8 
The tests have demonstrated that an eccentricity of 3 microns 
causes an error practically of the same value on the Y direction. 
The vertical bearings rolling on the support tube must have a 
maximum eccentricity tolerance of 5 to 6 microns in order to hold the 
error within 2 microns. 
Perpendicularity between the guides is measured with the help of a 
calibrated grid. As a matter of fact this operation is carried out after 
the adjustment of the optical path, and it will be described later. The 
corrections are made by working on the couples of horizontal bearings 
of the Y guide and support tube. Once having made this correction it is 
advisable to recheck the motion in the Y direction in order to make sure 
that the parallelism between the tubes has not been modified. 
Having finished the mechanical rectifications we now pass to the 
mounting of the optical groups (see fig.6) and to the necessary 
adjustings for these groups. 
The optical calibration should lead to the following results: 
- system of parallel rays in the telescopic path, 
- satisfactory immobility of the mark as seen from the eyepiece during 
the movement of the X carriage, 
- centering of the axial pupils in order to avoid deforming the exit 
pupil, 
- coincidence of the real image formed by the first lens with the 
floating mark. 
These conditions are obtained by working on rectifying means which 
the instrument is equipped with. 
The only two conditions which are interdependent are the immobility 
of the mark during X motion and the centering of the pupil. 
Schematically, the optical calibration and rectifying operations 
may be summarised as follows: 
(1) Mounting the two switching prisms for "base in" and "base out" 
there are no rectifications to be made at the time of mounting, 
the only check that must be made is the one regarding the 
correct subdivision of the total run between the two prisms in 
question. 
(2) Mounting the mobile optical unit, which levelling has already 
been carried out during the mechanical mounting and calibrating. 
We pass therefore to the mounting of the two rotating prisms. 
With the same level and following the same criteria described 
above which we used for rectifying the mobile optical unit, we 
can level the guide shaft between the prisms support and the