Got another puzzle for you: an f/3 8″ mirror that appears to have been made by laying a plastic layer on an unfinished thin mirror blank! I’ve never seen anything like it!

Anybody got any ideas?

Some background: Some weeks ago, Al P brought in to the DC-area ATM workshop the optics for a telescope that someone gave him a decade or so earlier. The telescope originally had some sort of thin, full-sized window that we determined was almost perfectly flat, so it had no corrective power that we could determine. He thought that the diagonal mirror had been attached to the window, but the rest of the telescope had long since disappeared. The aluminum layer on the mirror was in fairly decent shape considering how old the mirror was.

The mirror was very thin: 8″ across (20 cm) and only about 1 cm thick, so about 20:1 instead of the 6:1 diameter-to-thickness ration that used to be recommended in the 1930s through 1990s. The back of the mirror blank had circular grooves impressed into it, so many that at first glance it looked like an old-time glass LP record (rather than a vinyl one).

It was also very ‘fast’, with an focal ratio of almost exactly three (3)!

Unfortunately, the mirror was seriously undercorrected, and thus unusable if put into a telescope as is, even though a Ronchi test showed no signs of turned-down edge or of unwanted roughness or weird zones. A numerical Foucault test with Couder-style zones, repeated several times, revealed the lack of correction.

Eventually Al decided to try to refigure the mirror with an ordinary pitch lap, so he removed the aluminum layer with muriatic acid (HCl), and we remade a burgundy pitch lap and tried to get the lap into contact. It seemed to Al that the original bevel had pretty much vanished, so he used a fairly coarse sharpening stone to bring it back. We noticed a funny texture around the end of his bevel but weren’t sure what it was.

When we pressed the mirror against the lap, we immediately discovered that there was a huge amount of bumpiness and jerkiness – something was catching the lap, much like riding a Big Wheel trike on a cobblestone street. Plus, the pitch tended to stick to the mirror and had to be repeatedly removed with fingernails, turpentine, and paint thinner.

We tried rewarming and re-pressing the lap, with no improvement. When we ran our fingers around the edge of the glass, near the bevel, it seemed like there was a raised rim, almost like on a saucer. So Al got out a finer sharpening stone and increased the bevel all the way around, to about 3 or 4 mm wide. Still no improvement in the bumpiness, and the weird texture around the edge of the glass got even worse.

Then we tried removing all traces of upward-facing lip around the edge of the glass by taking a large sort-of-flat piece of 1/4″ glass, sprinkling some 220 grit and water on it, and stroking the mirror, face down, against the grit and piece of glass.

That also did not do anything to improve the bumpiness. Plus, it began to look to us more and more like this mirror had been made in a totally weird manner: a fairly rough piece of glass was hogged out to the correct curvature, then somehow coated with a smooth layer of plastic, then aluminized. If they did any figuring on it, they clearly did not use a pitch lap!

I attach a few photos that are badly out of focus because iphones don’t like to take close ups. The bright bars are LED fluorescent lights in the ceiling; the concentric rings or grooves are on the back of the mirror. Pay attention to the irregularly-shaped non-shiny areas, where we think the original plastic coating came off.


I am also going to link to a youtube video that I took through a cheap 60X – 100X LED microscope.

A few more clues: the plastic layer (if that’s what it is) does not seem to be removed with either HCl or turpentine or mineral spirits.

If anybody has any thoughts on this mysterious mirror, Al and I will be all ears.

Meanwhile he plans to create a new tool from dental plaster and porcelain tiles and regrind it to f/5.