Advancing from plane to solid angle in autocollimator calibration

Autocollimators have proven to be capable of accurate and traceable angle metrology. Their characterisation and calibration is central to making full use of their potential by correcting residual angle errors. In the recent years, great progress has been made at the Physikalisch-Technische Bundesanstalt (PTB) in this field. However, important challenges remain, such as (1) the extension of traceable calibration from the plane to the solid angle and (2) the calibration of the effects of optical path length changes. In this contribution, we will present our latest efforts at PTB to build up a novel device for this purpose, the Solid Angle Autocollimator Calibrator (SAAC). It makes use of an innovative Cartesian arrangement of three autocollimators (two reference autocollimators and the autocollimator to be calibrated) which allows a precise measurement of the angular orientation of a reflector cube in space. Important engineering considerations of the calibration device will be discussed. Strategies for the adjustment of the components and the determination and correction of residual adjustment errors will be outlined which might be of general interest.