Driving the design of a telescope for Advanced Virgo+ with back-scattered light characterization

In the context of the installation of new stable recycling cavities foreseen for the next Advanced Virgo+ phase 2 upgrade scheduled in 2025-2027, a new telescope will be developped and installed on the optical bench used to detect the interferometer output beam. In order to help identifying the best design for this telescope, a test campaign is being carried out on the scatter meter at LAPP to test refractive and reflective optics and quantify the contribution to back-scattered light of each type of considered telescope configurations- those with low-incidence curved mirrors or a simple telescope with lenses. This article will introduce this backscatter meter, show preliminary results obtained with this experiment and give some perspectives for the backscatter meter experiment.