Testing the constancy of the target axis with a change in focusing in precise theodolites and electronic tacheometers

The obtained measurement results show that for short targets of a few metres long, position changes of the target axis are considerable and even come close to lc (TC1610). Even in the case of the telescopes of precise T 2002 theodolites geared for close object measurements, target axis vacillations almost amount to 40cc. The course of vertical changes in the target axis vacillations is quite distinctive/typical/unusual/interesting? For all tested instruments the character of these changes was very similar. It is a phenomenon which is difficult to explain in a clear fashion. It should be also noted that the greatest changes take place with short targets up to about 10 m. Vacillations of the target axis above this length are considerably smaller and generally do not exceed 10cc. Although target axis movements expressed in terms of angular units are significant, their influence on accurate location of a point position is slight and does not exceed (especially for the horizontal component) a few hundredths millimeters. In spite of this changes in the target axis should be taken into consideration in designing high precision angular measurements for small objects.