On Mon, 26 Nov 2001 at 19:12:30 -0500, "Isaac Crawford" <[EMAIL PROTECTED]> wrote:
> You mean down to the photon!? Who's BSing now? We aren't > talking about a machine shop, we're talking about light. It is > literally impossible to grind lenses to within a photon's breadth > of correction. The criterion isn't a photon's breadth (I'm not even sure a photon has breadth), but rather the wavelength of the light (= photon). As it happens, a mirror ground to within 1/4 lambda (wavelength) behaves as though it were optically exact. I imagine that a similar phenomenon happens with lenses. And, yes, it is possible to form optical surfaces to this accuracy. Kodak has a proprietary method using a beam of argon ions to erode glass surfaces. This method has been used to correct the figure of the mirrors for the large Hawaiian telescopes and remove the normal errors in figure that result from mechanical grinding and polishing when the grinding tool overlaps the edge of the mirror. However, because of the dispersion of glass (variation in index of refraction with wavelength), a single lens element can only be optimized for a limited number of wavelengths, possibly just one. It will be out of focus for all other wavelengths. Mirrors differ in that reflection is essentially the same for all wavelengths, so a mirror figured to focus at, say, 400 nm wavelength (violet) will also accurately focus at 700 nm wavelength (red). -- Rodger Whitlock Victoria, British Columbia, Canada "To co-work is human, to cow-ork, bovine." - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
