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