Proposal for a universal test mirror for characterization of slope measuring instruments

Abstract

The development of third generation light sources like theAdvanced Light Source (ALS) or BESSY II brought to a focus the need forhigh performance synchrotron optics with unprecedented tolerances forslope error and micro roughness. Proposed beam lines at Free ElectronLasers (FEL) require optical elements up to a length of one meter,characterized by a residual slope error in the range of 0.1murad (rms),and rms values of 0.1 nm for micro roughness. These optical elements mustbe inspected by highly accurate measuring instruments, providing ameasurement uncertainty lower than the specified accuracy of the surfaceunder test. It is essential that metrology devices in use at synchrotronlaboratories be precisely characterized and calibrated to achieve thistarget. In this paper we discuss a proposal for a Universal Test Mirror(UTM) as a realization of a high performance calibration instrument. Theinstrument would provide an ideal calibration surface to replicate aredundant surface under test of redundant figure. The application of asophisticated calibration instrument will allow the elimination of themajority of the systematic error from the error budget of an individualmeasurement of a particular optical element. We present the limitationsof existing methods, initial UTM design considerations, possiblecalibration algorithms, and an estimation of the expectedaccuracy.