Front End #

In order to reach the scientific objectives, the RISTRETTO front-end should deliver:

• High strehl ratio > 60% on average,
• High contrast level (2.10^-4) on average.

In particular the second point has to be achieved “live”, i.e. not using data redutions tricks.

IFU #

The position angle of the planet is a priori unknown, thus there is the need for an IFU covering the whole area around the star.

The IFU is paved in an hexagonal fashion as shown in the following figure

Coronagraph #

The coronagraph is very specific in this kind of instrument. It has to provide “live” contrast of 10^-4, after injection into the 6 off-axis fibers of the IFU.

The selected solution is the PIAA (Guyon et al. 2003) coronagraph, that gives the advantage of giving a higher coupling of the telescope light into the spectrograph fibers at the same time as providing a good enough contrast in the fibers.

XAO System #

In order to obtain the required performances, one need an XAO system working in the Visible. Given the brightness of the target it is possible to make an AO system with a wavefron sensor in the infrared while keeping the visible light for the science.

The current design is to make a woofer tweeter system:

• LODM: 11x11 actuator,
• HODM: >= 40x40 actuator.

The main wavefront sensor will be pyramid wavefront sensor (possibly unmodulated) in the infrared. A second finer wavefront sensor (possibly Zernicke), also sensitive to phase discontinuities closer to the science band. It is also clear that a low order deformable mirror is necessary in the science band to tackle the NCPA.

With end to end simulations, with median paranal condition 0.75" seeing and 50° elevation we would get the following performances: