This is a new feature for my web site, where I show some of
the
projects that I have been working on. In some cases you'll
see a
finished project, but in others you'll see the unsanitized truth as I
clean up someone else's mess.
July 25, 2010
What
happens when a client takes delivery of a 25" F/4.2, 1.5"-thick mirror
from a professional optician with considerable experience?
One
would expect reasonably good performance from such an optic, and pretty
fast cooling.
What my client got was far from this. The first
optician produced an optic with serious astigmatism. The second
optician refigured it and improved it a bit, but the astigmatism was
still quite bad. I would be the third
optician to work on it. (Unfortunately stories like this are
not uncommon. I am the fourth
optician that another client has worked with.)
Here
is an animation of what the 25" primary looked like when I received it.
The animation is made from
images that I captured while doing Foucault testing.
I
rotated the mirror about 45º for each image in this animation.
With
this type of testing, if the mirror is a good figure of revolution, the
test images should basically look identical as the mirror is rotated.
Clearly, in this case, they are far from identical!
Watch
the triangular "notch" in the edge of the mirror - this is a piece of
tape that I placed on the edge of the mirror to make the rotation more
easy to track. As it rotates, the shadows at the top and
bottom
center of the mirror, where dark and light portions transition, should
be nearly identical, but clearly they change substantially.
This
is indicative of obvious astigmatism present in the mirror's figure.
The astigmatism was easily confirmed with my
figure-of-revolution
test.
Also, if you notice the "blotchiness" of the shadows,
that's simply surface roughness from poor figuring technique.
Sharp-eyed readers will also see a prominent scratch left
over
from someone's "work".
The second optician uses
interferometry, but still managed to miss the astigmatism
and probably did not see the roughness. From this the reader
can see that an
interferogram is no guarantee of quality unless it is done properly.
So, this mirror went straight to polish,
where it remained until the astigmatism was gone. I watched
the
coating polish off asymetrically as polish proceeded, and after many
hours all traces of the coating and original figure were obliterated,
leaving a nice near-sphere with no measurable astigmatism.
(The scratch would
not polish out in a reasonable time, so it was agreed that I would not
try to remove it, but is was partially polished away during my work.)
With the coating removed, I tested the glass for strain and found a
superb anneal. Therefore, the
astigmatism was polished in, and I had just polished it out.
Now,
you might think that after refiguring the primary mirror, this client's
optical problems would be over...... but you would be wrong.
At
right is the client's 4.5" secondary mirror that was
used with the 25" primary. Obviously the fringes are
not
straight - it was approximately 1/2-wave concave. (This type
of
interferometry is done by placing the flat on a reference flat and
allowing the two flats to equilibrate to the same temperature.)
So,
on top of the bad primary mirror, this poor client had purchased a flat
that was supposed to be good, but which was really contributing to even
more optical aberrations.
I stripped the coating, reworked the secondary mirror until it was
quite flat, and sent both optics off for coating.
The
results? The client is happy with his optics for the first
time.
He called to tell me he saw round stars and airy disks for
the
first time. After learning to cool his mirror evenly, there
is no
astigmatism visible due to the optics. I've been giving him
advice on improving his primary mirror cell. His other
comments
can be seen at the beginning of my
customer comments page here.
Please check back
for future installements of "In
the Shop".
