A Cautionary Tale

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.

Damaged Secondary
Damage to the Cerro Tololo Victor Blanco 4m f/8 Secondary. Image credit: CTIO
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.

Oops, a little too much power there…

How powerful is the K1 AO laser?

For someone who has lately been used to working around relatively harmless power levels, beams of a few milliwatts, this is a reminder that lasers are potentially very dangerous. With a milliwatt power level beam there is no danger in getting a hand in the beam, be mindful of your eyes, but otherwise not a lot of concern. This is vastly different, beam power here is measured in tens of watts… The bright yellow beam looks so innocent, appears so harmless. Give that beam a chance, a momentary slip, and it will burn you… badly.

Anything in the beam is at risk, even components we thought were robust enough to withstand the power levels. In this case a reflective ND filter that was to reflect most of the beam into a beam dump, allowing a small amount to continue up the beam train for use in alignments. So much for the ratings on the manufacturer’s data sheet, the beam punched through the coating and even started to melt the glass…

Burned Filter
A reflective ND filter burned through by the K1 AO Laser