Capabilities

The table and plots below document the main characteristics of 4MOST.

Parameter

Design value

Field of view

Shape: hexagonal
Size: 4.2 deg²
Diameter: 2.5 deg

Accessible sky (zenith angle < 55°)

30,000 deg²

Multiplex fibre positioner

2436

On-sky fibre aperture

Shape: circular
Size: 1.65 arcsec²
Diameter: 1.45 arcsec

Minimum distance between fibres

15 arcsec

Expected on-target fibre-hours per year

LRS: 3.2 Mh/yr
HRS: 1.6 Mh/yr

Overheads

3.5 min for the field acquisition (telescope preset), 4.4 min for each science exposure including attached calibrations

Detectors

Readout noise: < 2.3 e^-
Readout modes: normal (100 kHz), fast (400 kHz)
Binning modes (cross-dispersion × spectral): 1×1, 2×1, 1×2, 2×2, 1×4

Low-resolution spectrographs

Multiplex

812 (× 2)

Wavelength range

370 – 950 nm

Spectral resolving power

4000 – 7700 (see figure below)

Spectral sampling

≥ 2.5 pixel/FWHM

Radial velocity accuracy

< 1 km/s

Sensitivity

Limiting AB magnitudes for S/N = 10 Å^-1, 6 × 20 min exposures, mean seeing, new moon (also see figure below):

Wavelength (nm)	400	500	600	700	800	900
Limit (AB mag)	20.2	20.4	20.4	20.2	20.2	19.8

High-resolution spectrograph

Multiplex

812

Wavelength range

392.6 – 435.5, 516 – 573, 610 – 679 nm

Spectral resolving power

18,000 – 21,000 (see figure below)

Spectral sampling

≥ 2.5 pixel/FWHM

Radial velocity accuracy

< 1 km/s

Sensitivity

Limiting AB magnitudes for S/N = 100 Å^-1, 6 × 20 min exposures, mean seeing, 80% moon (also see figure below):

Wavelength (nm)	420	540	650
Limit (AB mag)	15.7	15.8	15.8

Spectral resolution

Left (click to enlarge): spectral resolving power of the low-resolution spectrographs in the blue, green and red wavelength channels. The dashed line shows the requirement. Right: same for the high-resolution spectrograph.