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. 

Bingo!

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.
2-2-12

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