GMACS Instrument Functional Requirements

The science objectives for GMACS impose specific requirements on the instrument. The principal requirement for the instrument is high throughput. Table 1 summarizes these requirements. Note that Table 1 includes "requirements" that are defined as the minimum necessary to fulfill the needs of more than 50% of the stated science objectives. Table 1 also includes a set of "goals" that are defined as desirable instrument characteristics for which there are also strong science cases.

Although no specific performance is required, the theme of nearly all the identified science objectives is observations of extremely faint targets (24-25 mag). In order to achieve reasonable SNR with the GMT in roughly one hour of integration under typical weather, seeing, and moon phase conditions, we calculate that the total instrument throughput (including atmosphere and the telescope) should be higher than ~20% overall. Clearly higher throughput is better. We adopt two channels as a requirement to meet the throughput goals over the necessary broad wavelength, simultaneous wavelength coverage.

Additionally, there is no specific requirement for the image quality produced by the instrument optics. However, GMT requires that we interface with MANIFEST and enable higher resolution (in addition to larger multiplex field) when GMACS is fed by fibers. Therefore, we adopt as a requirement instrumental image quality that will ultimately allow resolution 2-3 times higher (with a given grating and spectrograph geometry) than with a 0.7 arcsec slit width (the adopted fiducial). This implies a requirement on the instrument of 80% EE images at the final focal plane of better than 0.30 arcsec and a goal of 0.15 arcsec.

We also note that there was strong desire for rapid reconfiguration of the instrument to cover multiple resolutions and wavelength ranges during a single night of observations. We adopt as a requirement a mechanism capable of adjusting a single grating for use at multiple wavelength regions under remote control, with a goal of a mechanism that could exchange gratings and allow multiple wavelength regions during a single night under remote control.

The throughput requirements and goals suggest we use VPH gratings as dispersers. We assume these and describe potential performance below.

Parameter Requirement / Goal Comments
Field of View 30 arcmin sq. / 50 arcmin sq.
Wavelength coverage 370-950nm / 340-1000nm
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 2 / ≥2 resolutions Multiple wavelength regions
Slit Mask Exchange 20 / ≥20 Cassette style mask changer

Table 1: Summary of principal functional requirements and goals.

Design Trade Study Reports

Below are links to trade studies for various aspects of GMACS. For reference only, the working specifications of GMACS may have changed since these documents were created.

Spectrograph Parameter Study

VPH Grating Study

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

Astronomy Group
4242 TAMU
College Station, TX 77843-4242
Ph: (979) 845 7717
Texas A&M University - College of Science