VSTpol: the first large survey telescope for optical polarimetry

Replacing its unused ADC by a polarizing filter would transform the VST into the first large polarimetric optical survey telescope – VSTpol -, without affecting the current capabilities of the VST+OmegaCAM system. Scientific cases include: 3D mapping of the magnetic fields of the Milky-Way and Magellanic clouds, surveys of quasar polarization, polarimetric characterization and mapping of solar-system objects, follow-up of CTA sources and transients, polarisation measurements of bright transients identified by transient factories, such as bright gamma-ray burst afterglows, etc… The design of the VST limits the technical implementation of the polarimetric unit to a single-beam system: therefore, we will develop data-driven calibration methods to achieve a sub-percent polarimetric sensitivity and accuracy. Simulations show that a polarimetric noise level of 1.6 10-3 can be reached in 36 min of exposure time for V=16. In particular, this polarization sensitivity fulfils the specifications for the Cherenkov Telescope Array Optical Support Telescope. The proposed implementation is of low cost as polarizing filters of the needed size are readily available. It does not require major modification neither of VST, nor of OmegaCAM. The currently offered capabilities of the VST+OmegaCAM are preserved as the polarizing filter can be removed from the optical beam. Only small development efforts are needed on the design of the polarimeter unit to optimize accuracy and execution times, the calibration plan, and on the extension of the data reduction pipeline to extract the polarization signals.