This may be obvious to some of you doing optical tooling but just in case you pondered this, here are my thoughts.
Some autocollimators are calibrated so that the operator can measure the actual angle through which their reflecting mirror is tilted. Some instruments do not have any calibrated reticles; they just have a central dot or point where two crossed hairs overlap. When your retro image is centered on this point, your reflecting mirror is supposed to be aligned normal to your instrument’s line of sight. Same goes for auto-reflection. Some auto-reflection reticles are calibrated but some are not, especially if you had to make your own reticle.
Do you need to worry about calibration or not, in this instance? In many cases you do not. If all you are trying to achieve is aligning your reference reflecting mirror (or more likely, the device to which it is attached) so it is normal to the instrument’s line of sight, then you will not need to measure the actual angle through which it is tilted at any point in the process. You just keep working at it until you see that the retro image is centered, and call the job finished. This was the simpler case.
If you are given a task where the reflecting mirror has to be aligned normal just to within a specified tolerance, and when you have gotten it within that tolerance you are finished, then a calibrated method of measuring the tilt is much more helpful. Or if you are involved in some experiment where a normal to line of sight is not the goal, but knowing the actual tilt angle right now is your goal, obviously a calibrated method is the right choice. It matters because the application at hand will determine what kind of instrument you need to use. If you do not have the right kind of instrument in your lab cabinet, you might be forced to rent one or buy one to get the job done right. That can be expensive. Brand new calibrated autocollimators are really expensive, at least in my opinion.
In a pinch, you can make your own auto-reflection reticles which could be calibrated to any chosen degree of accuracy, then use auto-reflection observations to measure the tilt of the mirror. This is not as accurate as the results you could get with a good calibrated autocollimator, but it may be good enough for your application. How you would do this is the subject of another tip to be published in the future. I hope this little discussion will be helpful to someone finding themselves stuck with a new optical tooling task.
Optics Lab Tips and Tricks 