We describe a high-precision angle sensor and a new calibration system
for calibrating the linearity error of an angle sensor under in situ
conditions without the need for accurate reference instruments. To ach
ieve accuracy in the nanoradian (nrad) order, we elongated the critica
l-angle prism to increase the sensitivity, and we added a thermal cont
roller to the angle sensor. This served to suppress thermal drift of t
he Light source to improve stability. The proposed angle sensor demons
trated a stability of 3 nrad over 1 min, and the maximum repeatability
error of the calibration curve determined by the in situ self-calibra
tion method was approximately 35 nrad with a measurement range of 500
mu rad. (C) 1998 Optical Society of America.