Month: September 2020

Normally I do not discuss my experience using a particular piece of opto-mechanical hardware such as this. In the case of this item, I decided to deviate from my normal practice and talk about this item and my own experiences, because I felt it might be useful to others who might benefit from this information.

I had an experimental application which needed a high-angular- resolution adjustable mirror mount which satisfied certain specs. Other mirror mounts to which I had access did not work for this experiment. I found this item via an on-line web search of Newport MKS products. To my surprise, it seemed that this device could satisfy my requirements for my experiment if I could believe their stated specifications. Although this device can hold a round mirror which is limited to 25.4 mm (one inch) in diameter and I would have preferred a larger mirror capacity, I decided to give this device a try because its other specs were compatible with my experiment.

Therefore, I purchased one of these mirror mounts and received it quickly since Newport had it in stock. Before continuing with my discussion, I should emphasize that I did buy this device like any other customer, and I have no other business relationship, nor any other sort of connection with Newport MKS at all. My “review” of this product is therefore unbiased, and of course it is based only on my personal experiences with one (1) specimen of a particular product from this supplier. A sample size of “one” certainly does not qualify as a valid statistical sample for testing and readers will hopefully understand my reluctance to assume that my experiences will apply to all of these devices. However, with that caveat, I will proceed.

I needed a mirror adjuster which would have the highest possible angular resolution, adjustment sensitivity of 2 seconds of arc, high stability, very good repeatability, and low thermal drift. This particular product seemed to suit my requirements. This Newport SX100-F3KN-254, 3-Knob Suprema XTE Kinematic Mount was described as “the highest performing mount in the Newport Suprema family, as a result of its new actuators, its materials, and its design configuration.” The part of its design which really appealed to me was the fact that it incorporates three (3) threaded screw actuators each of which has 254 threads per inch. After doing a somewhat cursory search on the web, this seemed to be the finest thread-cut for screw actuators which I found, at that time. The other good feature of this mount was its construction with stainless steel alloy giving it a 38% lower coefficient of thermal expansion (CTE) than aluminum mounts, per the manufacturer. The fact that it used a kinematic mounting support design with three screw adjusters having the same 254 TPI threads gave me the option of tilting the mirror in two orthogonal angle directions as well as translating it along its “z” axis direction, which in terms of translation, I did not need to do.

Some of the technical specs which the manufacturer stated are:

Kinematic mount mechanism

Optic Diameter = 25.4 mm

No. of Actuators = 3 with Knobs to adjust them

Screw Thread for Actuators = 254 TPI

Material = Stainless steel

Feature = Low CTE, (low thermal shift)

Angular Range =  ± 7°

Sensitivity =  1.5 arc second

Bottom Support = Clearance hole for 8-32 screw is provided

The sensitivity spec was exactly what I was looking for. They added the comment that this spec is based on the assumption that the smallest possible manual angular adjustment which the user could make is a rotation of one degree of the adjuster screw knob, which sounded reasonable to me.

When the Suprema XTE Kinematic Mount arrived I inspected it visually and tried turning the adjuster screws. The manufacturer had stated elsewhere that in addition to the ultra-fine adjustment resolution of the 254 TPI screws, this mount had, (here I quote them exactly) “. . . the Suprema XTE mounts have consistent, smooth adjustability and feel, owing to the 3V kinematic configuration of the contact points. Because of the 2-point V-groove contact across each of the adjustment screws, the turning resistance is the same on each actuator.” They use polished carbide pads for those contacts. My observation agreed with their claim. The three screws turned very smoothly with what appeared to be identical resistance and there was no evidence of sticktion that I would find objectionable. They turned in either the CW or the CCW direction with the same ease. It seemed like the threads were lubricated with something but I did not investigate this.

For my application I would “push their operational specs to the limit.” I needed to adjust the one inch round mirror’s tilt in one direction only, but I wanted to be able to tilt that mirror from an arbitrary zero position by angles of 3 arc sec, 6 arc sec, 15 arc sec, and 30 arc sec. This would be done repeatedly for numerous trials, and I needed good repeatability between trials. It was not essential that the mirror could return to exactly the same zero position, as I could get around this. I knew I would not be able to do these tests unless I had some kind of reference method to help turn the screw knob through a known angle. But, I did not want to invest a lot of time or cost in preparing a superbly accurate method.

