Author Archives: Al

Optomechanics – Computational Zero is a Relative Term

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Colleagues:

You more astute readers have made a good point about my call for completeness and congruence in the modeling data for Nastran optomechanical models:  These properties are a necessary condition but, alas, not sufficient. 

The goal is to achieve computational zeros in the sums of a few dozen terms of a finite element model that may itself contain a few million terms.  But “computational zero” is a relative, not an absolute, term.  The engineer needs to demonstrate that the actual value of the computational zeros are small enough to be ignored.  Rigid body checks alone can’t do that.

And no technology is more sensitive to this issue than optics.  For instance: 

AEH was called to verify the predicted magnitude of dynamic jitter in a spectral imaging system.  AEH assembled an all-up meshed solid finite element Nastran model of the system, put the optical prescription through the AEH/Ivory Optomechanical Modeling Tools to generate the complete and congruent data that control the motions of the image on the detector and ran the rigid body checks.  Everything so far seemed reasonable.

Then AEH ran three-axis static gravity checks.  Considering the frequencies, displacements and accelerations the instrument would see AEH judged that the computational zeros, which dominated the rigid body checks, were two orders of magnitude too large compared to the static gravity checks.  Correcting this required three more significant figures in the optomechanical modeling data (both the geometry and the coefficients) for sufficient precision in the dynamic jitter analysis.

Fortunately, the software also produces both geometric and coefficient data with eighteen significant figures.  AEH was able to edit the additional significant figures into the Nastran database.  Finally, AEH ran the frequency response spectrum.  The image jitter proved to be safely within the system’s specified maximum. 

But, the engineer had to take the critical steps to demonstrate sufficient precision in the optomechanical modeling data.

AEH offers completeness, congruence and precision in those critical optomechanical analyses.

Well, the March Hare is here, “I’m late, I’m late, I’m late!”

And as Lewis Carol further wrote, “The time has come….”

See you all in Baltimore.

Al H.
3-17-14

Optomechanics – SPIE

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Colleagues:

On the evening of May 6th, during SPIE’s Defens+Security+Sensors Symposium, I’ll be hosting a meeting of the Optomechanical/Instrument Technical Group.

Our speaker will be Steve Rummel, Director of Product Technology for II-VI Infrared of Saxonburg, PA.  II-VI is a major supplier of optics for CO2 laser applications.  Mr. Rummel will discuss new developments in highly stable, heat resistant materials for use in high power applications, especially where a high quality optical finish is required on the surfaces.  He’ll review the microstructure and property data II-VI’s reaction bonded silicon carbide, discuss diamond-containing formulations for ultra-high heat load capability and present B4C containing versions which compete with beryllium.

This is the East Coast meeting of the premier group of optomechanical engineers that design and analyze the world’s optical instruments and systems. This gathering is open to all attendees to the Defense+Security+Sensing Symposium. Anyone who wishes to put an item on the agenda should contact the Chair, Al Hatheway, at aeh@aehinc.com.

The meeting will be from 8 to 10 PM in the Hilton Hotel associated with the Baltimore Convention Center.  Check the Symposium Program or the registration desk for the room’s location.

The Baltimore DSS Symposium is a terrific gathering of the technologists that “make it all happen.”  It’s only two months away.  Set the time aside now, you’ll be glad you did.

I look forward to seeing you all there.

Al H.
3-5-14

Optomechanics – Always be Analyzing

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Colleagues:

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.

1-17-14

Optomechanics – Save Time with Ivory

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Colleagues:

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.
11-12-13

Optomechanics – Ebony vs CodeV and Zemax

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Colleagues:

As a coda to last weeks missive I reserved the knottiest part for today, All Hallows Eve.

My optical design friends like to solve for the point spread function in a lens design code like CodeV or Zemax.  To achieve the same accuracy as my Nastran analysis they would require the use of the first 5,280 Zernike polynomials in the series.  With some effort the number could possibly be reduced to just 264 from among those polynomials.  Then they would trace the rays.  Have any of you ever used 264 Zernike polynomials in a lens design code at the same time?  Great Fun!

My Halloween treat for all of you is a direct solution, via AEH/Ebony and Nastran, to structurally deformed point spread functions.  Even More Fun!

Now, Beware the Great Pumpkin!

Happy Halloween.

Al H.
10-31-13

Optomechanics – Optical Analog, OA for Short

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Colleagues:

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
10-21-13

Optomechanics – Samuel Colt’s Principle of Interchangeable Parts

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Colleagues:

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.

Boo!

Al H.
9-23-13


Optomechanics – SPIE

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Colleagues:

We have a terrific program set up for SPIE’s Optics and Photonics Symposium in San Diego on August 25th through 29th, 2013.  We have a two-day conference, “Optomechanical Engineering 2013,” on Wednesday and Thursday (the 28th and 29th) with a poster session on Monday evening, a meeting of the Optomechanical/Instrument Technical Group on Tuesday evening, the 27th, and the greatest exhibit of the optics industry to be in Southern California all year on the 27th, 28th and 29th.

Our Tuesday night meeting will feature Phil Pressel’s presentation of some of the challenges he faced in designing the “Hexagon” space surveillance camera for the CIA.  The project was officially declassified in 2011 and Phil is publishing a book on his experiences on the project.  It may be available at the meeting and you might be able to get him to autograph it for you.

