Calibration of autocollimators

Autocollimators allow contact-free measurement of the inclination angles of reflecting surfaces via the angular deflec-tion of a reflected measuring beam. To date, PTB has calibrated these by means of the national primary standard for the plane angle, the angle-measuring table 220 (WMT 220). These calibrations are, however, only possible along one measuring axis of the autocollimator and at a distance from 250 mm to 550 mm from the reflector. In order to extend the cali-bration capabilities to spatial angles and larger distances, a novel calibration system for autocollimators, the Spatial Angle Autocollimator Calibrator (SAAC), has been set up at PTB.

The SAAC is based on a Cartesian arrangement of three autocollimators which are directed towards a reflector cube. Two of them serve as reference measuring systems; the third autocollimator is the object under calibration. It is located on a linear sliding carriage with which the distance from the cube can be adjusted from 300 mm to 1800 mm. By means of a two-axis tilting system, the cube can be tilted within an angular range of (3000  ×  3000) arcsec² around two axes that run vertically to each other. The autocollimator to be calibrated here-by records both tilting angles of the cube. Due to the Cartesian arrangement, each of the reference devices, however, only measures one of the two tilts as a practically plane angular deflection. These reference devices can be calibrated directly using the WMT 220. Measuring two plane angles thus ensures the traceability of the calibration of spatial angles to the national primary standard.

The SAAC makes it possible to calibrate autocollimators with respect to spatial angles and to characterize distance-dependent effects. Manufacturers who would like to improve their instruments, and users who apply them to measuring situations other than plane angles and fixed spacing, both benefit similarly from the new calibration facility. An example of this is the high-precision measurement of the shape of optical surfaces that are used to shape synchrotron or free-electron laser radiation beams.