The problem with the telrad during nose rotation is that the barrel of the rotating nose is slightly off the axis of rotation of the optical tube. The shim attempts to fix this. It is placed beneath the base of the telrad either in the front or in the rear depending on what is needed.
The shim should be very thin on one end and maybe 3/16 inch on the other, and wide enough to fit under the base. On my NGT-18, the barrell is about one degree off. The base of the telrad is 7.5 inches in length.
A little trig gives us the answer. what is the degrees(atan((3/16)/7.5))? Plot it into excell, and you will see the answer is 1.43 degrees.
This means that the shim 3/16" thick will fix any problem up to 1.43 degrees.
This means that the shim, when placed under either end of the telrad base will change the pitch of the telrad with respect to the rotating nose by anywhere from 0 degrees to 1.43 degrees. The shim may be placed on either end of the base depending on where the rotational error is. Experimentation will provide the answer to this.
Approach:
1) locate star using telescope and lock scope mark rotating nose position
2) adjust telrad to center star.
3) rotate nose 180 degrees and mark position
4) note alignment error
5) place shim under top side of Telrad base 1/4 way in
6) rotate nose to original marked position
7) adjust telrad to center star
8) rotate nose to 180 degree position
9) note alignment error.
10) If alignment error has increased remove shim and place in the back of the telrad mount. Do steps 6-9. skip this step from now on.
11) If alignment error has decreased, but still is off, push shim in another small amount. Repeat steps 6 through 9. If error continues to diminish repeat step 11.
12) if alignment error has inceased, pull shim out a small amount. repeat step 11.
