Gemini High Resolution Optical SpecTrograph (GHOST)

GHOST Instrument at the Gemini South telescope.

GHOST, the Gemini High-resolution Optical SpecTrograph, is the new Gemini facility instrument that provides world-class, high-resolution spectroscopic capabilities to the Gemini community. GHOST features a wide simultaneous wavelength coverage at high observational efficiency, enabling astronomers to investigate a broad range of science questions from the composition of the first stars to the characterization of exoplanet systems. A data reduction pipeline will be delivered with the instrument. Australian Astronomical Optics at Macquarie University leads the GHOST team, which also includes the National Research Council Herzberg (NRC-H), responsible for the construction of the spectrograph, and the Australian National University (ANU), responsible for the instrument control system and data reduction software.

GHOST consists of three primary components.

1. Cassegrain unit. Mounted on the axial port of Gemini South telescope the Cassegrain unit consists of the Main Structural Plate Assembly, Positioner Frame Assembly, Ballast Frame Assembly and Electronics Cabinet. The Positioner frame is the mounting interface for the Telecentricity Lens, atmospheric dispersion correctors (ADC) assemblies, positioners and integral field units (IFUs). The Ballast Frame is the mount for the ballast weights. These weights are used to provide a total Cassegrain unit assembly mass of 2000 kg. The electronics cabinets contain one of the distributed control systems used to control and monitor the GHOST system actuators and sensors. This distributed control system is based on a combination of standard Ethernet-based LANs and an industrial fieldbus (CAN) with an interface to Ethernet.

GHOST Cassegrain unit attached to Port 1 of Gemini South telescope.

2. Spectrograph bench. Located in the pier telescope laboratory, the spectrograph bench is isolated for image and wavelength stability. GHOST is an echelle spectrograph based on a two-arm asymmetric white-pupil design with a Volume Phase Holographic (VPH) grating for cross-dispersion and two detectors, one at the end of each arm: a 4k x 4k blue detector and 6k x 6k red detector. A third detector (slit unit camera) provides for object acquisition and active monitoring of the slit illumination. The bench uses active thermal stabilization to help provide the image and wavelength stability necessary for operational efficiency and radial velocity precision.

The GHOST bench unit.

3. Fiber cable. The 25-meter-long fiber cable transmits the light from the Cassegrain unit to the spectrograph bench.

GHOST provides simultaneous wavelength coverage from 363 nm to 950 nm. It has two selectable spectral resolution modes: a standard-resolution mode with resolving power R ~ 55,000 and a high-resolution mode with R ~ 80,000. The instrument will obtain a limiting magnitude of 17.5 at 450 nm.

GHOST will enable astronomers to investigate a broad range of science goals, from the composition of the first stars to the characterization of exoplanetary systems. GHOST will also provide crucial follow-up of interesting targets emerging from many ongoing and future surveys, such as Vera C. Rubin Observatory’s Legacy Survey of Space and Time, SkyMapper and GAIA. 

Quick Facts