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Smallest measurement uncertainties for autocollimators

High-resolution electronic autocollimators are used to determine small angular differences. Calibration of these devices requires appropriately small measurement uncertainties. With the PTB angle comparator now uncertainties as small as 3.4 · 10-8 rad or 0.007 arc seconds are achieved. This corresponds to a distance of roughly just 13 metres on the moon as viewed from the earth (distance approx. 384 000 km).

Calibration of a rotary table (index table) with autocollimator and precision polygon

Autocollimators are optical precision measuring instruments which determine small angular differences with the aid of a plane mirror. They are used both, in industrial metrology and in research and development. For example, they are deployed on machine tools or coordinate measuring machines to measure the angular deviations of guideways or angle measuring tables. Scientific applications, such as measuring angular differences in high-precision determination of flatness deviations (see PTB News 02.3), call for calibrated autocollimators with measurement uncertainties smaller than 0.01 arc seconds over a measuring range of roughly 300 arc seconds. Such autocollimators are also used in rotary balance experiments to determine the gravitational constant.

To meet these accuracy requirements, electronic autocollimators are calibrated in the Clean Room Centre on the PTB angle comparator, and thus directly traced back to the SI unit of the plane angle, rad. The optical measuring principle benefits from the laminar and extremely temperature-stable air flow in the Clean Room Centre. Metrologically, the key component of the comparator is a circular graduation pitch circle disc made of glass with 131 072 (= 217) radial graduation lines on 360°. The radial grating is incorporated on an air-cushioned precision rotor. It is scanned photoelectrically by eight reading heads using a novel reflected-light phase grating technique. A resolution of 0.0012 angular seconds can be achieved. By applying two separate and independent calibration procedures the angular deviations of the comparator can be determined with a hitherto unmatched uncertainty as small as 0.005 angular seconds (k = 2).

PTB can now calibrate high-resolution electronic autocollimators with an uncertainty down to 0.007 angular seconds (3.4 · 10-8 rad) (k = 2) and meet the present demands, in particular, in the field of research.

Contact at PTB:

Phone: +49-531-592-0