As suggested by its name, the Shaped Pupil (SP) technique consists in re-shaping the telescope pupil so to generate dark areas in the focal plane via destructive interference. Strictly speaking, “shaping'' here means constraining transmission to be either 0 or 1 across the pupil. 

In principle, the only binary mask is sufficient to create high contrast. However, there are two main drawbacks: first, the shaped PSF features a bright central core, which may easily saturate the detector. The second important drawback is stray light. 

In order to mitigate these effects, we can exploit SHARK-NIR intermediate focal plane to put a hard edge occulter that masks the PSF core. A Lyot stop in the second intermediate pupil plane then blocks the residual light diffracted by the occulter, as in the classical Lyot configuration. The three-plane configuration also allows to go deeper in contrast with respect to the apodizer-only configuration.

In a Gaussian Lyot coronagraph, the electric field amplitude in the focal plane is spatially modulated according to a Gaussian transmission profile. Continuous (greyscale) transmission profiles cannot be accurately reproduced with standard metal deposition methods in the NIR. For this reason, we decided to approximate the Gaussian profile using microdots, a technique named halftoning. Halftoning requires the theoretical Gaussian grayscale profile to be first converted to binary (only 0 and 1 transmission). To do so, we used a standard error-diffusion algorithm.

The Four-Quadrant Phase-Mask (FQPM) coronagraph suppresses on-axis starlight by means of a phase mask in the focal plane. The mask divides the FP into four quadrants and induces a π phase shift on two of them on one diagonal. Provided that the image of the star is perfectly centered on the common vertex of the quadrants, then the four outcoming beams combine destructively at infinity and the stellar light in the downstream pupil plane is totally rejected outside of the pupil area. This light is then easily blocked by means of a Lyot stop. 

This because being SHARK-NIR fundamentally a coronagraphic camera, it is severely impacted by the dust-scattered light.

The SHARK-NIR optical bench, initially empty, is hold by a dedicated handling in order to have the surface of the bench positioned perpendicular to the gravity vector and with the rotation axis at ~ 1 m above the ground.

Two alignment beams are foreseen. These are:

•  An on-axis, narrow and collimated laser beam, used for the materialization of the bearing rotation axis, the alignment of the refractive opitics and for visual aid in the alignment of all the reflective optics.

• A F/15 beam produced by a Zygo interferometer, simulating an on axis beam coming from LBT. Used for the fine alignment of the OAPs.

These two beams are of course co-aligned, following the procedure described in RD2, and it is possible to switch from one beam to the other by means of a motorized linear stage, which inserts a flat mirror in the beam. The insertion of the mirror in the optical path inject the light from the laser into SHARK-NIR, while removing the mirror we have the interferometer light entering into SHARK-NIR.

The whole alignment procedure is based on a mechanical/optical approach, meaning that as a first step we will mechanically pre-align all the optics to a mechanical reference representing the on-axis entrance focal plane of SHARK-NIR. This mechanical reference is a sphere, positioned and aligned by the SHARK-NIR mechanics manufacturer, whose center lies on the bearing rotation axis.

The mechanical alignment is based on the use of a Coordinate Measuring Machine (CMM) Space Plus 1800 from Tomelleri Engineering and rely on adjusting the position of the optics with respect to the sphere till reaching the nominal configuration.