G-CLEF is a fiber fed, optical echelle spectrograph that will be a first light instrument at the GMT. G-CLEF has been designed to be a general-purpose echelle spectrograph with precision radial velocity (PRV) capability. We have defined the performance envelope of G-CLEF to address several of the highest science priorities in the 2010 Decadal Survey.
The spectrograph optical design is an asymmetric, two-arm, white pupil design. The asymmetric white pupil design is adopted to minimize the size of the refractive camera lenses. The spectrograph beam is nominally 300 mm, reduced to 200 mm after dispersion by the R4 echelle grating. The passband is 3500–9500 Å. The spectrograph is primarily fed with multiple sets of fibers to enable four observing modes: High-Throughput (HT), Medium Resolution (MR), Precision Radial Velocity (PRV), and Non-Scrambled Precision Radial Velocity (NS-PRV). We also anticipate having an R ~35,000 Multi-object Spectroscopy mode with a multiplex of ~40 fibers.
The goal radial velocity precision of G-CLEF is ΔV < 10 cm/sec.
|Science Goal||Required Performance Parameter|
Detection and census of metal poor stars
Extended blue response to 3500 Å
|High z IGM and ISM/Gamma Ray Bursts||Rapid instrument changeover
Extended red response
|Exoplanet science||Very high resolution
Long term wavelength stability
|Detailed chemical composition beyond the Local Group||Long slit length for multi-object capability|
|Fiber Feed||PRV stability|
|Asymmetric White Pupil Design||PRV resolution, reduce risk and cost|
|Two Camera Design||Good red and blue response, reduce cost|
|Vacuum Enclosure Thermal/ambient index of refraction||stability|
|Short Fiber/Simple Feed||Good blue response|
|Modes||HT, MR, PRV, NS-PRV, MOS|
|Resolutions||23K, 35K, 108K, 110K|
|Calibration Lamps||Continuum, Supercontinuum, High Output ThAr, Low output ThAr, Ultrastable Etalon, Laser Comb|
|Telescope Aperture||25.4 m (7 × 8.4 m)|
|Echelle Grating||300 mm × 1200 mm, R4|
|Mosaic Cameras||Red, Blue|
|Camera Beam Diameter||200 mm|
|Spectrograph Field Derotation?||No|
|Band Limiting Filters?||Yes|
Description of Operation Modes
The G-CLEF instrument will enable a broad range of scientific missions by providing different measurement modes with performance characteristics individually optimized to address a variety of different science requirements. The following operation modes will be supported:
A High Throughput (HT) mode, in which the instrument throughput is maximized. Resolution is 23,000. HT mode is best for programs where signal-to-noise is paramount.
A Medium Resolution (MR) mode, in which instrument throughput and resolution are balanced. The resolution for MR mode is 35,000. This mode has been designed in mind for detailed stellar abundance studies.
A Precision Radial Velocity (PRV) mode, in which the instrument is optimized for accurate measurements of PRV. This PRV mode includes an optical double scrambler in the beam. The resolution for PRV mode is 108,000.
A second Precision Radial Velocity (NS-PRV) mode, in which the instrument is configured for PRV measurements, but the light is unscrambled. The throughput of this mode is improved ~20% compared to the PRV mode because of the lack of an optical double scrambler in the beam, and has the same 108,000 resolution as the PRV mode. This mode is for scientists who only care about achieving high resolutions without the capability of measuring precise radial velocities.
Each of the seven 8.4 m GMT primary mirror sub-apertures feeds an individual fiber in PRV and NS-PRV mode, meaning there are as many sub-apertures on the echelle image per order as there are mirrors on the GMT.
Each of these operation modes may be used with natural seeing or with adaptive optics using the on-instrument wavefront sensor . In addition to these G-CLEF capabilities, a Multi-Object Spectroscopy (MOS) mode will be supported by providing an interface to MANIFEST through a separate MOS instrument feed to the G-CLEF spectrograph. The MOS mode will have resolution of around 35,000 with a multiplex of around 40 fibers.
|Fiber Mode||Resolution||Fiber||# Science Fibers||# Cal/Sky Fiber||Resolution Element (pixels)|
|PRV||108,000||Hexagonal 100μ Core||7||2||7.5|
|NS-PRV||108,000||Circular 100μ Core||7||2||7.5|
|MR||35,000||Circular 300μ Core||1||2||20|
|HT||19,000||Circular 450μ Core||1||2||30|
|MOS||35,000||Circular 300μ Core||40||N/A||20|