Alignment methods for partial compensating lens of aspheric testing in a non-null interferometer

Careful alignment of optical elements is essential in interferometric tests. Misalignments of the key element largely influence the testing accuracy. For aspheric figure error testing, non-null tests achieve more flexible and economical measurements than the null ones. However, retrace error is induced due to the violation of null configuration, making the alignment difficult. In aspheric partial compensation testing, the partial compensating lens (PCL) as the key component needs careful adjustment. The aplanat alignment method is effective for the PCL adjusting with high accuracy employing a removable lens, which combined with the PCL as an aplanat. But its structure is complex. After describing this method, a PCL computer-aided alignment (CAA) method is posed basing on system modeling in a ray tracing software. The structure is simplified with computer calculations. The PCL tilt and decentration are easily aligned with a plane and a standard spherical mirror respectively, according to linear relations with wavefront coma aberrations on the detector. Alignment of the PCL was implemented with these two methods in an aspheric partial compensation testing experimental apparatus. Adjustment and aspheric testing results were presented in order. The CAA method is a generalized approach with simpler structure, while the aplanat alignment method is easy to carry out and suitable for industrial application.