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

TLDR

Synchrotron optics for third‑generation light sources and FEL beamlines demand slope errors below 0.1 μrad and micro‑roughness below 0.1 nm, requiring metrology devices with uncertainties lower than the surface tolerances and precise calibration. This paper proposes a Universal Test Mirror (UTM) as a high‑performance calibration instrument for such optics. The UTM provides an ideal calibration surface that can replicate a redundant figure, with design considerations and calibration algorithms aimed at achieving high accuracy. Applying the UTM eliminates most systematic error from the error budget of a single optical element measurement, overcoming limitations of existing methods and delivering the expected accuracy.

Abstract

The development of third generation light sources like the Advanced Light Source (ALS) or BESSY II brought to a focus the need for high performance synchrotron optics with unprecedented tolerances for slope error and micro roughness. Proposed beam lines at Free Electron Lasers (FEL) require optical elements up to a length of one meter, characterized by a residual slope error in the range of 0.1 μrad (rms), and rms values of 0.1 nm for micro roughness. These optical elements must be inspected by highly accurate measuring instruments, providing a measurement uncertainty lower than the specified accuracy of the surface under test. It is essential that metrology devices in use at synchrotron laboratories be precisely characterized and calibrated to achieve this target. In this paper we discuss a proposal for a Universal Test Mirror (UTM) as a realization of a high performance calibration instrument. The instrument would provide an ideal calibration surface to replicate a redundant surface under test of redundant figure. The application of a sophisticated calibration instrument will allow the elimination of the majority of the systematic error from the error budget of an individual measurement of a particular optical element. We present the limitations of existing methods, initial UTM design considerations, possible calibration algorithms, and an estimation of the expected accuracy.