The Template

This is used to gauge the curve on the mirror during coarse grinding. It should be given a radius of curvature of 98", although the completed mirror should have a radius of 96". The template is best made from thin sheet metal, 0.006" to 0.010" in thickness, and about 8" square.

To cut the curve, first nail the sheet metal to the floor. Drill two holes 98" apart in the ends of a long stick, and nail one end to the floor so that the other end is free to swing across the metal sheet. Insert the point of a strong knife blade into the hole at the free end, and cut through the metal in a single sweep. With the magnifier, examine the convex edge for burrs, and carefully file them off. The template
should be protected from bending by securing it between two thin boards, with its edge protruding about l/4". Since a difference of 0.001" in the sagitta of the mirror means a difference of about 1" in focal length, great care must be exercised in testing with the template; watch carefully for any streaks of light that might show up between its edge and the surface of the mirror. See Fig. 15 for
a diagram of the finished template.

The sagitta is the depth of the curve at the center, as measured between the mirror's surface and a straightedge laid across any diameter. It can be found with sufficient exactness from the value of r2/2R, where r is the radius of the mirror, and R its radius of curvature. For our 6-inch f/8 mirror, it is 9/192 or 0.047".

The channeling tool, grinding stand, mirror testing rack and stands