The wide field and atmospheric dispersion corrector (WFC/ADC) corrects for image aberrations. The system consists of 4 lenses with 2 counter-rotating prisms. The WFC creates a nearly unvignetted image of 2.6 degrees diameter in the focal surface of the telescope. The ADC corrects for atmospheric dispersion up to 55  degrees zenith distance and hence at larger zenith angles has poorer performance as object light gets dispersed in the focal surface and light at the end of the 4MOST wavelength range may miss the fibre. In simulations, the WFC has a quite homogeneous throughput over the whole field averaging to 84.6%. The plate scale of the WFC is about 59.4 μm/arcsec.

Two A&G cameras will be implemented for redundancy and in order to increase the sky coverage, but only one of them will be active at any time. There are four WFS cameras around the edge of the field of view. Each WFS camera (2048x2048, 13.5 μm pixels) samples a 27.6 mm × 27.6 mm patch of the field, equivalent to about 7.75' × 7.75' (60 arcmin²).

Left: arrangement of the 2436 science fibres in the focal surface and their mapping to the three spectrographs. In each column of spines, the fibres alternately feed LRS-A, LRS-B and HRS (as indicated by the colour code). All fibres are physically identical, i.e. all have the same core and cladding size. Right: the 4MOST AESOP fibre positioner with all the fibres in the spines back-illuminated with red light, allowing the metrology system to identify their position.

View of the fibre feed routing without showing the telescope, the fibre positioner or any mechanics. The cable chains contain the science, guide and fiducial fibres. After the loop boxes the conduits connect the science fibres to the three spectrographs. An additional cable carries stationary calibration and back-illumination fibres from the azimuth floor cabinet to the three spectrograph slits.

The Metrology System measures the location of all fibres in the focal plane and translates it to the targets on sky. The main components are a set of 4 identical cameras, imaging the whole field-of-view; and a back-illumination unit that illuminates all science and fiducial fibres, as well as the central fibres of the secondary guide bundles.

Two identical low-resolution spectrographs (LRS, left) and one high-resolution spectrograph (HRS, right), fed by 812 fibres each, provide simultaneous measurements of 2436 spectra. Each spectrograph has a blue, green, and red arm, in fixed configuration, and each arm is equipped with a 6k × 6k CCD detector. The LRS provide continuous wavelength coverage from 390 to 950 nm at a resolution of R > 4000. The HRS covers the ranges 392.6 – 435.5, 516 – 573, and 610 – 679 nm at a resolution of R > 18,000.

There are nine identical deep depletion detectors for all spectrographs and arms, with 6144 (H) × 6160 (V) pixels of 15 µm size. The readout noise is < 2.3 e^- (@ 100 kHz).

The calibration system uses a Fabry Perot (FP) etalon, illuminated by a Laser Driven Light Source (LDLS) to provide a regular comb of spectral lines for wavelength calibration. Additional wavelength calibration can be done with a ThAr lamp. The FP is in temperature-controlled vacuum and therefore very stable. The calibration light can be fed both through the telescope plus science fibres combination as well as directly through the simultaneous calibration fibres into the spectrograph slit to ensure accurate wavelength calibration. This will ensure an accuracy on stellar radial velocities better than 1 km/s.