Wednesday, December 3, 2008

Work At The Mirror Laboratory

Many of my friends and relatives know that we make telescope mirrors at the lab, but do not know any of the details. It is precision work, but nothing magical about it, so I'm hoping to post some of the activities we go through to make the largest telescope mirrors in the world.

For the past 2 weeks, we've been in the unusual position where there is no work being done on the GMT (Giant Magellan Telescope - 8.4 meters segment diameter) mirror itself, but rather on getting the polishing machine ready, as well as doing some work on the support system that holds the mirror properly in the polishing cell while we are grinding and polishing. Each of those tasks have manpower assigned to them, so the fabrication crew had time to closely examine the mirror for defects of any kind that need attention before proceeding with work. So late last week Kevin and I were on our bellies inspecting the mirror surface with flashlights and magnifiers to seek out flaws. Generally they were of 3 kinds - bubbles (which generally just needed some beveling so as to not become an issue), fractures (small ones can be safely ignored), and seeds (impurities embedded in the glass - when exposed on the surface they play havoc with polishing and need to be ground out). So the last 2 days, we've been bubble beveling (say that fast 3 times!) and using a dremel-mounted diamond tool to remove seeds.

Shown are the tools of the trade, including a hand loupe for a close up view of the defect, a head mounted magnifier used when grinding, water to cool the diamond tool, dremel with 2mm diameter diamond ball, a cylindrical stone with cone tip to smooth out the diamond finish, and of course, a flashlight to get some light on the subject.

Here is a typical seed - I'm not sure if they are flaked bits of refractory from casting or what, but many have associated fractures, since they are high-stress areas. Generally they need removal, or at least get them below the surface so they will not be exposed by further processing. We've got about .25mm to remove yet, so my goal is to grind them to a depth greater than that.

After a few minutes with the tool, the bulk of the seed is gone and the remaining fractures and seed is down the side of the beveled divot safely away from the surface. Once treated, they get the DK seal of approval (Dean Ketelsen) and a note on a logsheet for future reference. More from the Lab when something interesting comes along!


Tuguldur said...

very informative!

this place is on the 1st floor of optical sciences building right?

I've visited there last year, it was pretty amazing. Last year I was struggling to decide for a career as an optical engineer or as a planetary scientist...

I've built few small telescope mirrors in the size range of 8", but these big chunks of glasses are truly amazing.

Dean said...

No, the Steward Observatory Mirror Laboratory is under the east stands of the football stadium...

Tuguldur said...

so they took the mirror to steward mirror lab?

last year that 8.4 mirror was in the optic building. The guy was telling me that the actual mirror glass is only 4"... are we talking the same mirror?

Dean said...

The mirror you saw at OSC (Optical Sciences Center) was the 4 meter Discovery Channel Telescope, which is, in fact, only 4" thick. 20 years ago the plan was to make the then-7.5 meter telescopes at OSC, but as the competition increased the mirrors to 8 meters and beyond, there just wasn't enough room there and the Mirror Lab was born under the football stadium.