of view.

MANIFEST does not change the wavelength coverage, as the higher resolutions are not as a result of higher dispersion, but a reduction in the number of pixels in a resolution element.

Current Selection:

Grating:       Center λ: nm

Mouseover to Update:

Field Position: arcmin

Field of View*: arcmin2

Start λ: nm       End λ: nm

BW Fraction:

*Highlighted bars   Wavelength used to set grating angle.

Contact with questions or bug reports.