External right-angle measurement using a two-autocollimator system.

We report on a non-contact method for external right-angle measurement using two autocollimators. A precise mathematical model is deduced to evaluate and deduct the measuring error. The values measured with our method are very coincident compared with the results measured by a ZYGO interferometer. The measuring accuracy is superior to 0.1 arcsec for the right-angle errors within 3.0 arcsec and becomes 0.4 arcsec for the extended right-angle errors within 8.0 arcsec. This method can be widely used in situations for external right-angle measurement, such as angle measurement for the body of torsion balance and test mass in spaceborne laser interferometry.