To determine the correct sign of the deviation angle of a penta prism the following equipment is used:

1. An autocollimator, such as the Model D-638, Coordinate Autocollimator, that measures within the accuracy of the part to be measured.  That is, to measure the deviation of a penta prism in arc seconds then the autocollimator should be able to measure in arc seconds.
2. A flat adjustable mirror, such as the Model D-571, 5.8-inch Adjustable Reference Mirror.
3. A tip/tilt table, such as the Model D-263, 3 Axis Lab Base.

Figure 1.  How to determine the correct sign of the deviation angle of a penta prism

The steps needed in order to accomplish this task are as follows:

1. Make sure that the mirror is set to be 90 deg to the autocollimator.  This can be done using a master penta prism, such as the Model D-614 the angle of which has been calibrated or determined according to the application note “Measuring The Deviation Angle Of A Penta Prism”.
2. Arrange the mirror and the autocollimator as shown in the picture above.  Placing the mirror 180° across from the location shown will give the opposite sign for the deviation angle.
3. Read the deviation of the penta from 90° using the autocollimator.