Tuesday, April 13, 2010

Finished Perfection!

I make precision optics for a living. While a small 6" telescope mirror curve can be roughed out, ground, polished, and suitably accurate for a telescope can be done in a week, the near-30 foot behemoths we do at work go on for years! Generally, the start of a project to finish including the casting take 4 years or more. So while it seems our current projects have been going on a long time, it is nice to congratulate others in finishing a project.

I've blogged once before about the Optical Sciences Center, their optics shop, and the project known at the Discovery Channel Telescope. Since that last post, I've heard "through the grapevine" that the mirror was now finished, so I ambled over yesterday when I had a few moments to check it out. The rumors are true, the testing has been completed, and the final reports are nearing completion. The shipping box is having some slight modifications before loading and shipping to Flagstaff in a couple weeks. Eventually it will be installed in a telescope SE of there, becoming the 6th largest telescope in the continental US.

While I'm not privy to the final report, the engineer indicated it fully met the specifications, and while you didn't hear it from me, 80% of the light will fall into a .08 arcsecond circle. That is indeed very good for a telescope of that size.

What made this mirror more difficult is that the design chose to use a very thin mirror - only 1/40th of the mirror diameter. It is difficult to polish a smooth surface on such a thin, flexible mirror. Of course, you can take advantage of this flexibility by bending out some of the optical aberrations by using active optics applied through the 120 or so supports you can see through the telescope mirror. While polished with passive, hydraulic supports, when installed in the telescope, computer controlled supports that can slightly push and pull at the supports will correct minuscule figure errors, and correct support errors as the telescope moves across the sky. With the allowed bending errors subtracted, the residual errors are about 16nm rms, 16 millionths of a mm. Numbers this small are difficult to understand - the standard comparison is if you stretch the mirror from Atlantic to Pacific, the average height errors in the precision surface of this mirror would be about 20mm high, just over 3/4".

While the Optical Sciences Center's Optics Shop isn't "the competition", I take pride in their accomplishments, since I used to work there. Congratulations on a job well done!

1 comment:

Anonymous said...

currently watching the pbs show "100 years of the telescope".

definitely worth seeing again when the show repeats.