Optomechanics – How to Design with Ivory


When performance is crucial the engineer uses tools to assure it.  Let me show you:

Say, we need a hyperspectral design with a 60 cm entrance pupil and stabilized to less than 15 ur, rms, LOS error in object-space.  We have the physical prescription.  How to get started?

The first thing I do is run the physical prescription data through AEH/Ivory Optomechanical Modeling Tools and import them into MSC/Patran-Nastran to start a system model (1).  The pink lines represent the optics and the yellow lines are structural bar elements with lumped masses. 

It’s a crummy “visual” but a very important first step.  I validate the model with rigid-body checks.  Then I run the model through random vibration adjusting the properties of the bar elements (areas, inertias, materials, etc.) until AEH/Ivory-in-Nastran satisfies the LOS performance requirement (12.3 ur, rms, in this case) with a reasonable combination of properties.

(1) 12.3 ur, rms
(2) 12.8 ur, rms
(3) 11.9 ur,rms
(4) 12.7 ur, rms

Next I incorporate solid models of the optical elements and replace the simple bars with more complicated beam elements (rectangular and circular tubes, some tapered) and optimize their wall thicknesses to maintain the AEH/Ivory-in-Nastran LOS performance (2).  I now have an estimate of a housing geometry (masses, dimensions, thicknesses) that will meet the LOS performance. 

The CAD engineer has decided on a three-piece housing design; an objective, a compound elbow and a detector.  The first section we develop is the compound elbow that holds the grating.  The CAD engineer makes a model which I import into the system model.  When we have an elbow design that maintains the AEH/Ivory-in-Nastran LOS performance in the system model, (3), we move ahead.

We design the objective and the detector sections next, import them, one at a time, into the system model.  We run the model through random vibration adjusting properties to maintain the AEH/Ivory-in-Nastran LOS performance before proceeding.  Finally, (4), we have developed a three-piece optical housing design through an iterative engineering process that supports the required LOS stability.

That’s optomechanical engineering with tools.  No surprises.  No excitement.  Just getting where we need to go, working from the optical prescription.

Now, to play a little…

Oh!  Joyous Summertime.

Al H.

Optomechanics – Using Ivory for Early Structural Concepts


If spherical surfaces didn’t make pretty good images we’d live in a whole different world.  It seems that optics is an art that was just meant to work.  The mechanics?  Well, that’s maybe a whole different story.

I recently helped to demonstrate that a proposed optical system could be made to work.  It’s stability requirements were 2 1/2 times tighter than the earlier system on which the proposal had been based, and the earlier one had been a challenge in its time.  That extrapolation was a risk that the contractor had to eliminate very early. 

So, the CAD engineer and I shared a cubicle.  He collected information on all the stuff that had to go into the system.  I created a structural finite element model to analyze the image stability:  I started with the CodeV prescription, which I read into AEH/Ivory and then imported the Ivory file into Patran; I also imported into Patran the step-files (and ray bundle) for all the optical elements; I attached the Ivory file to the elements (Any time I moved an element I could then read the resulting motion of the image on the detector); finally, I imported into Patran the proposed flat honeycomb plate to which the optical elements were to be mounted.  The boring part was over.

And the real fun began.  I used Patran as a design tool:  I put cells around each of the optical elements; I tied the cells down to the flat plate; I ran a 6 DOF rigid body check and a 3 axis static gravity check in Nastran.  Everything behaved well computationally.  But in random vibration it was out of bed by ~3X.  There was work to be done.

With the AEH/Ivory data imported to Excel I could identify which elements were the big drivers of the image motions.  So, I started beefing up the bracing on those elements.  The CAD engineer was checking my work while he started his own modeling effort.  He guided me in positioning the optical mountings and I guided him in locating the other services (electronic, thermal, servo, mechanical) that had to work in proximity to the optics on the inner gimbal.  The bracing and the services all had to fit.  Ultimately, we (he and I) were able to reduce the image motions by over 3X and show safe margin on the stability requirements with everything on the gimbal.

All of this was done in the opening days of the project.  In fact, if you cannot make the optical system work when the design spaces are malleable, you will be unlikely to make it work later.  It only gets harder (and I’ve been there too). This early structural concept was itself malleable and would change over time as all of the disciplines agreed to the design.  It might be months before all the CAD interfaces would be settled.  Meanwhile, the project had a structural concept that promised to meet the stability requirements and could guide the detail mechanical design.  And an engineering tool for occasional spot-checks and trade-off studies.

