VSTPOL: upgrading the VST instrument control software for the new wide-field polarimetric mode

VSTPOL enhances VST’s capabilities by adding optical polarimetry via a linear polarized filter. This will make the VST the first large wide-field survey telescope with optical polarimetry. The project addresses the need for optical follow-up observations of Cherenkov Telescope Array (CTA) sources and transients. This paper describes software upgrades required for the new polarimetric mode. The current instrument control software, based on ESO VLT software 2011, manages pointing, acquisition, and active optics. The polarimetric mode necessitates two additional motorized movements: inserting the filter and selecting polarization while tracking the object. Traditionally, VLT systems use a Local Control Unit (LCU) on VxWorks for motor control, but this system is outdated. Since compatibility with modern hardware is crucial, we resorted to a PLC-based system, which are unsupported by the installed VLTSW. Fortunately, the ICS Fieldbus Extension allows for a dedicated Device Control Environment (DCE). This DCE, using an updated VTLSW release, acts as a gateway to control electronics, minimizing system-wide impact and reducing update-related risks.