Making a mirror is a simple process. We start with a clear piece of glass and grind a curve into it. Then we polish it until it shines. But to get the curve we need to use abrasives to grind away the glass. This means that the glass is fractured and looks pitted as we go. A 8x loupe was used to inspect these pits.
The first step is to chamfer the edge of the mirror so that the fracturing of the glass as we grind does not cause chips around the edge. Any small loss of area from doing this is small compared to what happens if the edge is not beveled in this manner. The bevel was made about 1/8 to 3/16 inch wide, approximately at 45 degrees. It can be seen in the lower photo.
Above is a close-up of the surface after using 80 grit. The surface
looks and feels rough at this point. You cannot see through it- it is translucent
and appears like frosted glass. We are done with this stage when the sagitta
shows that the depth is correct for our f/ ratio and the mirror is ground
out to the outside diameter. With the large iron tool we had available,
this was easy to determine by the feel of complete contact (no rocking).
Looking through the back of the mirror at the tool confirmed good contact.
The sagitta was then measured using 9 pieces of #20 lb paper (.004" each
= .036" depth).
After #80 grit, #120 grit abrasive was used. You can see that the pits are somewhat smaller in the photo below, taken after #120 grit was applied for 20 minutes. The pits are all of uniform size, right out to the edge of the mirror. It is important that you see the pits extending up to the beveled edge.
Notice the chamfered edge of the mirror. The bevel prevents chipping at the edge if the mirror.