Optische Distanzmessung : Doppelbild oder Distanzfaeden

In an attempt to automate the double-image distance measurement by photographing the rod image, the errors of telescope and prism must be either eliminated or corrected. In theory, corrections can be made, but in practice they do not appear to be feasible. A description is given of systems which are free of these errors, In these systems the deviations are caused by reflections. Especially in photography they show such disadvantages, that they are of no use in practice. A reasonable solution is obtained by the use of a rod which is marked from the centre into both directions. The results, obtained with this rod are satisfactory.

When measuring distances by the use of stadia hairs, focussing problems appear. These problems are discussed extensively. With the aid of a large series of observations, a formula has been found for the standard deviation in focussing.

A description is given of the possibilities for eliminating the influence of the incorrect focussing. Placing a stop behind the eye piece appears to be the best solution.

In the telescope with external focussing the stop should be placed in the rear focal plane of the eye lens. In the telescope with internal focussing the stop must be placed in the image of the plane, equally far in front of the focussing lens as the focussing lens lies in front of the crosshairs.

The distance measurements by means of double image as well as by means of stadia hairs, using a telescope with external focussing, turn out to be affected by temperature fluctuations in the same manner.

The telescope with internal focussing is more strongly affected by temperature changes.

Measurements of distances by means of stadia hairs with elimination of the influence of incorrect focussing are given. After this elimination both methods of optical distance measurement turn out to be equally accurate.