I’ll be teaching my day-long tutorial, “Optomechanics and the Tolerancing of Instruments,” on Monday the 26th.  This is the course in which I teach engineers the full theory behind the Optomechanical Constraint Equations that control the position, orientation and size of the image in an optical system.  I personally use the equations to tolerance the optical metering structure, size alignment mechanisms, evaluate thermal boresight shifts and analyze image jitter in dynamic environments.  I show my students how to do the same things.

Wednesday night is the Awards Banquet at which new Fellows will be inducted by the society.  That’s a grand event as well.

Also on Wednesday, the 28th, I’ll be presenting a paper in our conference and on Thursday I’ll be presenting a paper in Mark Kahan’s conference, “Optical Modeling and Performance Prediction VI.”

It’s a week-long hubbub of exciting events, networking and learning.  I hope to see you all there.

Al H.
7-31-13

Optomechanics – Keep Your Tools Sharp

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Colleagues:

If spherical surfaces didn’t make pretty good images our optical industry would be entirely different.  As befits a technology that basically works as intended, cliches and rules-of-thumb perform a yeoman’s service.  And they work!  I’m glad that many of you enjoyed my parable about “kinematic” mounts.  Well, that is, they work until they don’t, as in that misunderstanding between the laser physicists and the mechanical engineers.  Thanks for all of your comments.

More recently I’ve been inspired by some of my students to publish the optomechanical influence coefficients of diffraction gratings (i.e., the ratios of a spectrum’s motions to the grating’s motions).  Gratings are often simulated as mirrors.  But the grating’s influence coefficients differ slightly from the mirror’s and there are more of them.  I’ll present my results in Mark Kahn’s conference, “Optical Modeling and Performance Predictions VI,” at SPIE’s meeting in San Diego this August. 

Imaging spectrometers (using gratings of course) are particularly challenging to the optomechanical engineer because the images of both the far-field object and the near-field slit (the spectrum) need to be stabilized simultaneously on the detector plane.  The slit operates as a field stop and the two images behave somewhat (and sometimes importantly) differently.  “Mining” the resulting “data cube” requires close registration between the spectrum and the far-field object’s image.  The grating will work in my Ivory Optomechanical Modeling Tools software.

In San Diego I’ll also present a paper in my own conference, “Optomechanical Engineering 2013.”  This presentation will describe the use of my Jade Optomechanical Modeling Tool.  Jade models the subsurface cracks induced by grinding and polishing.  I use it to engineer, for structural safety, components made of glasses, ceramics and other brittle materials.  As an example I’ll show how I applied Jade to meter-class optical windows for a civilian transport-class aircraft.  The windows have been in service for years.

Engineers develop tools to keep themselves out of trouble.  In the public works domain these have developed into codes and standards that engineers are obligated (by their insurance companies) to follow.  Elsewhere, engineers develop tools for themselves.  In optomechanical engineering there are few rules of thumb to help.  There are, however, a few cliches. 

Summer is coming!  Get out the sunscreen and water skis again!

Keep your tools sharp and your wits even sharper.

Hasta luego, caimán.

Al H.
6-18-13

Optomechanics – The Trap of “Kinematics” vs “Kinetics”

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Colleagues:

A while ago I got a call from a sponsor who wanted me to go to a design review in Texas on very short notice.  On arrival I found my name was on the attendance list but no one knew why.  There were a lot of peculiar looks around the registration desk.

I took a seat at the back of the auditorium and quietly made notes.  At coffee break the manager who was funding the subcontract being reviewed came back to introduced himself.  He tried to “talk shop” but I had very little I could say as I’d not been briefed by my sponsor.  I was learning as I listened.

It was mid-morning of the second day that I discovered why I was there.  They had made the hottest doubled-YAG laser I’d seen, but it was unstable.  The laboratory system worked fine but the flight system lost power whenever a door in the room was closed.  Hmmm.

Well, at lunch break I chatted with the laser scientists, who were bewildered by the problem.  So I talked to the mechanical engineers and asked them what they did for the flight system that was different from the laboratory system.  They said, uniformly, “Nothing.”

After lunch, back in the design review, I discovered what “nothing” was.  The mechanical engineers had been directed to put the resonant cavity on a “kinematic mount,” which they did.  It worked great.  In the flight environment however the “kinematic mount” would fly apart so they bolted it together for flight.  The “stiction” in the “bolted kinematic mount” prevented the cavity’s return to its original geometry after a disturbances such as the closing of doors.

In my report to my sponsor I suggested that a simple redesign to replace the failed “kinematic mount” with a “kinetic mount” (i.e., flexures) would probably fix the problem.  But it was too late.   Within a short time the whole project was cancelled.

A warning to optomechanical engineers:  “Kinematic mount” is just a figure of speech.  “Kinematics” is defined as “the study of motion without regard to forces or masses.”  “Kinetics” is the study of motions of masses under the influence of forces.  When asked for a “kinematic” mechanism we should request the allowable motions.  We can usually work out the “kinetics” from there.  If you cannot find out the allowable motions be very, very careful.

I had a good class of students for my tutorial on Thursday at SPIE’s Defense, Security and Sensing Symposium.

Wasn’t Springtime in Baltimore gorgeous? 

Joy to all.

Al H.
5-14-13