If you are involved in the design of new optical equipment, and you expect to perform an optical alignment at some point to “get it all right,” it makes a lot of sense to design that equipment with the process of optical alignment in mind. Some engineers think this is too much trouble. I suggest that trying to do a meaningful optical alignment after the fact, on equipment that was never designed to support this task is a whole lot of trouble!
I have designed and built several optical sensor packages for use on large optical telescopes. These were all accomplished this way, i.e. with subsequent optical alignment in mind. In fact, doing it this way made it a cinch to use alignment tooling techniques during the assembly of those packages, and consequently, most of the alignment work was done by the time the equipment was completely assembled! The task is not difficult, as long as you are “in charge” of the project, or at least have a friendly ear from the person in charge. Make a good case for this, and decide to do this up front, before too much design work gets cast in concrete.
So here is what to do: Start off by understanding how you are going to look inside the equipment package with an alignment telescope or autocollimator; in fact, using both of these or a combined autocollimating alignment telescope (AAT) is best to obtain a complete alignment result. Decide how to properly support that scope relative to the entrance aperture of the optical package. A fundamental and obvious requirement is that you need to locate the AAT or other scope not too far away from the entrance aperture, and you can be certain that during subsequent alignment work, neither the scope nor the package will be able to move relative to each other. The smaller the angular precision you will be working to, the more stable should be the Line Of Sight (LOS) between the telescope and the optical package under test!
Often it is good to know were the pupils (or stops) in your optical equipment will be located in relation to mechanical parts. If you need to refresh your memory about this topic, grab a good classical optical text book and read about stops and pupils. This is important stuff.
Next decide where to locate reference targets or mirrors, or a combination mirrored target within your equipment. This is important so that you can trace your LOS through your optical equipment under test in order to align various components to this desired path. If the optical path is folded, which is often the case in complex equipment, you need enough reference targets to establish the correct LOS through each fold point, i.e. going into and out of every fold mirror. The type of reference targets (or mirrors) you make and their particular dimensions will depend on your overall optical package configuration. Obviously, the less space you have available inside, the smaller these targets will have to be. Also remember that you have to be able to install these targets during assembly, but then remove them after the alignment is completed without disturbing the rest of the optics which have now been aligned. This part can be tricky.
Regarding the reference targets (or mirrors) or combination mirrored targets, they may be circular in form factor and attach to prepared tapped holes at convenient mechanical surfaces in your package, or rectangular and attach via a flat base to a baseplate surface in the package, if there is a baseplate. The important thing to design for is accuracy and repeatability of their location. You may have to remove one or more of these and then reinstall them later as part of the alignment process. Their position must repeat. If you are not the machinist on this project, talk to your machinist about this. The best way to attach reference targets for repeatability is to pin them in with two pins. Targets may or may not need to attach via screws; it depends on the job. But in most cases small screws are fine; so #4-40 machine screws might be adequate. Remember you’ll need to remove these targets after alignment, so they must not fit too tight on those pins! It helps to have relief cuts in a few places along the edge of the attachment base, in case you need to pry the device off its attachment surface.
A good machinist will “get these concepts” readily and can help you design the best configuration for your targets, to facilitate their installation and removal. Also, all good machinists are used to working to tolerances of one-half a thousandth inch. But for this job, you may need tighter tolerances; your targets are going to define the precise location of the desired LOS throughout the package. Good machinists can work to much smaller tolerances than the old standard of one-half a thousandth. They just need to understand the nature of the problem and what they’re being asked to fabricate. In these cases, the machinist is your best ally; don’t hesitate to talk with them about the design concept before you hand them a drawing. They can save you time and money if they understand which parts of the reference target require a very tight tolerance on dimensions and which parts don’t matter. Usually, it’s the location of the pins and the actual “spatial reference point” on the target that are important for alignments.
Now about those alignment targets: What you need is something you can see with your alignment telescope, which is neither too small nor too large, and which will allow you to see past it to the next target. These are typically evaporated chrome on glass (transparent) targets or stretched crossed-hair targets placed over an aperture. If you need to establish both centration and angular alignment for your LOS at a particular plane, then you can use a mirrored target which provides both a spatial reference point for the LOS and a mirror surface to facilitate doing auto-reflection or auto-collimation at the same time. Of course, a mirrored target will not allow you to see past it, so you will be obliged to remove it at some point.
You can buy alignment targets from vendors of optical tooling equipment or make your own. In either case, don’t sacrifice quality to save money. You can fabricate your own cross-hair type targets by using thin monofilament nylon (fishing line) material stretched and bonded to an aluminum support. You can also use thin wire, but I prefer the monofilament nylon which will usually return to their original position if they are accidently brushed lightly during work. Wire may deform and stay that way, in which case they no longer provide a reliable reference.
Engineers usually have their own preferred way of doing this work. There is probably no ideal solution. In my own experience, I liked fabricating my own reference targets and working closely with our machinist to get them exactly suited to the specific application. Most of the time I used thin monofilament nylon material stretched a little to form a crossed-hair target, where the intersection of the lines established the spatial reference for the LOS I needed to set up. Obviously, the two lines require very accurate placement when installed. This is where a good conversation with the machinist about the problem can yield a productive result. The machinist can provide very thin scribe marks made with a milling machine to delineate the location of the two monofilaments. An alternative is to provide very small holes through the frame through which the monofilaments will pass, which will locate their positions and intersection point per your spec. Accuracy is the key, and it is amazing what a good machinist or instrument builder can provide.
In a scenario where your LOS alignment is super critical, you can use alignment targets which are adjustable on their supports, then align each one separately on the bench using optical alignment tooling techniques. When perfect, you can fix their location on their respective supports via bonding, etc. This is a lot more work, but can be accomplished and might prove to be an optimal solution to a critical alignment requirement.
The important thing to take away from this article is this: Plan ahead for doing an optical alignment while you are still in the early design phase of a project. Design that equipment package with the process of optical alignment in mind. Don’t try to do optical alignment as an after-thought, where no provision was made to use and install reference targets. Thinking this process through in advance and designing accordingly will save you much time and grief, and may help get the project done within the budgeted time and cost.
Optics Lab Tips and Tricks 