Joy and Happiness!

Ahhh… April.

Al H.

Optomechanics – Always be Analyzing


A dear friend who designs control systems likes to declare, “You have to know the answer before you do the analysis.”  He says it with a twinkle in his eye so you know he’s goading you.

But consider his declaration seriously. How does an analyst know he’s (or she’s) right?  OK, forget about “right,” how about “good enough?”

The short answer is that he or she may not “know”, but that by developing over time a keen “sense of smell” he or she can “kind of tell”.  One way to maintain that sense of smell is to continually make estimates and correlate test data whenever the chance occurs.

In this regard, and with the Holiday Season safely behind us I can pass along an anecdote:

I was managing a mechanical engineering department for a large optics company [the optomechanics key in this missive] and the management club decided to have its Christmas Party on board the Queen Mary, moored in Long Beach.  The dinner was preceded by a tour of the ship.  My wife and I decided to forgo the tour and arrived just as the diners were being seated. 

We joined a small group of program managers at a table near the band-stand.  The repartee was reasonably brisk, as it should be among a gathering of alpha-male managers.  But after a few minutes the hubbub subsided and I could see that a gentleman across the table who I’ll call Larry had a brochure in his hand.  Larry said to me (and I believe I quote him accurately), “Al, you’re so sharp.  Tell us [and he gestured to our peers around the table] how many rivets are in the Queen Marry.” 

I heard my wife take a deep breath.

Now, Larry and I had some history, of course.  Fun stuff like this.  And other less fun stuff.

So, I explained to him that I didn’t “know” but would estimate it for him.  I estimated the length and breadth of the ship, its number of decks and the sizes of compartments.  With that I estimated the length of all the joints needed to be riveted together.  Then, estimating the number of rivets per foot on the outside of the ship (which I had observed when day-sailing, with a friend, along her side the previous summer), I declared my estimate: 10,000,000 rivets.

Well, a hush descended on the table that wasn’t lifted until the band started playing.  My estimate agreed exactly with the number in the brochure Larry had picked up during the tour.  The rest of the table wouldn’t let me buy a drink all evening.

My response to my dear friend is that an engineer who does a lot of analysis needs to stay grounded in the meaning of the numbers by continually making estimates and correlating test data. Yes, even on a pleasant summer afternoon day-sail.  I believe he might nod his head and accept that.

Joy to all, including Larry, in the New Year!

Al H.


Optomechanics – Save Time with Ivory


Skeptics like to tease me about using my own software tools to create optomechanical models in finite element codes.  I could simply use the coefficients provided by the optical designer, they suggest.  In a way they’re right.  But I have found that my software allows me to enjoy more of my evenings and weekends.  Let me explain:

The task is to incorporate the optical image formation properties into a structural finite element model.  Rigor is required because small modeling errors can create large misleading results in the subsequent analyses.  A complete set of coefficients and congruent descriptionsof the geometries are essential for a properly formulated optomechanical model.  This allows the optomechanical engineer to validate the integrity of the entire system model with what are called “rigid-body checks.”

But, how to satisfy the “complete” and “congruent” criteria?  Well, I use Ivory.

Now, about the importance of those rigid-body checks:

First, structures:  A rigid-body check exercises the otherwise unconstrained complete model in three translations (Tx, Ty and Tz) and three rotations (Rx, Ry and Rz).  The check discloses malformed elements, erroneous constraints and other errors, which the engineer must correct to have confidence in subsequent analytical results.  It’s a tried-and-true method for checkout of structural models.

Then, optics:  In the optics domain the image motions on the detector during rigid-body checks should be either computational zeros or the effective focal length depending on the status of the object being imaged.  If the model’s image motions contain anomalies (motions other than 0. or the efl) in any of the whole model’s rigid-body motions then the model is poorly formed and the optomechanical engineer must correct it before relying on any subsequent results.  This is a tried-and-true method for checkout of optomechanical models.

