It should be noted that in addition to being sensitive to odd order harmonics, such front-face back-face measurements are also sensitive to numerous other sources of misalignment such as collimation error, non-orthogonality between the standing axis and the transit axis, etc.
Because the range and the vertical angle to the two targets are equal, we eliminate any sources that may produce horizontal angle errors that are functions of nominal range and nominal vertical angle (for example, those arising from a squareness error, collimation error, beam offset along the transit axis, etc.
Evaluate of the standard deviation of a collimation error
(pointing to the scale stroke) is equal to 2.
is a systematic error, and in practical work, the adjustment is carried to where the error is so small that it is considered to be negligible.
The component along T is equivalent to collimation error
Two-thirds of this shift was from the diagonal's collimation error
2 is so forgiving of minor collimation errors
that you will probably never need to adjust the mirrors.
Insisting on good-quality mechanics when you buy your binoculars ensures that they will last longer before collimation errors
become a problem.
The area of acceptable star images is essentially the same size as StarShoot II's chip, so even small collimation errors
result in uneven stars across the field.
To make matters worse, I've found that my eyes are far less tolerant of collimation errors
in the dark than in daylight--at night, relatively minor collimation errors
can noticeably degrade performance.