Reticles in autocollimators: change in image quality due to changed coherence properties

Autocollimators (AC) are used for different metrological measurement tasks. These instruments are compact and versatile and easy to use. Angle deviations of polygons, local slopes, fiducials, e.g., are measured with autocollimators. If calibrated electronic autocollimators are used, then the measurements can be performed with uncertainties down to 0.01 arc sec. The uncertainty might be increased further by one order of magnitude or more if surfaces with small apertures, aberrant surfaces or surfaces placed at long working distances are measured. To define the measuring window, respectively the limiting parameters, a model of the optical system was implemented. Aberrations, diffraction, temporal and spatial coherence are used. To increase the measurement range, a phase-shifting reticle can be used. Simulations made for small apertures show, that a significant reduction of the uncertainty can only be realized for a limited parameter range of the components used. For example, incoherent and coherent illumination are rather exceptions than standards. Partial coherence is used in a plurality of applications, e.g. for interferometry and space-bandwidth-limited wave front reconstruction, just to name a few. The system and the coherence function Γ must be tailored to provide specific operation. Almost all embodiments come with little changes of the light source's coherence properties only. For example, to use a spectral bandpass filter or to limit the size of the light source seems to be the standard solution for almost everything. Some error sources related to reticle imaging within an AC are identified. And the acceptable parameter range will be described.