Without a complete set of optomechanical coefficients and assured congruent geometries it is very difficult to tell whether any anomalies are artifacts of geometric differences or of inaccurate and/or missing coefficients.  Small imaging anomalies can create large errors in the analyses.  But even small (or perhaps “Especially small”) anomalies can be very time consuming to find and correct.  There are many potential sources of small errors.

That’s where my software lets me enjoy evenings and weekends.  I start with a complete set of Ivory’s coefficients and congruent geometries (from the optical designer’s prescription), check their validity in a simple finite element model (with rigid-body checks) before putting them into the structural engineer’s larger model of the system. 

When the two are married, Voila!  Bliss!  Well, fewer surprises anyway, and more evenings and weekends for me.

So, don’t spend your Holidays in front of your work-station when you should be with your family and friends.

The Season’s Cheer to you all.  I can almost hear the sleigh bells coming.

Al H.

Optomechanics – Optical Analog, OA for Short


It all began with the “Optical Analog,” OA for short. 

OA is what I’ve called my method for modeling the optical point spread function (PSF) in Nastran structural models of optical systems.  I started simple, modeling an axial chief ray and calculating its motion in object and image spaces when I tweek the structure with forces, displacements or thermal gradients. 

After a few successes it became clear that there was a lot more to be learned by modeling multiple optical rays through the system.  Their motions on the focal plane array would not only indicate image motions but also changes to the PSF (and therefore the OTF, a measure of image quality).

A typical application of the OA was to determine the optical effects caused by residual plastic strains in a light weight metallic primary mirror.  The plastic strains were caused by a sudden shock load.  The figure shows two views of the solution. 

The right side shows a 20 degree sector of the primary mirror model with 44 optical rays reflecting from it.  The mirror had 18 such sectors and the problem was axisymmetric.  The left side of the figure shows the results at the center detector of the FPA (blown-up about 5,000 fold).  The black dashed line shows the size of the geometric PSF before the shock load and the red dashed line shows the geometric PSF after the shock load.  The project had a strict requirement for “ensquared energy” on the detectors and I thinned-out the face sheet and webs until the results were just within the specification.

I wrote Ebony, a computer program, to assist in assembling structural models for OA analyses.  It’s one of my optomechanical modeling tools that I use to help guide mechanical designs.  I tend to put them to work in the early days of a project, while the concepts are malleable.  They’re also useful in “Red Team” assignments to find out, after-the-fact, what went wrong and what it takes to fix it.  AEH/Ebony unifies and couples the PSF to all of the structure in the Nastran model.

If you’re waiting for the beginning of Summer, some good news.  You have only eight months to go!

All Hallow Even comes first, of course.

Joy and good health to all.

Al Hatheway

Optomechanics – Samuel Colt’s Principle of Interchangeable Parts


Well, San Diego’s history now.  Whew!

Thanks to the OMTG Program Committee for an absolutely terrific two-days of papers (plus a poster session).  Thanks to Phil Pressel for an awesome evening presentation on the Hexagon camera system.  And thanks to Eugene Arthurs and the SPIE staff for keeping it (all the rest of the Symposium) together:  What a herd of cats!

And if you missed the Exhibit Hall, well that’s your problem, understandable but still your problem.

Back to optomechanical engineering.  One of the mechanical engineer’s duties on an optical project is to survey the available mechanical design space looking for problems.  The mechanical design space includes dimensions, temperatures, stresses, deflections, tolerances, pressures, masses, damping, friction, durability, service life, stability, ….  Oh, I shouldn’t leave cost out of the design space either. 

In my practice of the mechanical engineering arts I’ve become a disciple of Samuel Colt.  He’s the guy who introduced the principle of interchangeable parts to the manufacture of his infamous .44 caliber revolver in 1841.  Up to that time firearms were assembled by a gunsmith who would grind, file and polish all the manufactured parts until they fit together and operated to his satisfaction.  Their weapons were very expensive.  On the other hand the Colt revolver’s price was so low that “The Great Equalizer” became available to almost everyone.

My tolerancing method applies Colt’s principle to optical products.  Using influence coefficients from my AEH/Ivory Optomechanical Modeling Tools, I calculate the maximum worst-case assembly errors between the image and the detector in all seven registration variables:  Tx, Ty, Tz, Rx, Ry, Rz and dM/M.  I include the tolerances on the lens design variables (R1, R2, t and n) in addition to all the mechanical dimensional tolerances.  Then I tweak all the tolerances (in a spreadsheet) so that the pain is equally shared between the mechanical suppliers, the optical suppliers and the assembly technicians.  And, all the manufactured parts get used as-is. 

