Grinding Telescope Mirrors, Then and Now
Posted by Darin Hayton on 01/27 at 02:06 PM
NPR recently reported on efforts to grind enormous mirrors for the Giant Magellan Telescope. The article, “Want To Make A Giant Telescope Mirror?” is fascinating for various reasons, not least is the fact that these mirrors are being made underneath the football stadium at the University of Arizona in Tucson.
What I found interesting is the way that craft or tacit knowledge clearly plays such a large role in making a good mirror. In our modern era, we seem to assume that machines, computers, and other technological innovations have removed the subjectivity, the skilled craftsman or craftswoman, from the process. But what comes out in this article is the enduring role for craft knowledge.
The master craftsman, Roger Angel, who has been making telescope mirrors for 30 years, or its the technicians arranging—arranging not simply throwing into the mold—chunks of glass (pictured above), or those grinding and polishing the mirror, success depends on just knowing when you’ve done it right. Or, as Angel says, “Not only is it devilishly hard to grind and then polish an aspherical mirror, it’s hard to know when you’ve done it right.”
This recalled Jim Bennett’s talk a few years ago at the ICHST in Budapest (and perhaps again more recently) on John Mudge’s efforts to grind mirrors. Jim traced some of Mudge’s efforts to codify tacit and perhaps oral knowledge. Although a physician, Mudge pursued interests in grinding and polishing mirrors. In 1777 Mudge was elected Fellow of the Royal Society and awarded the Copley Medal for his article on grinding mirrors: John Mudge, “Directions for Making the Best Composition for the Metals of Reflecting Telescopes; Together with a Description of the Process for Grinding, Polishing, and Giving the Great Speculum the True Parabolic Curve.” Like his modern counterparts, Mudge created an apparatus to minimize the role of the technician and the possibility of errors.
In the end, however, Mudge could not erase the tacit knowledge of the technician. His advice ultimately ended in subjective decisions like when “When by working in this equable manner, … you have nearly got out all the vestiges of the turning tool, and brought them both nearly to a figure, it will be then time to give the same form to the metal.” Clearly, “nearly” is a judgement call. So too do his instructions for grinding rely on tacit skills: “In order to give a proper figure to the hones, … some common flour emery (unwashed) with a good deal of water must be put upon them, and the bruiser being placed upon the hones and rubbed thereon with a few strokes of a light hand ….” How much is “some common flour” or “a good deal of water”? Or how heavy is “a light hand”?
In the end, tacit skills reassert themselves. While our technologies are different and perhaps more expensive, those technologies are still created, maintained, and employed by people who have “convince themselves they’ve polished their mirror properly.”