Reader: My supervisor asked me to “align” some lenses on the bench for a test we need to do. He has a green laser pointer (the kind used to point to stuff during meetings and presentations.) He suggested that I just use that laser pointer to “accurately align” these lenses, and that it should be a “piece of cake” job, which should take me less than 1 hour. I have never aligned optics before, but I have a strange feeling about this. Seems too easy. What do you think about this?
Tony: Well my friend, this is a tricky question to answer, because the answer depends on so many variables. Actually your question will prompt me to write several future posts about “Optical Alignments.” This is a large subject, by the way! It deserves a proper and technically-detailed discussion.
I’ll attempt to give you a quick answer, although I would prefer to get much more details from you regarding the nature of the optics and your testing requirements. I personally never used a laser pointer to produce an optical alignment. Most “laser pointer” devices are NOT good optical beam-quality sources, since they are intended simply to be used to point at something! (Partly this is due to the kind of laser used in pointers.) They are not even close to what engineers and scientists think of when they require a “laser” for testing work! Your suspicion that your supervisor’s suggestion seems too easy is well-founded. Unless they tell you in detail what they want you to do, and that the test is only a “quick and dirty” test, whose results are only preliminary and not critical to any design, project, or product, you should be wary of this approach!
You say you need to accurately align some lenses on the bench. That’s actually a limited description of your task. Did your supervisor mention to what angular accuracy those lenses needed to be aligned? Does the term “align” mean they are all to be arranged in space so their respective optical axes are all co-linear? Does their common optical axis need to be co-linear with any other devices such as a light source or a detector? Do they need to be arranged in space so as to simulate their position inside some future housing or system? Are you concerned with lens tilts? Is the goal to provide an optimal image of some test object? In each case, we still need to answer the angular accuracy question! If this test is really a “quick and dirty” one where a “rough alignment” will suffice, maybe the laser pointer could be useful, but I still have some concerns about this. Did that supervisor tell you what to do with the laser beam and how to determine that those lenses were actually aligned to something by means of a crude laser beam? People talk about using lasers to “align optics” a lot. Often, they have never done a critical optical alignment with special optical telescopes designed for that purpose. Talk is “cheap,” compared with actually performing a test in the lab. Also remember that the laser (especially a green wavelength laser) may be dangerous to your eyesight, at full power, especially after passing through an unspecified series of lenses. Never ever look directly into the oncoming beam from a laser, but especially after its beam passes through some lenses! The beam could be focused, increasing its power density on your retina!
If your task is to align lenses to only about one degree of angular accuracy, you might possibly be able to do that with that suggested method. But that is a rough alignment for sure! If you must perform an optical alignment of several lenses to a straight line axis in space to a few minutes of arc or worse, to a few seconds of arc, you are going to need to use some sophisticated alignment telescope equipment and other ancillary equipment to get that job done correctly. With the right alignment telescope and/or an autocollimator, you can accomplish alignments to one second of arc or better. I’m not suggesting it is easy, but I’ve done it hundreds of times. However, you do need that specialized equipment, and they are expensive!
I have not really explained how to do any specific “Optical Alignments.” In the mean time, ask your supervisor for more details about that test. More on this subject will follow in subsequent posts!
Optics Lab Tips and Tricks 