Reader: Hi there. Can you explain the differences between Autoreflection and Autocollimation? Also, why would I choose to use one of these methods over the other?
TD Reply: Yes, let me answer your first question. Fundamentally, all this stuff has to do with the operation of alignment telescopes and autocollimating-type alignment scopes, in the pursuit of doing optical metrology. Assuming that is clear, I’ll go ahead. When you are using Autocollimation mode, the light beam from your scope is projected out toward the “target” mirror as a completely collimated beam; all the rays are parallel to each other, and to your line of sight. Then the target mirror reflects that beam back (again collimated) toward your scope. (We omit the rest of the details for now.) When you are using Autoreflection mode, this is not true. What you do in this case is, using that target mirror out in space somewhere, you focus your alignment telescope on a physical target which is usually located on the very front of the telescope, typically in the form of a precision reticle formed on glass. Many commercially available alignment telescopes come equipped with an Autoreflection reticle on the cover glass plate in front of their objective, at the very front of their cylindrical barrel. The difference is that the optical beam is focused at a finite distance, not at infinity, as in the case of a collimated beam. That finite distance will be just twice the distance from the front of your alignment telescope to your target mirror. So this will hopefully answer your first question.
Now for your second question: Why use one or the other method? This answer will take more space. Suppose the only instrument you have at hand for optical metrology is an alignment telescope which does not incorporate Autocollimation operation. In that case, while you cannot do Autocollimation with this instrument, you still can use Autoreflection to your advantage. Maybe your scope has its own Autoreflection reticle installed on the front. Then, you are good. Otherwise, you can make your own reticle for this. You can purchase a precision reticle on glass from several vendors, or if time or money prohibit that purchase, make a custom crossed-line reticle using thin monofilament line, stretched and centered over a metal frame. Make that frame so that it will attach accurately to the front of your instrument’s barrel so it coincides with the line of sight of that alignment telescope. That is really important!
Then, focus that scope on the reticle, by reflecting the beam from the target mirror you have installed in your equipment under test. The reticle (as well as the front of the scope’s barrel) will look smaller because they appear to be at a distance equal to 2X the distance from the scope to your mirror. At first, the reticle will probably be way off axis from the desired line of sight. Adjust the target mirror by tilting it in both vertical and horizontal directions as needed, until the Autoreflection target is exactly centered relative to the reference reticle inside your telescope. Now, that target mirror is “fairly-well” normal to your desired line of sight, established by the alignment telescope. How accurately “normal” will it be? That will depend on many factors. These include, how well you fabricated your home-made reticle, how accurately it is installed on that scope, how co-linear it is with the line of sight of that scope, and how far away your scope is from the mirror. But with careful workmanship and techniques, you can often get the mirror normal to a line of sight to within less than one minute of arc, perhaps to half a minute of arc, under the best of conditions.
Even if you have at hand an instrument which does incorporate Autocollimation mode of operation, as well as alignment and full-focusing adjustment, you can still utilize Autoreflection mode to your advantage. Often it is really difficult to find the autocollimated return beam target in your scope, because the target mirror might be tilted so much that the return beam target is completely out of the field of view. In this case, you can use Autoreflection mode first, to get the mirror close to the normal-to-line-of-sight condition. This works because with Autoreflection, the visible field has less sensitivity to the tilt of the mirror. Thus, you may be able to locate the image of that Autoreflection target and front of the scope barrel, and tweak it until it is close to alignment with the line of sight. Then switch over to Autocollimation mode of operation, with collimated light. Now you will be able to see the return beam target in your scope well enough to complete the adjustment of the target mirror until you have achieved the desired accuracy of alignment of your mirror relative to your line of sight! This can be done very accurately, with the right techniques.
Just one last aside comment: That Autoreflection target does not have to be installed directly on the alignment scope barrel, it is just normally done that way for convenience. It can be located a bit in front of your scope, if necessary. But in that case, you must be sure that the intersection point of the crossed lines is exactly coincident with the desired line of sight, otherwise you may not get the desired results!
TD
Optics Lab Tips and Tricks 