My compromise was to prepare a scale with a computer drawing program which had radial lines and tick marks indicating angles of 0°, 5°, 10°, 15°, and 20°, with added tick marks indicating half-way values between these. After printing this on paper I checked it with a draftsman’s protractor and found it to be good enough. I used some mechanical components I had in stock to make a pointer and attached it to the screw I was going to adjust. It attached directly to the adjuster screw shaft behind the knob so that it would not slip. I placed the scale on a card backing and attached it to the assembly with brackets I had in stock, almost touching the metal pointer which was made from precision micro-shaft material. This arrangement provided a means to rotate the adjuster screw by known angles (approximately of course) with unknown repeatability. Not a perfect solution; but I determined by experiments that this would work for my tests if I exercised some patience.

Proceeding with my planned experimental application, I used the Newport SX100-F3KN-254, 3-Knob Suprema XTE Kinematic Mount to tilt the 1 inch diameter mirror, and examined the results with a calibrated autocollimator capable of reading mirror tilts directly to 0.2 arc second. About 30 trials were made initially. These trials were aimed at becoming familiar with the operation of the mount and its added scale and pointer. (The autocollimator was known to work as advertised and to be accurate.) Further trials (about 50 over several days) were made in support of the experiment. The results do seem to confirm the specs given by Newport MKS for this device.

In short, it seems that an operator can make a change in the mirror angle as small as 1.5 arc sec with good reliability, if and probably only if they can be assured that the adjustment knob is rotated accurately through the chosen small angle. In my experiment it became obvious to me that what limited the accuracy of my results was not the performance of the kinematic mount, but definitely my ability to set the pointer on that shaft to the appropriate tick mark on my scale. My home-made scale was surely not a perfect rendition of what might be a professionally made scale.

An example data result from one of my tests is given here:  A set of trials tilting a mirror surface by 15 seconds of arc from a common zero:

Theoretical value sought = 15.00 (sec)

Number of data values (trials) = 29

Mean value = 14.9532

Range = 0.8

Median value = 14.99

Modal value = 15.00

Standard deviation = 0.20889

After analysing this data, I “monkeyed” with the mount and autocollimator and verified that I could not set the pointer on the desired tick mark perfectly, and that was a clear error source. Also, the autocollimator is capable of reading a tilt angle directly to 0.2 arc sec, but recording data to two decimal places meant I was estimating the second decimal place.

My conclusion:  The SX100-F3KN-254, 3-Knob Suprema XTE Kinematic Mount from Newport MKS is capable of doing what the manufacturer says it will do! The person using it will need to devise a method of accurately turning the adjuster knobs by a small incremental angle. Alternatively, the user can use a good autocollimator to optically measure the tilt angle that is produced by some uncalibrated adjustment of a knob. I rate this device as a very useful and reliable mount for small angle adjustments of a mirror, for applications where a 1” diameter mirror is suitable.

Thanks for reading. Do good work!

End

Are alignment telescopes and autocollimators hard or easy to use?

Good question! The answer is not simple and it is non-trivial. Before getting to the answers I propose to give, I’ll say this about those instruments: They are (both of them) good, useful, adaptable, and in some applications also indispensable measuring instruments. My own work experience with autocollimators and alignment telescopes has spanned over 40 years, and I guess that comment gives away some fact about my age! I have used various types of both instruments made by different manufacturers. All my experiences with them were pretty good.

When I got the fortunate opportunity to work hands-on with developmental optical equipment, I sort of accidently “fell into” the application of both types of instruments while doing accurate optical alignment. I discovered in less than the first week that I loved doing that stuff, and worked to learn about alignment, practice it, use it, and continue to the present day. Thus, my own personal story with alignment telescopes and autocollimators is one which includes a lot of hands-on experience, exploring different equipment, and learning some lessons the hard way.

