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A magnetic mirror is used to check the perpendicularity of any flat surface to a line of sight using the optical principles of autoreflection or autocollimation.  The 6202 mirror has a circular front surface with a 2½" (63.5mm) diameter.  Its reflecting surface is flat to within ¼ wavelength of light over any area not greater than 50 mm in diameter.  A single magnet is cemented to the back of the mirror.  Its contact surface is ground parallel to the mirror surface within 3 arcseconds.  This type of mirror is also a convenient accessory for performing instrument calibrations in the field.  You may wish to look at other mirrors on this website.

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Left: The back side of the 6202 showing
the magnetic mount.