To make the optical axis of an autocollimator, such as the Model D-638, Coordinate Autocollimator, parallel to an optical bench one needs an adjustable double-sided mirror, with high quality flatness and under one arc second parallelism - such as the Model D-616, Plane Parallel Adjustable Reference Mirror shown in the picture.

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

1. Place the autocollimator and mirror as shown in Figure 1 above.  Check to see if the return image is coming back into the autocollimator.  If not adjust orientation of the mirror and/or autocollimator until the return image is seen through the autocollimator's eyepiece.
2. Rotate the mirror 180° in the plane of the optical bench.  Check the autocollimator's eyepiece for the return image.  Adjust the mirror and/or autocollimator until the return image can be seen with the autocollimator.  The final steps to complete the alignment process may now be taken.
3. Record the autocollimator's elevation reading with one side of the double-sided mirror facing the autocollimator and then with the mirror rotated 180°.
4. Calculate the difference between the readings.  Adjust the autocollimator's elevation by half this difference and the mirror's elevation by the other half.
5. Repeat steps 3 & 4 until the autocollimator's elevation deviation reads zero with the mirror rotated to either side.  The optical axis of the autocollimator is now parallel to the optical bench.