When I describe this principle someone is usually perplexed at how I can do this without using the statistical distribution of each dimension.  I point out that I can put the statistical distributions into the calculations if I choose but they won’t change the maximum worst-case assembly errors.

Scrap is another one of the problems that mechanical engineers work to avoid.  Thank you Samuel!

Well, I bought some candy corn this morning.  All Hollow’s Eve is on the way.


Al H.

Optomechanics – A Collaborative Art


Optomechanical engineering is a collaborative art, a fascinating blend of optics, machine design, structural mechanics, servo controls and heat transfer.  I tend to emphasize my (mechanical) contributions in these missives.  But enough about me.  This time I want you guys to stand up and take the bow.  Let’s list at a few topics from the recent past:

Membrane optics research (of 2-28-12)
My contribution of designing some test facilities and helping with the tests was nothing compared to the conception of the telescope it was intended to support.  My thanks to the telescope designer, the lab technicians who ran the tests and the structural engineers who interpreted the results.  (Applause)

Tensile stresses in ring mounted glass lenses (of 8-31-09)
A dear friend and colleague persisted in his belief that glass was too fragile to be mounted in metal rings.  A survey of the literature showed no solution for this load condition and the nearest ones, point load and line load, were unreasonable.  So, I got out my pencil (remember those?) and developed the solution for ring loading.  My thanks to my dear friend.  (More applause)

Mounting mirrors with elastomers (of 2-6-12)
The optics community has been searching for the perfect “athermal” mounting scheme for years.  Guess what, there isn’t one.  This is one of my contributions to the lore.  Love (and Timoshenko) made me do it.  My thanks (posthumously) to Alexander and Stephen.  (More applause)

Stabilizing lines of sight (of 7-12-11)
I teased the servo engineers, the structural engineers and the “optikers” somewhat mercilessly.  It was entirely rhetorical.  They were the heroes of the story.  No one gets down to microradian stability levels on moving earth-bound vehicles unless they all have done a very good job.  My thanks to the servo engineers, structural engineers and “optikers.”  (Still more applause)

Co-inventing a remote sensor (of 4-17-12)
An optical designer friend thought my nanometer-class structural actuators with his lens design skills would be the solution.  He was wrong.  The best approach was an entirely optical solution, with his lens design skills and somewhat more complicated optics.  It worked.  And I got to do the mechanical design!  My thanks to my optical designer friend.  (And yet more applause)

My list is nearly endless.  And each of you has been a stimulus, a catalyst and a joy to have as a friend and a colleague.

Now, all of you, step forward and take a bow (or two or three).  (Deafening applause)

Thank you all for allowing me to participate in your adventures.

Rejoice on our Independence Day.

And happy Summertime to all.

Al H.

Optomechanics – Elastomeric Mounting of Mirrors


Please allow me to complete the discussion of my engineering tool for elastomeric mounting of mirrors.

So, according to my previous missive, the elastomer reduces the shear stresses on the back face of the mirror by two to three orders of magnitude compared to a rigid adhesive.  That’s all well-and-good but how do we know that it’s good enough?  Of course those of you who have picked up the source reference (SPIE: 6665-03, 2007) know the answer.  You also know why there are no dimensional quantities (inches, millimeters, etc.) for the mirror in my equation,

In the derivation I assumed that the gravitational sag of the mirror was a reasonable budget for the figure errors induced by the mounting method.  When I equated the deflection of the mirror due to gravity to the deflection of the mirror due to the thermally induced shear stresses on the back of the mirror the mirror’s dimensions (thickness and edge length) dropped out leaving only the adhesive’s thickness, t, the environmental temperature change, DT, the difference in coefficient of thermal expansion between the mirror and the mount,   Da , the modulus of rigidity (shear modulus) of the elastomer, G, and the specific weight of the mirror substrate, sVoila!

So, that’s my engineering tool.  But plugging numbers in is the easy part.  Now the engineer has to go to work.  You’ll find a discussion of the engineering considerations in the source reference also.

Thank you for your patience. 

‘Tis mid-winter and Valentine’s day is nigh.  Ah, the joy of good company!  Thank you all, again. 

