Category: Technical Stuff

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!

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!

Reader:  OK, so I need to build a testing setup of some sort, which will be permanent. That means it will sit in my lab and be ready for use, so I can just walk in and make measurements in a few moments, without wasted setup time. It should be permanent for at least a year or more. My boss suggests I use an Optical Breadboard Bench and set up my components on that. Is this the best way to go, in your opinion, Tony?

Tony:  Well OK, in my opinion, NO it’s not the best way to build a permanent testing setup. Unless you need the actual features of a Breadboard Bench, which I discussed in my last post, I recommend you do not use that Breadboard. Why? Several reasons come to mind, which have all happened to me before. First and foremost reason: You just got that testing setup perfected and it works really great. It has been 2 weeks in the lab, left undisturbed and willing and able to help you test stuff. Suddenly, some engineer with way more clout in the company comes in and NEEDS that bench for his testing. Since the boss respects his clout more than yours, your equipment comes off the bench and his goes on. (In companies, clout counts, and not necessarily common sense.)

Second reason, with Breadboards, those evenly spaced 1/4″ x 20 tpi holes seem convenient at first glance, but when you go to install your hardware exactly where you want it, those holes always seem to be in the wrong place. Off by 3/32″ or some other equally dumb amount. Either you have to monkey around with several adapters to get stuff to fit right, or you have to file holes into slots, so they are almost breaking out from the metal surrounding.

Third reason, hardware made for breadboards often rely on post holders and posts to hold components. It’s supposed to be convenient. Well maybe in the university lab it’s convenient to set up some demo or some experiment for teaching principles to students. But in a real-life testing setup, when you’re counting on it to work right every time, not so much. The first time that manager person comes into the lab to show your brilliant test setup to some customers, and bumps the detector (sitting on a post in a post holder) with their elbow, your detector (which took all day to align) is now facing the calendar on the wall instead of the light beam’s axis! Hey, you can easily bump it with your own elbow too.

Reader:  Well what am I supposed to do, Tony?

Tony:  Get yourself a nice thick piece of aluminum tooling plate which will be as long and as wide as you would ever need. Aluminum tooling plate is pretty flat on both sides, too. Attach your optics, adjusters, electronics, detectors, etc. exactly where YOU want them to be, by drilling and tapping that plate for any size screw you think is appropriate for the task. (Does not always have to be 1/4-20 screws.) YOU are making the decision on where those tapped holes shall be, not some mechanical engineer who is just designing a product to sell. For goodness sake, do use machine screws though. Wait, you don’t know how to drill and tap holes in aluminum? For goodness sake, you had better learn how to drill and tap for machine screws, and soon! Unless you have a good technician or a good machinist who is always at your beck and call, you had better get proficient at drilling and tapping, in aluminum, steel, and soft plastic.

One final thing. To support your optics and equipment, do not plan to use posts and post holders, unless there is a compelling technical reason for using them! Learn how to mount stuff on brackets which are firmly bolted in place and can not rotate or move the first time a person (you or the manager) accidentally bumps them with their elbow. You can buy nice construction components or construction kits, with brackets and cubes, etc. If the equipment is shaped weird or is big and heavy, make your own brackets. Learn how to design and make your own brackets and mounting hardware to suit a specific application. Mount components on that tooling plate rigidly, as though it had to go into space.

Reader:  Machine metal? Are you nuts?  Tony, I am terrified of using a drill press, much less a mill or a lathe!

Tony:  OK. Machine tools are dangerous and you should always get proper training on how to use them, safely!  So, in that case, get that good technician or a machinist who is always at your beck and call to fabricate the parts you need. If your company, school, etc. does not have a machine shop and machinist at all, you can design the parts, even if you have to do it on the front of an envelope with a pencil, and take it to a local machine shop in your town, to get a quote. Most machinists can make things like “brackets” to hold optics or components on a plate, without any trouble. By the way, when you find a good machinist and shop you trust, do cultivate their friendship and give them your business. They can be life-savers in industry.

Mount your optical testing stuff on that plate. Do your testing. If that engineer with all the clout comes in and needs the wooden table you have your tooling plate sitting on, (sigh) you and your technician can pick up the plate with its permanently mounted testing setup and equipment, and put it on your desk in your office for a few days, if need be, while that guy does his tests in the lab. Here’s the point my friend:  If that tooling plate and its rigidly mounted components are really put together well, you should be able to pick it up, move it to your desk, then bring it back later, and still have it aligned and working fine!   Do good work!

As the title implies, do I always need one of these, and which one do I need, and why oh why are they so expensive?

