The 4MOST facility will be running spectroscopic Public Surveys on VISTA nearly full time, and consists next to the instrument also of all the software to schedule and observe many surveys in parallel and the Data Management system to reduce, analyze, validate and publish all the data including higher-level data products (radial velocities/redshifts, stellar parameters, abundances, etc.).
The critical components of the hardware of the facility are the telescope with a wide field corrector, the fibre positioner system with a fibre run to the spectrographs.
To achieve the main science goals, 4MOST shall be able to obtain:
- radial velocities of ≤2 km/s accuracy of the faintest stars observed by Gaia by obtaining spectra of 19 (goal 19.5) r-mag stars with S/N=10 per Angstrom at resolution R=5000 in a ≤1 hour exposure,
- abundances of up to 15 chemical elements of ≤16 V-mag stars by obtaining spectra with S/N=140 per Angstrom at resolution R=20000 in a ≤2 hour exposure,
- redshifts of 22 r-mag galaxies and AGN in a ≤2 hour exposure in dark time.
The science requires that, within a 5 year survey, 20 (goal 30) million targets shall be observed at R~5000 and 2.0 (goal 3.0) million objects at R~20,000 (these numbers include community targets). In this period at least a 15,000 (goal 20,000) degree2 area on the sky shall be covered at least two times (goal three times).
This table summarizes the main instrument specifications and current design values.
|Field-of-View in hexagon||>4 degree2||>5 degree2||4.1 degree2|
|Fibre multiplex per pointing||>1500||>2400||2436|
|Smallest target separation||<30″||<15″||<17″|
Low-Resolution Spectrograph (LRS)
|Spectral resolution||R> 4000||R>7500 @800nm||R>4000–7800|
|Wavelength coverage||400–885 nm||390–950 nm||370–950 nm|
|High-Resolution Spectrograph (HRS)|
|Photon detecting percentage (in 1.0 arcsec seeing)||>15%||>20%||>15%|
|Spectral crosstalk (after data reduction)||<0.05%||<0.02%||TBD|
|Fibre aperture diameter||1.4″±0.1″||1.4″±0.1″||1.45″±0.05″|
|Observing efficiency||Overhead < 20%||Overhead < 10%||<18%|
|Available sky area (Zenith angle)||2–55 degrees||2–70 degrees||2–55 degrees|