ESPRESSO
ESPRESSO is the ESO/VLT high-resolution spectrograph to measure precise radial velocities with the main scientific aim of detecting and characterising Earth twins in the habitable zone of solar-like stars.
ESPRESSO is a highly-stabilized fibre-fed échelle spectrograph that can be fed with light from either a single or the four VLT Unit Telescopes (UTs) simultaneously. The instrument is installed at the incoherent combined Coudé focus (ICCF) of the VLT. The light from an astronomical source is redirected from the telescopes to the detectors through three components of the ICCF facility: the UT Coudé trains, the front end units, and the spectrograph itself. The Coudé Trains (CT) bring the light from each telescope to the Combined Coudé Lab (CCL) through 13 optical elements, including mirrors, lenses, and prisms. The four Front Ends (one for each UT) receive the light from the CTs and feed the spectrograph entrance fibres. The Fibre Link transports the light from the Front Ends to the vacuum vessel. The latter is thermally stabilized at the mK level. The light is then going through the optical components of the spectrograph and split up into a red and a blue spectrum which are recorded on the corresponding science detectors.
The spectrograph is fed by two fibres, one for the target and the other one for simultaneous calibration (either the sky or a simultaneous reference: Fabry-Pérot, Laser Frequency Comb, or Thorium-Argon lamp). The light from the two fibres is recorded onto a blue (380-525nm) and a red (525-788nm) CCD mosaic. ESPRESSO can operate in three main instrument configurations: High Resolution 1-UT (HR), Ultra High-Resolution 1-UT (UHR) and Medium Resolution 4-UT (MR). The main characteristics of these modes are summarised below (for more details on the characteristics of the instrument, see the Instrument Description).
|
HR (1-UT) | UHR (1-UT) | MR (4-UT) |
| Wavelength range | 380–788 nm | 380–788 nm | 380–788 nm |
| Resolving power (median) | 140,000 | 190,000 | 70,000 |
| Aperture on sky | 1''.0 | 0''.5 | 4x1''.0 |
| Total efficiency |
11% | 5% | 11% |
| RV precision (requirement) |
< 10 cm/s | < 5 m/s | < 5 m/s |
| Limiting V-band magnitude* | ~17 | ~16 | ~20 |
| Binning | 1x1, 2x1 | 1x1 | 4x2, 8x4 |
| Spectral sampling (average) | 4.5 px | 2.5 px | 5.5 px (binned x2) |
| Spatial sampling per slice | 9.0 (4.5) px | 5.0 px | 5.5 px (binned x4) |
| Number of slices | 2 | 2 | 1 |
* based on an approximate S/N per binned pixel of 10 in one hour.