  The KWISP opto-mechanical force sensor has been built and calibrated in the
INFN Trieste optics laboratory and is now under off-beam commissioning at CAST.
It is designed to detect the pressure exerted by a flux of solar Chameleons on
a thin (100 nm) Si$_3$N$_4$ micromembrane thanks to their direct coupling to
matter. A thermally-limited force sensitivity of $1.5 \cdot
10^{-14}~\mbox{N}/\sqrt{\mbox{Hz}}$, corresponding to $7.5 \cdot
10^{-16}~\mbox{m}/\sqrt{\mbox{Hz}}$ in terms of displacement, has been
obtained. An originally developed prototype chameleon chopper has been used in
combination with the KWISP force sensor to conduct preliminary searches for
solar chamaleons.