The quick answer is NO, you do not always need to use an Optical Rail or a Breadboard. For example, many good opticians do excellent work in their shop using very simple testing aids, such as lots of wooden blocks. I’ve seen opticians test large mirrors which were supported on wooden stands and blocks. It really depends on the type of testing you need to do.

When do you really need an Optical Rail?  When you have a bunch of optics hardware which is made specifically to reside and slide on a rail, and these are what you need to use, you probably ought to have that rail. Sounds kind of dumb, I know. But I have been faced with that problem in a lab. We had a bunch of really nice rail-mounted testing hardware, but no rail to be seen anywhere within 3 miles. That is just frustrating. If you need to slide one or more optical elements or detectors, etc. so that they move (smoothly) relative to one another, and then can be locked down where you want them to stay, you probably ought to have that rail so you can do your work. If you want to keep some optics fairly aligned to one axis, it might work. If you cannot afford to purchase a full-on Breadboard, or you do not have a crane to pick it up and move it around your building, there are some inexpensive Optical Rails with various attachments and hardware mounts which can be obtained from optical supply houses. If you must do your testing on a wooden bench, etc., then the Rail might be a helpful aid.

When do you really need an Optical Breadboard Bench? If you need to mount lots of optics and components all spread out on a 2-dimensional surface, and you want to be able to attach them to it (breadboard style) with all the same fasteners (usually 1/4-20 machine screws in the USA) then you probably will love a Breadboard Bench. They have a matrix of accurately spaced 1/4-20 tapped holes all over the flat surface, which by the way, can be quite flat! What usually happens is a lab is set up with a comfortable budget, so they buy a couple of massive 4 ft x 8 ft benches with super good support legs. (These wind up being heavy enough to use as anchors for the Queen Mary I.) Then 90% of the time, the optical setups they do require a tiny fraction of the space available on them. Oh well; I should not fret about stuff like that. These benches can be obtained in many styles, including some which have clever internal mechanical structures which reduce vibrations a lot, at least over certain frequencies!  In case you simply do not have the budget, or perhaps do not have enough space for a ship anchor in your lab, you can also buy small Breadboards which are made to order, and are thin and light weight, which can be better than other options. I once had one I could pick up and move myself. If you plan to do serious interferometry where you will be counting fringes or using software to analyze the fringes and patterns, you are going to wish you had a good, heavy, monster bench, and have it mounted on an air piston isolation suspension. Unless you are working inside a subterranean cave, you will probably experience nasty vibration problems with any flimsy support table for your interferometer setups.

Why are they so expensive?  Cause they are complex to build. If you don’t believe that, try making one yourself. And because the manufacturers have a captive audience.

OK, I have to do a test on a lens I have. I have this gadget called a lens holder which looks like a large open clamp, with an adjustment screw, which when turned tightens the rubber covered clamp tines around the edge of a lens.  It kind of reminds me of a clamp I used in chemistry lab to hold some glassware equipment? Is this thing the right way to hold my lens? Do I need a more sophisticated holder?

Well, we older folks have all been there at some time.  The true answer is, it really depends on what you are trying to do or measure regarding that lens. If you intend to make a quick and dirty measurement of the focal length of the lens and you are going to use a wooden desk ruler and a 3″ x 5″ index card to see the “image” etc., well then that clamp they gave you, which might be designed to hold an Erlenmeyer flask in the chemistry lab, could be good enough. You might have a budget of zero. We would understand.

But, if you are trying to do a sophisticated test or measurement, you will benefit from having a more sophisticated holder. There are so many what-ifs to answer.

Do you need to hold a bare glass lens or a lens in a barrel or housing? Do you need to tilt it around some axes in order to align its optical axis with something? Are you going to measure its image in air with some kind of measuring microscope? Are you going to try to “focus” that lens’s image to a fixed focal plane in space or on a detector you have? Or, (good grief) are you going to have to do interferometry using that lens? The Erlenmeyer flask clamp may not work at all in these cases. This post will get horribly long if I try to consider all of these possibilities in detail. So, to keep this one short, let’s say:

There are many kinds of opto-mechanical lens holders available. If you expect to make very small angular adjustments or very small linear movements with that lens assembly in order to make really accurate (or repeatable) focusing, or (good grief) interferometric measurements, then you are going to need a good, sturdy, stable, adjustable, lens holder, which may cost much more than that lens you have to work with! Even when the boss tells you that your measurement budget is “zero,” the fact is that your time spent in the lab is costing way more than zero. Time wasted on making a “serious” measurement or a “go/no-go test” on a lens will be worse than wasted, if your results of the test are erroneous because they gave you a cheap clamp to use.  Do good work!