The LATT is a prototype designed and manufactured funded under an ESA - TRP. The goal of the LATT project was to demonstrate the technology scalability from the adaptive secondary mirror (see for instance the adaptive M2 of the LBT and VLT) to an active, segmented primary for space telescope.

The prototype is a spherical mirror, 40 cm in diameter, controlled by 19 voice-coil actuators. The concept is based on a thin glass shell as optical surface, plus an ultra-light reference body with no optical property. The system has been qualified in thermo-vacuum test, on the vibration platform and on the optical bench for full optical characterization. The main features of the prototype are a low power consumption per actuator (< 55 mW), a 1 mm available actuator stroke (valuable to recover deployment errors) and an areal density lower than 16 kg/m2. The LATT project ended in 2015 after the final review at ESA. During a later informal follow up phase, we discussed the OBB features beyond the specific requirements and boundaries of the LATT project, thus identifying the main drivers and technological challenges for the SPLATT activities.

The P-WFS is a pupil plane sensor widely studied and adopted for AO system on 8m-class telescopes and also for the ELTs.  It implements the knife edge test and is basically composed by a double pyramid prism, with its tip placed at the telescope focal plane. The light beam is split by the four faces, then a relay lens produces four pupil images on a detector. Any deviation from the diffraction limited PSF on the pyramid tip results in an intensity unbalance amongst the 4 images, so that it is possible to evaluate the WF slope by comparing the 4 images. The PWFS, then, exploits the full telescope resolution to sense the WF: the larger the diameter, producing a sharper PSF, the better the WF sensitivity, which in turn is spatial-scale dependent (more sensitive at the lowest spatial scale). As a second point, the PWFS is intrinsically sensitive to phase steps, i.e vertical offsets in the WF map; so that it allows measuring and recovering the differential piston (and differential alignment in general) amongst the mirror segments after deployment and tracking them in close loop during the observations. A large sensitivity means that a robust SNR can be achieved with a fast integration time, so that the "open loop" stability of the telescope can be relaxed.