Brunson's 6204 is used for the same purposes as the smaller 6202, but this mirror provides a much larger field of view. This is an advantage when you are autoreflecting or autocollimating at greater distances. Its circular front surface mirror is 4" (101.6 mm) in diameter. Its reflecting surface is flat to within ¼ wavelength of light over any area not greater than 50 mm in diameter. Three magnetic feet are cemented to the back of the mirror (see photo below). Their contact surfaces are ground parallel to the mirror surface to within 3 arcseconds.

Below is the back side of the 6204 showing the magnetic mount.