Ray-tracing is a standard way to analyze optical designs. The technique allows the optical designer to follow the path of each ray of light through a system of lenses and mirrors. While ray tracing used to be done with pencil and paper, it is now done on a computer screen. What I had never seen was this process done in the physical. But I can figure out how…
Take a few laser line modules, a bit of sheet metal and paint, a bit of circuitry and we can do this!
In the photo you can see the idea… Five red laser line modules are aligned across a table. Stick a lens section in the beams and you can observe refraction as it happens. Using a double convex lens, all of the beams converge to a focus. A clear demonstration of the basic principles of optics!
At Keck we regularly move pieces of glass up to two meters across and weighing hundreds of pounds. These optics are nearly irreplaceable, visions of catastrophic damage to one of these pieces of glass is the stuff of nightmares. An observatory is built around the telescope, hundreds of tons of steel supporting the all important optics. While damage of any sort is a concern, much of the critical equipment can be repaired without major issue. It is the optics that are much harder and more expensive to replace. While these pieces of glass could be re-manufactured, it would probably take a year or more to accomplish.
Last week the unthinkable happened at the Cerro Tololo Victor Blanco 4m Telescope in Chile. A secondary mirror was being removed from the telescope when the handling cart tipped over and injured two workers. Fortunately the injuries were not very serious. The secondary? It suffered severe damage, a 20cm crater in the front surface.
At Keck we had recently undertaken a full review of our optics handling procedures. Every step of the process, every piece of equipment was subject to scrutiny. The procedures reviewed by a committee of internal and external reviewers. The goal was to prevent just this sort of incident, to protect our invaluable glass.
Photos of the damaged CTIO secondary and descriptions of the incident are a powerful example of what can go wrong. Something that will be in the back of everyone’s mind next time we are moving a piece of big glass.