optical flat


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optical flat

[′äp·tə·kəl ′flat]
(optics)
A disk of high-grade quartz glass approximately 2 centimeters thick, with a deviation in flatness usually not exceeding 0.05 micrometer all over, and a surface quality of 5 microfinish or less; used in determinations of surface contour and in comparison of lineal measurement.
A plane surface, with deviations from a plane surface generally not exceeding one-tenth of a wavelength of light, used to redirect light in a telescope or other optical instrument.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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Their material can be polished to the mirror finish of a quartz optical flat, and has the optical reflectivity needed for interferometric calibration measurements.
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The upper section is a detachable spar made from a 33-inch length of heavy-walled 1-inch-diameter steel tubing that connects the 3-inch-diameter tube to the housing of the optical flat. Short lengths of precision-ground stainless-steel shafting pressed into the ends of the spar slide into mating sleeves located on the undersides of the 3-inch-diameter tube and the housing of the optical flat, where they are retained by knurled thumbscrews that bear against milled flat surfaces.
A folded Herschelian reflector
I already had a 4 1/4-inch optical flat for reflecting starlight upward to the refractor's objective.
A 3-inch Gerrish polar telescope
The drive and optical flat are protected from the elements by a 4-by-6-foot weather-stripped cover.
My large fixed-eyepiece telescope
Surface [R.sub.5] need not be extremely flat; it can show up to about 30 interference fringes when placed in contact with an optical flat, but these fringes should be fairly circular (within one or two fringe spacings).
A short Newtonian with a spherical mirror
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