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.

Sensitivity

Expected 4MOST point-source sensitivity for the S/N per Å values and sky conditions indicated in the legend. The solid lines are for a total exposure time of 120 min, whereas the dashed lines are the limits for 20 min exposures. The approximate conversion to S/N per pixel is obtained by dividing the HRS values by 3.3 and the LRS numbers by 1.7. This plot is representative for an entire 4MOST survey, not just for the optimal conditions. The sensitivities shown are those required by typical science cases, i.e. for obtaining detailed elemental abundances of stars (orange), stellar parameters and some elemental abundances (dark blue), stellar radial velocities (light blue), and galaxy and AGN redshifts (black: 90% completeness, grey: 50% completeness).