Performance of the first mechanically-coupled twisting heliostat: encircled energy vs angle of incidence, and closed loop tracking

The first automatic, mechanically-coupled twisting heliostat, with 8 m² reflector, was completed and tested on-sun in January 2024. It was set up on a target-oriented dual-axis mount, with the target-axis aimed at a target 113 m due west. The shape-twisting is purely automatic, made by a mechanical cam coupling to the cross-axis, which turns as the angle of incidence of the sun’s rays on the reflector change throughout the day. Three unsaturated images were recorded at different times of day of the sun on the target, reflected at angles of incidence of 5°, 49°, and 68°. The measured FWHM of the three images is very similar, up to 1.13 m or 10.0 mrad, compared to the ideal solar disc of 9.3 mrad. The encircled energy measured for all three images showed ≥ 85% within 1.34 m diameter (11.8 mrad). We report here also on a novel, closed-loop tracking camera for the target-axis mount, which employs a semitransparent beamsplitter fixed perpendicular to both the mirror surface and to the plane of incidence. During 2.5 hours of measurement of closed loop tracking, errors were ≤ 0.36 mrad rms in both tracking axes.