August 13, 2011:  Why you should look at the back of a mirror

The back of a mirror is often neglected, and that can lead to trouble.

Some manufacturers take for granted that the back of the blank is Blanchard ground flat or to a figure of revolution shape when it comes from the glass supplier.

However, this is usually not the case in my experience.  The grinder may not be perfect, or possibly the glass was annealed after machining resulting in a minor change in the shape of the glass as stress was released or the glass sagged in the kiln.

Take, for example, the 16" mirror blank in the images below.  The first image (left) shows a grid of marker lines that I drew on the back to observe areas that had ground.  The second image (right) shows the mirror after some grinding.  Clearly the back of the mirror had a saddle shape, and it is being ground away.

Before grindingAfter grinding

A mirror with an irregular back shape has a greater chance of being ground or polished to an astigmatic shape, especially in the case of a thin blank like this one.   So, I always grind the back of thin mirrors to be sure that they are figures of revolution.  The mirror back may be slightly concave, flat, or convex, so long as it is the same shape across any diameter.

Typically I grind down to 12 to 5 micron grit to get a nice surface finish.  This grind is not as long as the grind for the front surface where larger pits will cause later problems.  A few large pits are no big deal on the back.

I also grind the backs of most of the larger, thicker, high-precision telescope mirrors that I make so that they will slide more easily on the mirror cell support points, lessening stiction/friction and "pull" on the glass and allowing the cell to perform as it should.

I don't usually grind the back of mirrors that come in for minor refiguring, as this could affect the figure of the front and could dramatically increase the price of the refiguring.  I also may not grind the back of mirrors that have larger or "looser" tolerances specified, because it is unnecessary for the mirror to meet the specification.

How do you tell if the back of a mirror has been ground?  You look for the lack of a pattern - if you see faint or obvious lines or arcs in the surface and it appears to have been machined in some way (not shiny or very rough), these are probably the marks left by Blanchard grinding.  Grinding with abrasives will leave no lines.  For properly fine ground surfaces, the surface texture will be so fine that you can't really see the pits with the naked eye.

So, the first step to a good optical surface in my shop is usually to take care that the opposite surface is properly prepared.  These details are very important in the manufacture of high-quality mirrors.

Please check back for future installements of "In the Shop".

Mike Lockwood
Lockwood Custom Optics

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