Yes simple rigs can give you a sphere, nature gives that one to us. You can do it by hand up to a meter or so in diameter. It took a few decades to develop optical tests and parabolizing strokes to take the sphere and grind it to a parabola, and nowdays, secondaries can be hyperbolas. A sphere does not make a good imager unless the focal length is huge compared to the diameter of the mirror. It's why old telescopes are long. But then f5 and greater system are still parabolic primaries. I don't think you can go spherical only until you start playing with f10 or greater. you could use a Dall Kirkam, but you need some image field correction still.
The machine tools used in mirror grinding are not encoded to twice the limit required on the glass as called for by Mr Nyquist. All the optical grinders I've worked with are close to run open loop along these lines. There is a model by which glass is removed. Foccult testing and Hartmann testing give you the high and low points on a mirror. Strokes with a given grit are computed to remove the high surfaces, and then run on a machine that may only be encoded to 0.010 inches/tick. You run the stroke, then run another optical test, then compute another stroke ..... You keep going until the error is within acceptable limits. Here's the machines I worked on. http://mirrorlab.as.arizona.edu/ Andrew > > Message: 1 > Date: Thu, 26 Aug 2010 17:22:15 -0400 > From: Przemek Klosowski <przemek.klosow...@gmail.com> > Subject: Re: [Emc-users] Mirror Grinders. > To: "Enhanced Machine Controller (EMC)" > <emc-users@lists.sourceforge.net> > Message-ID: > <aanlktikupatkroy7vud0jajrrf560s3qa6v671ny0...@mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1 > Well, the beauty of the eccentric mirror polishers is that > the > kinematics of the polishing process make the mirror surface > ideally > spherical. It is possible to machine optical surfaces but > you need > subwavelength accuracy i.e. better than 500 nm. ------------------------------------------------------------------------------ Sell apps to millions through the Intel(R) Atom(Tm) Developer Program Be part of this innovative community and reach millions of netbook users worldwide. Take advantage of special opportunities to increase revenue and speed time-to-market. Join now, and jumpstart your future. http://p.sf.net/sfu/intel-atom-d2d _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
