To Mirror Or Not To Mirror

In my previous post I mentioned using crossed hair targets as well as mirrored targets for doing alignments. It occurs to me that some folks doing optical alignments may wonder whether they should use a mirror-backed form of alignment target, and why they would use them, or just stick with the less expensive stretched-filament type of crossed hair targets supported in air. So here’s a discussion of this topic.

We know from basic geometry that any two points in a three-space will determine a straight line through space. So as long as you are trying to generate a single straight line within a piece of equipment (it does not matter whether it is optical or mechanical equipment), you could argue that you need only 2 reference points in that package to define your desired line of sight (LOS). By the way, all optical tooling alignment work is based on setting up some desired line of sight (LOS) by means of optical observation. The equipment and instrumentation vary a bit. When you have to fold a beam, then a new LOS will be required.

A mirrored target is just some sort of precision alignment target with a very high quality flat front surface mirror placed behind it. Sometimes, the target is actually fabricated right on that front surface mirror. You can buy various versions of these mirrored targets from vendors of optical tooling equipment; usually they are the same folks who make alignment telescopes (ATs). You can also have a target custom made to suit your needs for special applications. It is not easy to make your own, so this is not a recommended way to go, unless you have a sophisticated optical fabrication shop as part of your facility.

If all you need to do is set up a LOS between two points in your equipment under test, and the exact direction in space is not too critical (so long as it goes through those two points) then you can do this with a good alignment telescope and two stretched-filament crossed hair targets supported in air. You can make these yourself. A series of LOS observations with your AT will establish the axial LOS within your equipment under test.

But some applications are more complicated. A common situation in optical equipment requires a LOS through a plane which not only goes through a reference point, but also is normal to that plane. In this case a mirrored target may be the best choice. Then you can use auto-reflection or auto-collimation instruments to set up your LOS. This application mandates that the front surface mirror is located accurately so that it establishes the same “plane” you care about. This can be tricky. This is when an honest discussion with your machinist about the problem can save you lots of grief.

In a project I did fabricating a sensor package for a large optical telescope, I needed to get the optical components within it aligned to the theoretical optical axis of the main telescope, which would enter this package at dead center of a round aperture and normal to the plane of that aperture. I built my own mirrored target in which the mirror was removable. Once I got the LOS from my autocollimating alignment telescope to coincide with the reference center (point) and also be normal to that mirror, I removed the mirror and could then look past the crossed hairs to other components within. The mirror had to go back in place with high repeatability when I needed to use it. That is difficult to do when you are talking about LOS angular repeatability of 2 arcseconds or less, but it can be done. It worked. My assembly and alignment project was successful.

“Whether to mirror or not to mirror” is a decision which should always be driven by your application. Do good work!

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Designing Equipment With Alignment in Mind

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.

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To Calibrate Or Not

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.

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Autoreflection and Autocollimation Compared

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!


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Photon Counting Task

Reader:  Hello. My company assigned me to a project which involves doing photon counting. We’re just getting started on this. During our first staff meeting, some people suggested using an expensive professional-grade CCD camera as our prime detector. But one engineer suggested a photomultiplier tube was the best detector to use, since we don’t need to acquire any spatial-resolved data for this task. Some staff at the meeting said this was an old-fashioned solution. What’s your take on this?

Tony’s Reply:  Since I don’t know any of the details about your project, except that you stated that you “Don’t need to acquire any spatial-resolved data . . .” my reply to your question is pretty easy.

A CCD camera, regardless of its quality or cost, is not mandated, because you have no need to record photon arrival along with spatially-resolved information, i.e. imagery data. If you basically need to do photon-counting of some weak light flux, then a photomultiplier tube, suitably chosen for this task based on its specs, and calibrated by the tube manufacturer is an ideal way to go. Here’s why:

The classical photomultiplier tube (a glass vacuum tube with a photocathode and electron-multiplier dynodes) is inherently the closest thing to a perfect photon detector. This detector may be considered “old fashioned” by some younger engineers, but the fact is, the photomultiplier tube (or PMT) is still the ideal photon detector. For this short blog I won’t go into all the reasons why it is unbeatable.

