Autocollimator and angle dekkor

Autocollimator

An autocollimator is an optical instrument that measures very small angular tilts (down to 0.1 s) of a reflecting surface at a distance. It combines a collimator and a telescope.

Principle: A target (cross-wire) at the focus of an objective lens gives a parallel beam. If the beam strikes a plane mirror exactly normal to it, the beam returns on itself and the reflected image falls on the target. If the mirror tilts by an angle $\theta$, the reflected beam turns by $2\theta$ and the image shifts by

$$x = 2\,\theta\, f$$

where $f$ is the focal length of the objective.

 lamp--> [cross-wire] --|beam splitter|--> objective --->| mirror
                               |            (f)  parallel |
                           eyepiece  <---- reflected beam-|
                           micrometer

Light from the lamp illuminates the target, passes through the beam splitter and objective, goes to the mirror, returns and forms an image of the target in the eyepiece. The shift is read by a micrometer eyepiece graduated in seconds of arc.

Uses: checking straightness and flatness of machine beds and surface plates, squareness, parallelism of slideways, angle of prisms and polygons, and calibration of angle gauges and indexing tables. The mirror is fixed on a carriage moved along the bed.

Angle dekkor

It is also an autocollimator but with a fixed illuminated scale and a fixed graticule with a cross-line in the eyepiece. The reflected image of the scale is seen superposed on the fixed graticule, and the reading gives the tilt directly in minutes. Angle dekkor measures angles up to about $\pm 1^\circ$ with a least count of 1 minute (and 0.5 min estimated).

For measuring an angle of a component, slip gauges and a sine bar are used to set the working face to a known angle, and the dekkor reading shows the difference from the nominal angle (e.g. checking angle gauges, taper plugs, and bevel angles).