Tests of a 6.0" x 0.8" quartz disk
by Michael E. Lockwood
Earlier this summer a seller of quartz disks offered to give a 6"
diameter disk to anyone who would test it and post results. I
thought this would be an interesting piece to test by various methods.
I sent the seller
a note and offered to perform some tests. The seller agreed, and
disk arrived less than a week later. No other payment was
in exchange for this work.
Packing, phsical description
The disk was received, packed very nicely in foam in a box. A
of photos of the disk are shown below, along with an image of the
and packing that it arrived in in excellent condition.
The disk was polished on both sides. Both sides had light
scratches and pits on the surface, but overall the disk was in very
The edge was fairly smooth and the disk had a slight bevel to
chipping, but some texture from cutting the disk remained on the edge
the form of smoothed bumps.
The physical dimensions of the disk were measured with calipers and a
micrometer, and are summarized below:
Diameter - between
5.990" to 5.998"
between 0.8001" to 0.8018", wedge of
The wedge is enough that the quartz could not be used for an optical
without reduction of the wedge through grinding.
The disk was tested for strain and no
strain was detected. A
of the test is shown below. Uniform darkening indicates an excellent
Next, the flatness of each surface was measured by placing an optical
in contact with it. Both sides
were found to be significantly
on the order of about 7-8 fringes, or approximately 3.5 to 4 waves.
figure was also a bit irregular. These are more reasons that this
could not be used for a window in the condition it was received.
of the contact interference testing using monochromatic light are shown
for both sides of the disk.
Transmission - collimated laser light
The first test of the quartz in transmission
was done by placing it
a 4.25" F/4.5 Newtonian telescope (see my telescopes page) of known
quality and a source of collimated (parallel) laser light. Below
is an image
the 4.25" system (only) under test showing its good correction with a
ronchi grating near the focus.
This laser light is just like light coming from a star - it is
and is focused by the telescope into a star image. The image
by the telescope was photographed as best I could through an eyepiece,
and then the
was introduced between the telescope and collimated light and
photographed again. The introduction of the quartz caused a
defocus of the image (due the convexity of the faces) and some
in the image (due to wedge and irregularity of the convex sides).
As this 4.25" F/4.5 is a very low-power telescope, it is not a very
test for a window. Therefore, failure in this test indicates that
not suitable for use in transmission without further work to flatten
parallelize the two faces. The focused image is shown on the left
without the quartz, and on the right with the quartz between the
and collimated light.
Transmission - using reference sphere
Finally, the disk was tested in transmission using a very accurate
mirror with a radius of curvature of 246.5". The quartz was
in front of the sphere approximately 1" away from it. A photo of
the test setup, the
(coated) spherical mirror with the quartz sitting in front of it, is
below. Thus, this constitutes a double-pass test in transmission
the quartz disk. The light from the test device passes through
quartz, bounces off the spherical mirror, and then passes through it
on the way back to the tester/analyzer.
Below at left is an image of the sphere under test without the quartz
front of it. It shows a fairly good null. Thus, any
caused by the quartz will appear as dark or light spots when it is
in front of the sphere. At right is an image of the quartz in
of the sphere, with the tester still positioned at the radius of
of the sphere. It is noted that one side of the quartz darkens
the other, indicating that the quartz has refracted the light and light
through it comes to focus at a different location than the sphere.
this case, the radius of curvature
was shortened by approximately 2" by
convex faces of the quartz forming a weak lens.
Below is a series of images taken as the knife edge is brought into the
showing some of the irregularities in the surface of the quartz disk.
By moving the knife edge toward the
spherical mirror approximately 2",
knife edge is close to the radius of curvature of the quartz/sphere
and the surface irregularities of the quartz may be more readily seen.
image below shows this quite clearly. Note the semicircular
features that are
on the surface of the quartz or due to subtle variations in the
of the glass throughout the disk. Without first de-wedging the
and then figuring both sides to at minimum a figure of revolution (flat
best, but a figure of revolution will do), I cannot say more about the
of these features. I think they're on the surface, though.
I plan to de-wedge the blank and figure one side to an accurate flat in
future, and then figure the second side to fairly close to flat.
No idea when this will be done.
1) This blank would make an excellent substrate for a
flat mirror, an uncoated
optical flat (with one or two figured faces), a Newtonian primary
or a convex Cassegrain secondary mirror.
2) While it is possible that it may work well as an optical window
after further grinding, polishing and figuring to get both sides flat
and parallel, I
say for sure unless I did that work and tested it again in
transmission. I am not likely to do that - most likely I will
flatten one side or use the quartz for a Cassegrain secondary mirror.