Science requirements:

1. Field corrector sub-system

In order to enable the exploitation of the full 25 arcminute field of view (FOV) of the VLT Nasmyth focus, the use of a field corrector is necessary. The corrector is formed by two large lenses with approximately 110 mm of thickness in the axis and a diameter of 880 mm. The first lens is a plano-convex and the second is a symmetrical biconcave; this option allows minimizing manufacturing costs without influencing performances. To provide a glass internal transmission better than 95% for the full MOONS wavelength range (800 nm to 1800 nm), the selected material is Fused Silica. Compared to a no-corrector option, this corrector improves the image quality over the full FOV by a factor of 8, better than 0.1 arcsec (80% geometric energy for full FOV), the exit pupil is practically concentric to the field curvature and the field curvature is reduced to half (from a radius of 2090 mm to 4210 mm).

  1. 2.Fiber positioner sub-system

The fibers for science observations are deployed on the focal plane via a fiber positioner. Due to the different requirements on sky subtraction for bright and faint targets, the pick-off system must be able to allocate the 1000 fibers both independently – e.g. all on different targets – or in pairs to perform the cross-beam switching, in which each target has got its dedicated sky fiber at a distance of few arcsec. Another key requirement for the positioner is the reconfiguration time, which should be < 5 minutes in order to have acceptable overheads. For the Phase A study two possible implementations have been considered: a micro-mechanical pick-off system and a pick and place spine system.

        2.1.Micro-mechanical pick-off system

   In the micro-mechanical pick-off solution the idea is to cover the focal plane with modular fibre positioners each of which has a fixed patrol area. In this concept design each pick-off unit has got two rotating arms (as shown in Figure below). To cover the Nasmyth focal plane with ~1000 units, each positioner should have a physical size of ~30mm in diameter, which corresponds to ~1 arcmin on sky. In order to have a high allocation efficiency of the fibres on targets, some overlap between neighboring patrol fields is needed, with one fibre being able to patrol up to the centre of the neighboring cell.


  1. 3. Fiber sub-system

At the entrance of the fibres, the optical aperture conversion from F/15 beam of the telescope to F/3.65 is realized by the coupling with a microlens. In fact, injecting at a fast F ratio allows the change between the entrance and output F ratio to be minimised. In fact, a system using an input of F/3.65 with an output of F/3.5 looses less than 2.5% in transmission. The input aperture on the sky of the fibre is 1-1.2 arcsec, which correspond to a physical core diameter of 140-170µm.  In the phase A study only Polymicro fibres are investigated, since they are well known and are used in various astronomical instruments.

At the output end, a bundle of 10 to 15 fibres is grouped into sub-slits, which are then arranged to form the entrance slit to the spectrograph. The fibres inside the spectrograph enclosure are kept at cryogenic temperature, similarly to APOGEE (Brunner et al. 2010).

  1. 5.Spectrograph sub-system

A complete description of the MOONS spectrograph is presented by Oliva et al. (SPIE 2012).  The baseline design consists of two identical cryogenic spectrographs. Each of them collects the light from over 500 fibers and feeds, through dichroics, 3 spectrometers covering the ”I” (0.79–0.94 μm), ”YJ” (0.94–1.35 μm) and ”H” (1.45–1.81 μm) bands, simultaneously (see Figure below).  The low resolution mode provides a complete spectrum with a resolving power ranging from R>4,000 in the YJ-band, to R>6,000 in the H-band and R>8,000 in the I-band. A higher resolution mode with R>20,000 is also included. It simultaneously covers two selected spectral regions within the YJ and H bands. The whole spectrometer is in a vacuum vessel cooled to cryogenic temperatures.  Each spectrograph, utilizes two Hawaii-4RG-15 devices, from Teledyne Imaging (one for the YJ and one for H-band channel) and one 4k x 4k optical CCD from e2v technologies for the I-band channel.