Angular Measurement
 Field Use of the
    Theodolite
 Taking Measurements
 Errors of Construction and
    Adjustment
 Collimation Error
 Horizontal Collimation
 Vertical Circle Index
 Plate Level
 Optical Plummet
 Miscentring
 Parallax in Telescope

Parallax in Telescope

View Through Theodolite Telescope With Cross-hairs Unfocussed

View Through Theodolite Telescope With Cross-hairs Focussed

Parallax Error occurs when the cross-hairs of the telescope are not focussed cleanly upon the target. There are two problems caused by parallax in the telescope. The obvious one is that if the cross hair is not focussed correctly then you will not be able to see it at all. The less obvious one is that if there is some parallax in the telescope (caused by the cross hair not being focussed correctly) then it is possible to look ‘around’ the cross hair. This can be seen during observation by lifting your head up and down slightly and seeing whether the cross hair moves with respect to the target. If the hair seems to shift then there is some residual parallax. The parallax is removed by focussing the telescope onto a target at infinity and then focussing the cross hair sharply onto the target.

The point at which the blue lines on the image above converge indicates the focus point of the incoming image. Because the telescope is not focussed properly, the image has not focussed on the reticule. For an accurate angle reading to be made, the focus point of the telescope needs to coincide with the reticule. The distance between the focus point of the telescope and the reticule is the parallax.

Failure to eliminate parallax can cause errors of a few tens of seconds.

This page completes the 'Angular Measurement' section of Choose a Topic. Use the next button to select another topic.