Yes Tiny Tim, thank you too.

Al H.

Optomechanics – Bridge the Chasm (between the optical and mechanical domains)

Mid-winter greetings!  Condolences to my Northern California friends about the snow-pack in the Sierras.  Perhaps we can import some from Europe.  It’s one of the things they seem to have in surplus this year.

Structural mechanics is the very nexus of optomechanical engineering.  It has been since at least 1638, the year that Galileo Galilei wrote in his journal, “If I push on this beam how far will it bend and when will it break?”  With that query he became the recognized father of the structural mechanics art.  That was some 28 years after he had turned his telescope on the heavens to become the father of astronomy.  It took even that great man a long time to recognize that the structure of his telescope was essential to stabilizing the planetary images on his retina.  That nexus remains nearly as obscure and difficult today as it was then.  I discuss this situation in my dinnertime talk, “Bridging the Chasm.”

The awkwardness between optics and structures was brought home to me again in a recent project.  I had an opportunity to help a colleague evaluate the stability of the images in a spectral imager.  I built-up the instrument’s structural model from step files generated by the CAD engineer.  I had some challenges in the meshing processes in Patran:  unresolvable singularities, wicked element geometry and that sort of thing.  Checking out the full mechanical model with six degree of freedom rigid body motions and three axis static gravity loads helped to correct the problems in the elastic model

The optical designer gave me a set of influence coefficients he’d prepared in Zemax and I modeled those into the Nastran deck manually.  I was unable to get reasonable results from the optical model in the check-out runs.  In the analysis runs the image motions were wildly, incredibly, out-of-bed.

I was able to show, using an Ivory-generated set of influence coefficients that the structure was behaving reasonably.  The Ivory model should have behaved somewhat like the optical designer’s model, but it didn’t.  The optical designer and I sat down and went through all he and I had done.  I, for some reason, could not relate his optical coordinate systems to my structural model coordinate systems.  We finally agreed to prepare a new set of influence coefficients based upon revised simple coordinate systems. 


Once we took the time to “bridge the chasm” the modeling problems disappeared and engineering could begin.  There are a lot of opportunities for misunderstanding and misinterpreting numbers moving between the optical and mechanical domains.

From optical image correlators to off-axis spectral imagers Ivory has proven to be my indispensable optomechanical engineering tool.

Al H.

Optomechanics – Adhesive Mounting of Mirrors


Let me revisit my engineering tool for adhesive mounting of mirrors:

Those of you who have studied the source reference (SPIE: 6665-03, 2007) know now that it guides the engineer to elastomeric adhesives for the mounting.  The reason for this is that hard adhesives have a high modulus of rigildity, G. This fact leads to large optical distortions of the mirror surface due to differential thermal expansion and contraction between the mirror and the mount.  But, for elastomers their low modulus of rigidity tends to isolate the mirror from the differential expansion and contraction of the mount.  The reduction in surface distortion may be a factor of between two to three orders of magnitude using an elastomer compared to a hard adhesive.

Simultaneously, Poisson stiffening tends to stabilize the position of the mirror’s surface.  It increases, by a similar factor of 100 to 1,000, the apparent tension/compression modulus, K‘, of the elastomer between the mirror and the mount (comparing K’ to the Young’s modulus, E).  The bulk modulus, K, for a silicone elastomer is typically in the range of 150,000 psi to 200,000 psi whereas its modulus of rigidity may be as low as 180 psi to 200 psi.  In thin adhesive layers the apparent tension/compression modulus, K’, approaches the bulk modulus, K.  Since the Young’s modulus would be about 570 psi, which becomes a Stiffening Factor of about 300 (see above).  The low modulus of rigidity assures small shear stresses in the bondline due to thermal expansion and contraction while the high bulk modulus stabilizes the mirror’s surface in the optical path.

Perhaps you begin to see why this tool is really not a rule-of-thumb.  It is an engineering technique for tailoring the thickness, t, of a specific elastomeric adhesive, G, to the properties of the mirror, the properties of the mount and the thermal environment the assembly will see in service.  It also requires the engineer have some understanding of the Poisson stiffening effect in thin bondlines.

I hope the Holidays left you all refreshed and eager for the New Year.  Here we go again!

Al H.