VSTPOL: the control electronics design for the new polarimetric mode of the VST

The VST (VLT Survey Telescope) is a 2.6m telescope installed in the ESO Observatory of Cerro Paranal, equipped with a wide-field imaging camera operating in the visible band (OmegaCAM). One of the goals of the Cherenkov Telescope Array Plus (CTA+) program, included in the EU Recovery Plan (PNRR), is to upgrade this ground-based optical facility adding a new polarimetric mode to allow the follow-up and monitoring of the CTA transient sources. The VSTPOL design aims to replace the actual electro-opto-mechanical system connected to the back side of the primary mirror cell of the telescope with a new system, consisting of two motorized functions: a linear exchanger mechanism to switch between the traditional imaging mode and the new polarimetric mode; a rotating device equipped with a polarimetric filter, replacing the unused ADC functionality, that enables tracking to compensate for the field rotation, following the movement of the OmegaCAM. Here we present the VSTPOL control electronics architecture, based on the new ESO electronics standards. All the control electronics are hosted in a wall-mountable and properly cooled enclosure installed on-board of the telescope: Commercial Off-The-Shelf (COTS) industrial components (e.g. Beckhoff PLC and EtherCAT fieldbus modules) represent the core of the system to increase the overall reliability and maintainability.