Sub-μrad angular stability measurements by use of Long-Trace-Profiler-based systems

High accuracy angle measurement at the sub-mu rad level requires extremely high instrument stability. in order to reach sub-mu rad stability (0.1 are second or less) over long time periods, it is necessary to maintain the test object and almost all of the optical components in the measuring instrument in very steady positions. However, mechanical force relaxation, thermal expansion, and asymmetric structures produce angular and Linear displacements in the system, resulting in angular measurement error. A Long-Trace-Profiler (LTP)-based stable equipment is used to test precision angular stability with sub-mu rad resolution. Long term stability over 15 hours has been measured on different kind of mechanical structures. Temperature monitoring during the tests is extremely important. Some test results showing the effects of thermal variations are presented, which indicate that temperature stability on the older of 0.1 degrees C is absolutely necessary for repeatable sub-mu rad measurements. The optical method, using optics with an even number of reflecting surfaces (for example, a right angle prism, pentaprism, or rhomboid prism) to reduce the influence of existing angular displacement, is introduced and the comparison measurement is presented. An optical fiber transfer line is able to reduce the laser angular shift from about 10 mu rad to a level of 0.3 mu rad rms. Careful system configuration, design and operation are very important for the sub-mu rad angle stability.