As for doing photon-counting, there’s a list of questions you will need to answer before you buy components and set up your photon-counting system. PMTs come with many different photocathodes (hence different spectral responsivities); you need to decide which is best for your application. PMTs come in different physical package sizes and shapes. You need to decide, etc. For most serious photon-counting jobs the PMT will need to be cooled to a low temperature (we’re not talking liquid nitrogen here) so it probably ought to be installed inside a special cooling chamber with a suitable window, which will transmit the spectral band you are interested in. This affects what kind of form-factor PMT is appropriate to use. Other specs for the PMT need to be considered, based on the photon-counting task at hand. Finally there is the topic of the electronics hardware you need to support the PMT, in order to get reliable results for the actual photon-arrival-rates which you want to record. But lots of researchers have done this with great success, most notably astronomers and nuclear instrumentation engineers!

So, my quick answer was:  PMT – Yep, that’s the way to go. But the real answer for your task will be longer, because there are a lot of engineering decisions you need to make, based on your application. Best thing to do is contact some PMT manufacturers and talk with one of their Application Engineers about this. They are going to be really knowledgeable on PMTs.


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Optical Alignment – Question #2

Reader:  Thanks for the previous reply. I have more detailed information for you this time, and another question. My supervisor tells me that our lenses which I have to test will each be mounted in its own metal cell. Our test needs to align them to a common optical axis and test them working as a system to produce an image of a test target. The target will be back-lit and use white light. The goal of this test is: determine the correct spacing between each lens which will result in the very sharpest image of that target. This has been calculated already using software, but we need to convince ourselves that the designer was correct! Finally, one of our other divisions has an alignment telescope which we can borrow from them to help with this testing. It is a Davidson Optronics D-275 Model Telescope and it’s in good calibration. How can I use this instrument to align those lenses and help make this test work?  Thank you!

Tony:  Yes, you can use that alignment scope for this test. First, you need to ask your lens designer if it is safe to assume that these lenses when mounted in their respective cells do not suffer from any “wedge” problem. If they do not have any significant “wedge” issues, then the optical axis of each lens will be co-linear with their mechanical axis. (Note that it is not a given that this will always be the case. I have seen purchased lenses which met all the procurement specs, but had bad wedge issues, because this had not been specified during procurement; a bad mistake. As a result, their optical axes were skewed with respect to their mechanical centerline axes.)

Next, make a cross-hair reticle to mount on each of the lens cells. This can be as fancy a reticle as you can afford to make. Or simple. You can have these made and mounted in your machine shop, if you have the time, and you need them to be perfect. Otherwise, make your own reticles with mono-filament nylon fishing line from a sporting goods store. Use a line which is around 10 mils thick. Mount two pieces of this mono-filament nylon line on each lens cell so that they cross at the mechanical center of each lens. These lines do not have to be at exactly 90 degrees to each other. The important thing is that their intersection (crossed-hairs) is as close to the exact center of the lens as possible. Mount them under some tension, so they do not flop or flex. This concept assumes that the lens does not exhibit any wedge! When they all have cross-hair reticles mounted, continue on.

Before you begin to mount these lenses on your test bench, first set up the D-275 Telescope behind the place where you expect the image plane to be, so that it’s looking directly at the center of your test target, and so that its line of sight is passing directly through the center of your image circle in the image plane. Mind you, this is not easy to do, it will take some effort and time, but it is critical. So do this part well!  What this will accomplish is the line of sight of that D-275 Telescope will represent (take the place of) the actual optical centerline of the lens system you will be testing. (Remember, you can focus the Davidson D-275 scope anywhere in space from infinity down to 16 inches away from the front of the lens barrel; so take this into consideration when setting up the scope.) Note, from this point on, do not move the D-275 Alignment Telescope in any way! It is now your reference line of sight. When you are sure you have this condition set up, then continue on.

