Strain Testing Glass

With some simple equipment, you can check your future mirror for strain before you spend countless hours carefully crafting its surface to within nanometers of a parabola.  Strain can cause glass to change shape over time, destroying the figure that was given to the surface.  Severe and even mild strain should be completely removed by annealing, but this is not always the case.  Also, mechanical operations performed on the glass (generating, slicing, significant grinding, cutting, etc.) can cause strain or release it, the latter causing at best a subtle change in the shape of the mirror, and at worst, breakage of the glass.

A simple test for strain generally requires three things:
1) a pair of polarizers (polaroid material), one piece large enough to cover most of the glass to be tested, and
2) a source of diffused white light
3) an uncoated mirror
I recently bought several pairs of polaroids, about 8" x 10" in size, from Surplus Shed (www.surplusshed.com).  I also built two light boxes, one approximately 8"x10" square and the other approximately 19" x 23".  Both boxes are 7.5" high, to permit incandescent 60W light bulbs to fit vertically.  One light bulb illuminates the small box, and four light the large one.  Here are two pictures of the larger box, without and with the white plexiglass cover that serves as a diffuser.

Open box lighted box

The smaller light box is completely covered by one of the polarizing films.  Then the pieces under test are set on top of the polarizer, so that the polarized light passes through them.  The tester views the light through another polarizer, and he/she rotates it to the point where it makes the background the darkest.  At this point, variations in brightness of the light coming through the mirror are either due to marks on the mirror front or back, or due to strain.  This test is best done with the front and back of the blank smooth and coated with a little oil to make the blank more transparent if necessary.

The larger light box, shown above, is covered with a larger polarizer.  I got it from a company in Taiwan called 3DLens.  It cost ~$70, arrived after a few weeks, and is of excellent quality.  When lit, the plexiglass in the larger box appears nearly uniformly lit.

Below is a photo of two small blanks under test.  The blank on the right is 3" in diameter, and shows only generation marks on the back.  The blank is the same shade of gray all over, indicating that there is very little strain in it.  The blank on the left is a 4.5" that had been dropped - the large chip on the right side shows some color and brightness variations near it, indicating quite a bit of strain.  A faint bright strip radiates downward and to the left from the fracture, indicating some strain in the rest of the blank.  Overall, this blank is not badly strained, and will serve its function just fine in a low power telescope.  The fractured area is to be covered.

Small blanks under strain testing

The photo below shows an 8" mirror with a little bit of strain.  The strain is indicated by the pattern, resembling a plus sign (+), made by the darker area.  This indicates minor to mild strain, as the difference in brightness is not that much.  This level of strain can even be caused by having the blank at a different temperature than the air, and the strain will disappear as the blank equilibrates.

This blank is not bad, and I prefer to work on mirrors with this level of strain or less (preferably less).  The rectangular feature in the center of the mirror is leftover adhesive from a label that had been stuck to the back of it by the coater, and the other features near the edge at the 1 o'clock position and extending from 5 to 9 o'clock are on the back surface of the mirror.  All of those are to be ignored, and only the behavior of the mirror as it darkens (due to the polarizer being rotated) is important.

8" mirror with mild strain

Over the last few years, I have obtained many molded blanks and machined/generated blanks from United Lens, and to date, NONE of them have had any strain that is detectable using this test.  Some opticians claim there is no good source of well-annealed blanks, but I disagree!!!

Below is an image of a 25" x 4.5"-thick blank.  We think it is Pyrex.  It is a stunning example of what this simple test can show.  The blank is larger than the large light box, so parts of the mirror are not illuminted with polarized light.  Some strain and numerous striae are visible in this blank, the worst striae I have ever seen.  However, the blank as a whole is fairly stable, judging by the figure on the surface which has been there for quite a while.  So, as long as it is not mechanically altered (regenerated, sliced, ground, etc.), I don't believe the blank's shape will change much.  However, I can't say that for certain, and it's just the impression I get for this particular blank.  Every piece of glass is different, so test them all and be careful.

Strain in a large thick blank

There are other ways to produce polarized light (such as by using reflected sunlight) and this technique, largely borrowed from others, is not the only way to test for strain.  However, because I test quite a few mirrors, the investment in the setup has paid for itself many times over and has provided photos like the stunning example above.

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