There are many established technologies for precise characterization of the mirror geometries available. The paper presents a high precision measuring setup based on a single beam homodyne laser interferometer. The single beam interferometer is moved by a linear stage between the reference and measuring surfaces and delivers the differences between them. The reference mirror defines an absolute accuracy of the method. This point based method allows a high spatial resolution of the mirror shape and is suitable for measurements of the free form mirror geometries as long as the radius of curvature does not exceed the maximal toleranced tilt.<p> </p>The measuring results have been obtained for optics with dimensions of up to 50 mm and have been verified both for plan mirrors and for mirrors with radii of curvature in the range between 6 m and 10 m. A repeatability of the measuring results in sub-nanometer range can be shown.<p> </p>Especially for mirrors with a very big radius of curvature the knowledge of the exact position of the each measuring point on the surface is important for minimizing the errors of the mathematical fitting algorithms. Therefore a triple beam interferometer has been used for measurements of the stage position. The tight synchronization between all interferometer channels of 0.1 ns allows very fast “on-the-fly” scans of the surface.
A three-degree-of-freedom measurement system for the acquisition of the straightness and roll errors of a moving linear
stage is described. The horizontal (Δx) and vertical (Δy) straightness errors are obtained by measuring the lateral displacement
of a triple prism with a laser beam and position sensitive detectors. From two simultaneously performed vertical
straightness measurements the roll angle (Θz) can be calculated. The system consists of a cable-free reflector head
and a detector head. The position sensitive detectors have been calibrated using a precision x,y-stage equipped with two
plane mirror interferometers. Different position sensitive detectors are compared with regard to position sensitivity, linearity,
null-shift stability and sensitivity to the intensity profile of the detected laser beam. In combination with an already
known triple-beam plane mirror interferometer, additional information about the linear position (Δz) and the pitch (Θx)
and yaw (Θy) angle can be obtained from three parallel linear measurements. Thus all six-degree-of-freedom geometric
errors can be measured simultaneously.
Systematic errors of the three-degree-of-freedom measurement due to misalignment of the laser beams and geometric
errors of the triple reflectors are discussed. An approach for correction of those errors caused by the triple reflectors is
shown. The method is based on determination of the reflector geometry and calculation using the additional information
(Δz) acquired by the interferometer. Furthermore the metrological properties of the proposed system for the measurement
of straightness and roll are compared to other measurement principles. Experimental results demonstrate the
measurement capabilities of the system.