NavList:

Hi Igor,

One possibility is that the entire index-arm assembly (micrometer, arm, mirror) is tilting a little bit in the index bearing.  That is, there may be too much radial play in this bearing.  Can you very gently rock the index arm with your fingers, exerting torque around the index-arm long axis, and see if anything happens in the eyepiece?  If, after letting go, you see a difference between counterclockwise torque and clockwise torque, then it may be time to very gently tighten the nut on the tapered bearing.

Just a guess.  It could be something else.  (Are the screws tight on the index-mirror-cell-to-index-arm joint?)  Maybe Bill Morris's book has some discussion; it certainly discusses the internals of this bearing, with photographs.

Your diagrams show a side error of only a couple of arcminutes, so the error in actual sights would be very small---you could just ignore it.  But it would be worth tracking down just to keep the instrument in top shape.

Cheers,

Peter