Characterization of the error budget of Alba-NOM

The Alba-NOM instrument is a high accuracy scanning machine capable of measuring the slope profile of long mirrors with resolution below the nanometer scale and for a wide range of curvatures. We present the characterization of different sources of errors that limit the uncertainty of the instrument. We have investigated three main contributions to the uncertainty of the measurements: errors introduced by the scanning system and the pentaprism, errors due to environmental conditions, and optical errors of the autocollimator. These sources of error have been investigated by measuring the corresponding motion errors with a high accuracy differential interferometer and by simulating their impact on the measurements by means of ray-tracing. Optical error contributions have been extracted from the analysis of redundant measurements of test surfaces. The methods and results are presented, as well as an example of application that has benefited from the achieved accuracy.