The GMT-Consortium Large Earth Finder (G-CLEF) is a fiber-fed, optical echelle spectrograph selected as the first light instrument for the Giant Magellan Telescope (GMT) currently under construction at the Las Campanas Observatory in Chile. G-CLEF is vacuum-enclosed and fiber-fed to enable precision radial velocity (PRV) measurements, enabling the detection and characterization of low-mass exoplanets orbiting solar-type stars. The passband of G-CLEF is very broad, extending from 3500Å to 9000Å. This provides good sensitivity at blue wavelengths for stellar abundance studies and deep red response permitting observations of high-redshift phenomena.

G-CLEF’s radial velocity (RV) precision goal is 10 cm/sec. This high level of precision is necessary for detection of Earth-sized exoplanets. It also imposes challenging stability requirements on the optical mounts and spectrograph support structures especially when the operational environment is considered.

The accuracy of G-CLEF’s PRV measurements could be adversely influenced by a variety of factors including:

• Normal daily fluctuation of the surrounding air temperature
• Weather-related ambient pressure variations
• Seismic inputs and machine-made vibration
• Micro gravity-vector variations caused by normal telescope slewing

For these reasons we have enclosed the spectrograph in a vibration isolated vacuum chamber, within a well-insulated thermal enclosure, supported on a self-leveling platform. Additional design constraints posed by the GMT telescope include:

• A limited space envelope
• A thermal emission ceiling
• A maximum weight allowance

Of course, manufacturability, serviceability, cost, and use of available technology wherever possible are high priorities. The above considerations must be managed while ensuring performance requirements are achieved.

• Composit Optical Bench:


G-CLEF will be the first ground-based PRV spectrograph to have a composite optical bench to exploit that material’s extremely low coefficient of thermal expansion and high stiffness-to-weight ratio.
• Thermal Control System:


The design also incorporates a unique thermal control system that enables sub-milli-kelvin level thermal stability over diurnal time scales.
• Optical Design:


The spectrograph camera subsystems are divided into red and blue channels, split by a dichroic beam splitter. There are two independent refractive spectrograph camera designs. The designs minimize lens fabrication risk while maximizing performance as measured by point spread function size, throughput and immunity from ghosting and scattered light. Exposure meters, positioned to harvest out-of-order light otherwise unseen by the detectors, are implemented at each channel.
• Systems Engineering Approach:


G-CLEF has been conceived and designed within a strict systems engineering framework. As a part of this process, we have developed a powerful tool set to assess the predicted performance of G-CLEF as it evolves through various design phases. The control software is being developed in a model-driven context that has been adopted globally by GMTO for all telescope systems.

G-CLEF has recently passed preliminary design review (PDR) and a subsequent gate review of the spectrograph optical design. The project is on track for Critical Design Review (CDR) in 2017. Delivery and commissioning to the GMT is planned for 2021.

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