Optomechanics – Completing the Zemax Prescription with Ivory


Well, I’m just back from a week in San Diego for SPIE’s Optics+Photonics meeting.  WOW!  Everybody seems to have some way to “model” optomechanical behavior but nobody seems to know how to verify that the results are right.  That’s why I created “Unified Modeling” (the Ivory and Ebony Optomechanical Modeling Tools):  to provide the engineer verifiable confidence in the results. 

A good friend of mine likes to declare “You need to know the answer before you do the analysis!”  Cute, huh?  It always goes over well from the back of the room during a CDR.  Well, the next-best-thing (I have found) is to have used the same procedures and theories to analyze a case (any case) for which you already know the answer.  One of my favorite test cases is called a “six degree of freedom rigid body check.”  You put the model through three translations (X, Y and Z) and three rotations (Rx, Ry and Rz).  In optical systems it’s easy to predict the image motions on the detector.  They’re either 1.0, computational zeros or some predictable fraction of the back image distance.  And, you don’t need a finite element code.  You can do it all in a spreadsheet.

AEH analyzed the stability of a hyperspectral sensor.  Our client provided the optomechanical influence functions based upon the Zemax optical design.  The random analysis showed that the image stability was out of specification by an order of magnitude.  A subsequent six degree of freedom rigid body check showed large values in stead of computational zeros.  A review of the influence functions showed that they were incomplete.  AEH put the Zemax prescription into Ivory to get a complete set of influence functions.  This six degree of freedom rigid body check showed computational zeros across the board in a spreadsheet!  And subsequent Nastran random stability analysis predicted the performance would be within specification.  Which it ultimately proved to be during qualification testing.

Unified modeling provides traceable modeling performance and helps to keep an engineer’s tools sharp.

School starts next week.  Good luck to all the children!

Al H.

Optomechanics – Early Adoption Matters


At AEH we study closely the ways things fail.

It is often said that an engineer’s job is to make things work.  Well, that’s nice.  Tinkerers can do that too.  What’s really required from engineers is to make things work every time.  That’s a little different discipline. 

So, as engineers AEH studies how things fail in order to better know how to prevent bad things from happening:

In optical systems, virtually all “optical” failures result from some defect in the mechanical implementation.  These failures are never corrected by changing the optical prescription.  Well, almost never:  There was the Hubbell primary mirror fix.

Optomechanical problems are best spotted early, while the design resources (size, shape, mass, arrangement, interfaces, etc.) are malleable.  Those resources can quickly become depleted, even unavailable, as they are claimed by other interests:  bearings, servos, electronics, cryogenics, subcontractors, etc.

Early detection requires special tools for the optomechanical engineer.  To assess optomechanical problems the optics and mechanics must be coupled by the engineer, hopefully from the first publication of the prescription, perhaps during the proposal effort even. 

The results of this early optomechanical coupling may only be estimates, but they’re essential.  They give the engineer who uses them a sense of how to guide the design to his desired…, no, to his required destination: 

Spot-on performance with a trouble-free service life.

Early assessment of optomechanical problems is one way we help our clients.  AEH has the tools:  longhand, ten-key, spreadsheet and Nastran. We’ve got all that plus Ivory, Ebony and Jade to interpret the optomechanics for you.

Of course, we can often help after problems materialize and the corrective options have become more restricted.

Joy to all for the Autumn season.  It has arrived!

Al H.

Optomechanics – Ebony vs CodeV and Zemax


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.

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