Strain in telescope mirrors and mirror blanks

What is OK, and what is bad?  I try to clarify that question.

All images and text Copyright Mike Lockwood, 2011

Stress in a mirror blank is analogous to a land mine from a forgotten war.  The mine is there, but it might never be found, or it might be a dud, and might not harm anyone ever.  Or it might blow up.

Similarly, strain can cause the shape of a precision optic to change, wrecking the figure.... or it might not ever affect it.

Strain within glass causes the polarization of light to change (rotate) as it passes through the glass.  Measurement of the amout of rotation allows a difference in optical path (in millimicrons per centimeter) to be calculated.

Typically less than 5 millimicrons per centimeter is precision annealed, and more is not.

It is very difficult to put a number on strain without buying and using a compensator (Babinet-Soleil or similar type) that can directly measure the rotation of the polarized light, and quantify the strain.  These compensators are expensive, so the average ATM won't buy one.

So how can you tell how much strain is too much?  Maybe the images below will help.  Just be aware that this is not an exact science, and I'll try to err on the side of caution.

Let's start off with a good strain test - one that if I saw it, I would have complete confidence that the glass was very, very stable.  You can see it below.  There is some residue left from three glue spots, and the pattern of generating marks and something the mirror was sitting on is seen, but overall the "greyness" of the mirror is quite uniform.

As the generating marks are ground away (front AND back, ideally), that will relieve the slight residual stress that they cause.  There are a few bright inclusions in the glass, but they are small enough to be of no concern.

Good strain test

Now let's see what a little bit of strain looks like - note the lighter regions near the edges of the blank.  Stress often shows up in glass as a four-sided symmetry, and the "gray" portion of the mirror looks vaguely square.  As it gets worse, it looks more like the dreaded "cross" that some opticians speak of.  Some inclusions within the glass again show up as bright spots, one of which has four-sided symmetry itself, though again none are large enough to be of concern.  Again, faint arcs from the generating are visible, but not of concern.

This blank made a very good telescope mirror.

Weak strain in a mirorr blank

The mirror below came from an Asian telescope manufacturer.  The marks from what I believe was the pouring of the glass into the mold are visible on the right and bottom right, inward to near the center.  This is likely strain due to inhomogeneity, or the glass not having the same properties throughout, which could have been due to uncontrolled cooling of the last part of the pour, poorly mixed glass, or other things.

I informed the client of the situation, but he advised that he had not seen astigmatism with temperature changes.  That is a good indication that the glass is not being affected by the strain.  I refigured the mirror and it has been a superb performer thus far.

Glass with non-homogenous area

The blank below shows some irregular strain.  It actually looked worse to the eye, so it was sent back for reannealing.  (The square in the center is a label that I stuck on it.)  It might have been fine in a telescope, but why take a chance?  I had a thinner 22" blank that showed four-sided symmetry a little stronger than this, and it polished out with astigmatism.  I believe that the symmetry makes it more likely for strain to cause astigmatism, rather than some other irregular distortion that may be less noticeable in images.

So, I'll go out on a limb here and theorize that a little irregular strain is less objectionable than a little "cross" type 
(symmetric) strain.  Think of the symmetric strain like putting the blank in a vise an squeezing along two sides, or hanging it from a bad sling causing potato-chipping.  All of the surface contributes to a non-round star shape.  The irregular strain is like putting some weights randomly around the mirror, and is likely to cause a smaller error in terms of peak-to-valley error, and a less noticeable one because stars are more likely to appear round in a probabilistic sense.

Some strain in a large blank

This is the transition point where I would rather have a blank reannealed rather than risk the figure changing in the future - blanks below this seriously need annealing work, in my opinion......

The blank below shows moderate to bad strain.  Note the "cross" pattern, again with four-sided symmetry.  Off the top of my head, I don't recall what blank this was.

Moderate to bad strain

This is just bad.  The strain pattern is off-center (asymmetric) and quite strong.  This glass was doomed from the start.

Asymmetric strain in large blank

Finally, we have the weirdest piece of glass I have seen with the strain test.  Let's call it "worms".  I save this piece of glass purely for illustrative purposes!  It is also the one that appears in my In the Shop segment "Strain Testing in the Digital Age", in which I use an iPad as a source of polarized light.  (LCD TVs are also good sources of polarized light that have become widely available in recent years.)

Bizarre strain

I hope that helps some ATMs out there decide if their glass is posessed.  You can get by with some strain, but if you see highly irregular features or a strong "cross" it is probably best to have the glass annealed (if possible) or get another piece to work on.

For more info on the strain-testing light source and equipment, see this page.

I check all blanks that I work on, including mirrors that come in for testing (if they are uncoated at some point).  Disturbingly, though, most of the cases of astigmatism that I have seen/fixed have been polished into blanks with good anneal, rather than being caused by strain in the glass.

Just remember that it is still possible for the shaped of strained glass to change over time, so consider your mirror investment carefully.  I would not personally use glass from the import shown above for any client's, or my own, new mirror.  It's not worth the risk.

I wish you clear, dark skies, and strain-free glass.

  -Mike Lockwood, Lockwood Custom Optics

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