ESPRESSO front-end guiding algorithm

Abstract

This paper presents the Espresso Front End Guiding Algorithm. ESPRESSO, the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, will be installed on ESO's Very Large Telescope (VLT). The Front End (FE) is the subsystem that collects the light coming from the Coudè Trains of all the Four Telescope Units (UTs), provides Field and Pupil stabilization via piezoelectric tip tilt devices and inject the beams into the Spectrograph fibers. The field and pupil guiding is obtained through a re-imaging system that elaborates the halo of the light out of the Injection Fiber and a telescope pupil beacon. The first guiding algorithm that we evaluated splits the FP in four areas and computes the sum of the photon counting of each pixel in that area. The unbalancing of the photon sums will give the centroid misalignment information that will be handled by the Instrument Control Software (ICS). Different algorithms and controllers architectures have been evaluated and implemented in order to select a strategy that enables the FE to guide up to 20 apparent magnitude in V band.