Back to the question, with a quick answer first. Using these instruments can be easy or hard; it depends upon your experience, the tasks required by your application, but mostly it depends upon your attitude towards this work. Yeah, really, your attitude. That’s because some people find this kind of work a crushing bore and a pain, while other persons find it a joy to do and an inspiration. Evidently, I fell into that latter group. If the reader has worked in optical engineering for at least a little while, you probably have recognized that some optical engineers never pick up a wrench or a screw driver in their line of work, and do not care to. Others use all the tools they can get their hands on, and get their hands dirty and may occasionally drop a lens on the floor. That is “life” in this particular vocation. I was the kind of engineer guy who liked the tools, the grease or chemicals on my hands, and who spent many hours or days adjusting mirrors and lenses in equipment I also designed. (Fortunately I did not drop many lenses on the floor.)

If you are about to use an alignment telescope and/or an autocollimator for the first time but have zero prior hands-on experience, here are a few suggestions. If it is possible, talk with some person who has been doing optical tooling with these instruments. A few discussions like this can save you a lot of time and wasted effort by putting you on the right track. My own experience went like that. When I embarked on my first job at a large observatory, my experience with optical tooling techniques was close to nil. There I met a much older gentleman who was also an engineer at this site and had worked for the Argus Camera Company (the original one) in Ann Arbor, Michigan. He had a long history of optical tooling and alignment with Argus and was putting his background to good use at this observatory. I offered to become his assistant with what was then his current project. Once he observed that I was a quick learner and that I respected optics, we became good technical associates and he taught me how to use alignment telescopes and autocollimators. This saved me from learning by mistakes and put me on the right track.

In the event you do not have this option, be sure to read the equipment user manuals which you have and any other information you can find on the world wide web. The main point here is: be sure you understand exactly how the instrument works and what you can expect from it in terms of its accuracy and its intended use in tooling applications. These instruments are often quite different from one model to another, and their actual performance specifications are genuinely important, so you ought to be familiar with what they say.

The next step for a novice would be, play with the instrument on the optical bench or even on your desk if you have to. Get used to operating its controls and learn how to interpret what you see in the images. (Of course, for a person who is not a novice and has lots of experience with these, you probably will not need to read this article at all.)

After a few experiments you will surely notice that both alignment telescopes and autocollimators are extremely sensitive to movements and therefore to any potential instabilities in their support and retention mechanisms. They simply cannot be subject to bumps or vibrations or any retention method which is not secure or not repeatable, in the event you will need to remove and replace the instrument in your application. It is always best if the alignment telescope and/or autocollimator is not removed from the setup at all, but some applications might require this to happen! In any event, pay attention to repeatability.

When you are in the midst of your measurement procedure, if anything unexpected or strange turns up in your data, don’t hesitate to call the manufacturer of the instrument you are using at that time and ask them questions. One good 15 minute discussion with a knowledgeable tech support engineer or technician at the manufacturer’s level can save you days of frustration and save the project some money.

Back to the original question: “Are alignment telescopes and autocollimators hard or easy to use?” The simple answer: They are not hard to use. They are made to be somewhat easy to use. The longer answer is this: The more experience you have using them, the better will be your own personal answer to that question. In terms of practical results of measurements or alignment tasks, experienced users tend to get better results more quickly than a novice might. That is the same idea for almost everything in hands-on engineering; this is nothing new.

A final few words on the subject of task assignments from management to engineers. (This can be considered slightly divergent from the topic.)

Often, engineers will find that they have been given a task by management which is beyond their personal experience level or even beyond the realm of possibility! When that happens, it is not necessarily a sign of malevolent intentions by the management. It could be. More likely, this happens because managers are adept at managing things, but may not be aware of the real “nuts and bolts” of engineering processes. If it seems that your task cannot be accomplished with commonly available optical tooling equipment, talk it over with other engineers. Then, talk it over with your management as well, when you have a clear idea of the problems inherent in the task you have been given. Avoid diving into a project which demands high levels of skill and instrumentation, but which has a budget restriction that severely limits your acquisition of that instrumentation or anticipates skill levels which exceed yours. Of course it’s true that adding new skills to your skill set often is nearly free of charge, and is usually good to pursue.

Procuring brand new alignment telescopes or autocollimators can be expensive. Be careful using old instruments which were purchased used from an unknown source by means of internet sales. If you are about to use one of these, be sure to confirm that it works and that it is calibrated. On one occasion in my past, it was suggested to me that I use an “older” auto-collimating telescope which had been obtained using the method I just mentioned above. I checked it on the bench and discovered that it did not work at all! It’s always best to be sure you have a viable instrument.

You have all my best wishes for success.