Instrument Characteristics and Observation Modes


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.

Table 1. Core science case programs for G-CLEF.
Science Goal Required Performance Parameter
Abundance studies
Detection and census of metal poor stars
High resolution
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
Table 2. G-CLEF performance parameters required to achieve key science objectives.
Spectrograph Property Motivation
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
Table 3. G-CLEF overall characteristics
Parameter Value
Resolutions 23K, 35K, 108K, 110K
Passband 3500–9500 Å
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
Input f/# f/8
Camera Beam Diameter 200 mm
Spectrograph Field Derotation? No
ADC? Yes
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.

Table 4. Observation Mode Characteristics
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