As you insert each lens and cell into your test bench, refocus the alignment scope as needed until you see the reticle on that lens cell in sharp focus. Move or remount each lens cell until that cross-hair intersection is located exactly on the center of the thin black internal reticle of your alignment scope. You may have to use a bright light source to illuminate the lens reticles to see them well. Repeat this step with each lens & cell until they are all mounted on your test bench.

When you have all the lenses aligned to the line of sight of the D-275 scope, you can be fairly certain that their optical axes and mechanical axes are co-linear and aligned to your test target and image circle. Note that when you have to remove the Alignment Telescope, (I assume you will have to remove it to proceed with your image quality evaluation), it would be ideal if you had it mounted in such a way that it could be replaced back on that test bench and returned to the exact position in space where it was before. That’s in case you need to repeat the alignment. Otherwise, you will have to realign your line of sight again! This may be tricky to do, but it’s well worth the effort to get it right the first time! (Experience speaking here!)

Of course, if your lenses need to be in very close proximity, so that the reticles interfere with lens surfaces, then you will have to cut them off prior to final adjustments of the lenses. This is where you would appreciate a “cleverly made” reticle which can be removed and reinstalled any time. We do not always have that luxury. You may need to remove them anyway, to prevent them from optically perturbing your image evaluation. It depends on the details.

Then, just move those lenses back and forth, but only in their axial direction, until you find their optimal spatial positions for the best image results. I am assuming that you have a mechanical means for precisely adjusting the axial position of each lens. You are going to need that!  I have omitted some details in this overview. Since your company has done other optical testing, it’s fair to assume that you will have the ability to work through some of the detailed engineering steps, which you are likely to encounter in this work.

Good luck with your testing. I hope this information will help you.  T.D.

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Speaking About Telescopes; I Have a Question

Reader:  Speaking about telescopes, I have a question you might be able to answer. I recently looked at some Internet photos of the Hobby-Eberly Telescope at McDonald Observatory, which is in Texas. The photos of the primary mirror which is a segmented mirror in the shape of a hexagon do not show any opening in the center. It appears to be pretty solid, except for the segmented pieces. Question:  How do they get light to the Cassegrain focus, which should be behind the primary mirror? Is there some trick they use which is not obvious? Is this telescope not a Cassegrain scope? Your “About Me” page says you worked at astronomical observatories. Thanks for any light you can shed on this (no pun intended).

Tony:  You asked the right person this question, I suppose!  As it turns out, I worked at the Hobby-Eberly Telescope (we referred to it as the HET) at McDonald Observatory. While there, I was their on-site optical engineer for that scope. So, I definitely know the correct answer to your question.

The HET is not a Cassegrain telescope. It is a Prime Focus type of telescope. Therefore, there is no secondary mirror at all. There are mirrors up near the Prime Focus, but their purpose is to reimage the pupil of the scope on a corrector surface (also a mirror.) More on this shortly. The primary is segmented as you said, incorporating 91 individual mirror segments, each of which is a hexagon shape, one meter across the flats. So, the resulting primary is enormous, as you might imagine. That’s what makes the HET a wonderfully useful telescope for doing spectroscopy of faint objects, which is its main reason for existence!

The figure of each of the mirror segments is spherical, and they all have the same radius of curvature, so when they are adjusted correctly, they all fit into the same global sphere. Of course, the resulting large spherical mirror will suffer from spherical aberration. This aberration is corrected very nicely by a “corrector mirror” which has an aspherical surface. Its figure is “just right” to cancel the spherical aberration. This is located up close to Prime Focus. The image at Prime Focus is analyzed by a spectrograph, which is how data is acquired by the astronomers, for whatever research they happen to be doing. When I worked at HET, there was a low-resolution spectrograph located in the Tracker Assembly right at Prime Focus, and two other systems, a medium-resolution and a high-resolution spectrograph located in the basement below the scope. Light was fed to these spectrographs by means of fiber optics originating at Prime Focus and terminating at the appropriate spectrograph optics below. I don’t know if that configuration has been changed, now. I believe the HET is being used for a new research project.

Hope this helps you get a better understanding of the Hobby-Eberly Telescope. Thanks for your question.

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