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!

TD

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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.

End

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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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A Question About Telescope Collimation

Reader:  Hello. I am an optical engineer by trade and a newbie amateur astronomer. I bought a new Schmidt-Cassegrain telescope (SCT) recently. It is a “….” (brand and model deleted by T.D.)  I collimated this scope twice, using a bright star, per the manufacturer’s instructions. I have 2 questions for you:  First, why does everybody in this hobby refer to this process as “collimating a telescope?” My training in optics engineering tells me that this has very little to do with a “collimated” lens! And second, although I followed their instructions exactly and it appeared that I accomplished the goal of this adjustment, I am not happy with the images I get. They seem a bit “soft” to me; if you know what I mean. I expected this big SCT to deliver nearly perfect (diffraction-limited) images!  Am I doing something wrong? Help!

Tony:  Nice to hear you’re getting into astronomy as a hobby; it’s a good hobby. I can answer both of your questions. First, your question about the use of the term “collimating” is a good one and you are correct. For those readers who may not be familiar with this technique, the process known as “collimating a Schmidt-Cassegrain telescope” is a misnomer, in the field of optics, because it’s actually a procedure for “aligning” the secondary mirror of the SCT to the primary mirror. It is truly an alignment task. It’s only necessary to do because, in the case of many mass-produced amateur SCT scopes, their secondary mirrors (usually spherical figures by the way) are heavy, but poorly supported in the center of the scope’s corrector plate. Because of their inadequate mechanical support, they tend to get misaligned to the primary mirror (also a sphere) when the scope is jostled in transportation, or by rough handling.

But, you are correct; the term is a misnomer, in the sense that we “optics” guys use it. A telescope used for astronomy is normally “collimated” anyway, by virtue of the fact that it makes images of objects that are located at infinite conjugate. So it’s the reversed version of a “Collimator” which you might use in a lab. When you adjust that secondary mirror by tilting it, you’re just aligning it to the optical axis of the primary mirror which normally does not tilt.

Your second question requires a more delicate answer! In my career, I have examined a lot of telescopes used by amateurs (as well as some big-boy, professional observatory scopes!) Mass-produced amateur scopes (especially SCTs) often do not provide the exquisite, diffraction-limited imagery you might expect as an engineer working in the optics field! They are still good, in the sense that they don’t suffer from chromatic aberration, and they offer fairly large apertures for modest cost.

But, for one thing, they have an obvious central obscuration (the secondary) in their pupil. That means they suffer from apodization, by definition. This means their MTF (Modulation Transfer Function) is altered such that energy is stolen from the higher frequencies and added into the middle-range frequencies. (You can look this up in a good optics text.) The resulting apodized image is a peculiar animal. It just does not look like a classical-diffraction-limited image does. But even the big boy Cassegrain scopes used by professionals at observatories have the same issue. They usually have a secondary mirror component which is their central obscuration.

The other fact is, because they are mass-produced in a factory, and their manufacturers need to make a serious profit, their optical elements may not always be perfect. They can have some zonal defects, even if they are nearly perfect spheres! The images which result will vary a lot from one specimen to the next, even from the same lot of products made the same week! I observed this to be a fact, in my personal experience, in a previous occupation.

So, you may be adjusting your new SCT in the correct manner, and not doing anything wrong. If you continue to be uninspired by the imagery you are getting, contact the manufacturer of the scope and discuss this performance matter. Also, you might look through similar scopes which other amateurs are using, and do some comparative observations to see where your scope falls in the area of imagery performance. You might find you are doing as well as the rest are. Hope this information helps you!

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Optical Alignment Confusion

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!

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Unplanned Trips

It is rather remarkable how an unplanned trip to a hospital can alter your Blog plans!

While on a recent driving trip (to Yellowstone National Park and elsewhere) I suffered an illness which ultimately put me in a hospital. Had not planned on that. Most people don’t. To keep this post short, I wound up having emergency surgery and getting stuck in that hospital for 10 days. After returning home, had a different illness which required another surgery and a stay in a local hospital. Still recovering from these. Sure can mess up a person’s attempts at doing a Blog.

I will try to get back to some serious technical writing again, soon! That technical stuff is rattling around in my head!

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