Why is the Vertical angle of my total station changing depending on the Horizontal angle? 

Recently I have performeda series of surveys and I have started to question the accuracy of individualface left measurements due to the fact that the vertical (Vt) angle of thetotal station that I am using changes depending on where I aim it even though Ihave not touched the telescope.

At first I thought that this may be the dualaxis compensator at work correcting for minor errors in the level of the stationbut in this case should my display not show a constant Vt angle?

The stationalways returns to the same Vt measurement when returned to the same Hz anglesuggesting a systematic error in the station is there any way to modal it andremove it from previous readings? is this a common error? 

Does any one know of any literature that covers this problem ?

Thanks