This procedure uses two autocollimators such as the Model D-638, Coordinate Autocollimators and two mirrors like the Model D-571, Adjustable Reference Mirrors to measure the deviation angle of very high precision penta prisms that have deviations from a true 90 degrees under 5 arc seconds.

Using the penta prism to be calibrated, position both the upper and lower (or right and left, depending on configuration) mirrors so that the projected reticle return image is within the field of view of each autocollimator.

When the autocollimator on the left observes its projected reticle's return image with the penta prism in either position 1 or 2 and the autocollimator on the right observes its projected reticle's return image with the penta prism is placed in either position 3 or 4, the calibration measurements may be made.

1. Record measurement 1 with the autocollimator on the left and the penta prism oriented as shown in the figure above.
2. Rotate the penta prism 90 degrees clockwise and make measurement 2 using the same autocollimator (the one on the left).
3. Rotate the penta prism another 90 degrees clockwise and note measurement 3 using the other autocollimator (the one on the right).
4. Rotate the penta prism another 90 degrees clockwise and record measurement 4 using the autocollimator on the right.

The following calculation provides the penta prism's deviation angle:

Deviation Angle from perfect 90°  =  [ (3+1) - (2+4) ] / 4

The above formula cancels out all possible misalignments between the two mirrors and/or the autocollimators and the mirrors.

Furthermore by taking 4 readings and dividing by 4 the uncertainty error of the measurements is reduced by a factor of 2.

The penta prism's starting point could be 1 or 3.   Starting from 2 or 4 yields the opposite sign for the deviation angle.   Therefore, it is important to take the measurements in the order described above.