The GMT-Consortium Large Earth Finder (G-CLEF) will be a cross-dispersed, optical band echelle spectrograph to be delivered as the first light scientific instrument for the Giant Magellan Telescope (GMT) in the mid 2020's. G-CLEF is vacuum-enclosed and fiber-fed to enable precision radial velocity (PRV) measurements, especially for 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 passband provides good sensitivity at blue wavelengths for stellar abundance studies and deep red response so as to permit observations of high-redshift phenomena.

The design of G-CLEF incorporates several novel technical innovations and methodologies when compared with previous PRV instruments and instrumentation for ground based telescopes in general. G-CLEF has recently passed preliminary design review (PDR) and a subsequent gate review of the spectrograph optical design. More details on the design of G-CLEF can be found in Szentgyorgyi, et al., 2014.

Overview of the G-CLEF subsystems. The G-CLEF Front-End Assembly (GCFEA) focuses the light from the telescope onto the optical fibers which run to the spectrograph (GCSPECT). The GCFEA also controls instrument flexure, and sends light to the wavefront sensor for adaptive optics. The fiber feeds (GCCFIB) send the light from the GCFEA or MANIFEST instrument to the spectrograph, where the light diffracts off the echelle grating and into a dichroic beam splitter which splits the light into red and blue; two independent CCDs image the red and blue light. The spectrograph itself is enclosed in a thermally controlled environment which keeps the optical bench at 20 °C to microkelvin accuracy.

In particular, G-CLEF will be the first PRV spectrograph to have a composite optical bench to exploit that material’s extremely low coefficient of thermal expansion and high stiffness-to-mass ratio. The design also incorporates a unique thermal control system design that maximizes instrumental immunity to ambient temperature fluctuations. This system has been prototyped and has demonstrated sub-millikelvin thermal stability on diurnal time scales.

The spectrograph camera subsystem is divided into a red and a blue channel, split by a dichroic beamsplitter, so there are two independent refractive spectrograph camera designs. These designs minimize lens fabrication risk while maximizing performance as measured by point spread function size, throughput and immunity from ghosting and scattered light. The exposure meter has been implemented as a subsystem of the camera and harvests out-of-order light at the volume phase holographic gratings, which cross-disperse the two spectrograph beams after the dichroic.

The spectrograph optical design. The spectroscopic light path is shown in a solid line. The zero order path, i.e., light undeviated by the VPH, is shown with a dotted line. The red camera optical elements are shown, while the blue camera optic mount is shown.

The control system software is being developed in model-driven software context that has been adopted globally at the GMT for all telescope systems.

gclef-fcc gclef-fcc-on-gmt
The flexure control camera system. Some light from the telescope is reflected off the slit mirror on the fiber selector system into the flexure control camera (FCC) system. This camera system is used for two purposes: to focus the telescope light on the slit mirror, and to correct for instrument flexure which may cause the target light from the telescope to drift off of the optical fiber entrances. The image on the flexure control camera is analyzed; corrective positions are sent to the G-CLEF tip-tilt tertiary mirror to counteract instrument flexure, centering the telescope light back on the optical fibers.

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 has evolved through subsequent design phases.

Instrument Characteristics

G-CLEF Property Value
Spectrograph Beam Diameter 300 mm
Echelle Grating Facet Count 3
Spectrograph Focal Ratio F/8
Collimator focal length 2400 mm
Fiber Output Focal Ratio F/3
Pupil Xfer focal lengt 1600 mm
Camera Beam Diameter 200 mm
Pup.Xfer Mirr./Cam. Reduction 450/1600 = 0.2813
Camera Focal Ratio F/2.25
PRV Slit Image Size 0.075 mm
Camera Focal Length 450 mm
Baseline Pixel Size 0.010 mm
Passband 3500Å-5400Å (blue) & 5400Å-9000Å (red)
Camera lens count (blue/red> 8/7