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 the disk arrived less than a week later.  No other payment was received in exchange for this work.

Packing, phsical description

The disk was received, packed very nicely in foam in a box.  A couple of photos of the disk are shown below, along with an image of the package and packing that it arrived in in excellent condition.

Quartz diskPackage
Quartz disk

The disk was polished on both sides.  Both sides had light scratches and pits on the surface, but overall the disk was in very good condition.  The edge was fairly smooth and the disk had a slight bevel to prevent chipping, but some texture from cutting the disk remained on the edge in 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"
    Thickness - between 0.8001" to 0.8018", wedge of 0.0017"

The wedge is enough that the quartz could not be used for an optical window without reduction of the wedge through grinding.

Strain Testing

The disk was tested for strain and no strain was detected.  A photo of the test is shown below.  Uniform darkening indicates an excellent anneal.

Strain test of quartz disk

Surface Flatness

Next, the flatness of each surface was measured by placing an optical flat in contact with it.  Both sides were found to be significantly convex, on the order of about 7-8 fringes, or approximately 3.5 to 4 waves.  The figure was also a bit irregular.  These are more reasons that this quartz could not be used for a window in the condition it was received.  Images of the contact interference testing using monochromatic light are shown below for both sides of the disk.

Fringes for side A Fringes for side B

Transmission - collimated laser light

The first test of the quartz in transmission was done by placing it between a 4.25" F/4.5 Newtonian telescope (see my telescopes page) of known good quality and a source of collimated (parallel) laser light.  Below is an image of the 4.25" system (only) under test showing its good correction with a ronchi grating near the focus.

Ronchi test of 4.25" telescope

This laser light is just like light coming from a star - it is parallel, and is focused by the telescope into a star image.  The image formed by the telescope was photographed as best I could through an eyepiece, and then the quartz was introduced between the telescope and collimated light and the star image photographed again.  The introduction of the quartz caused a noticeable defocus of the image (due the convexity of the faces) and some asymmetry 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 tough test for a window.  Therefore, failure in this test indicates that the quartz was not suitable for use in transmission without further work to flatten and parallelize the two faces.  The focused image is shown on the left below without the quartz, and on the right with the quartz between the telescope and collimated light.

Focused image, no quartz With quartz

Transmission - using reference sphere

Finally, the disk was tested in transmission using a very accurate spherical mirror with a radius of curvature of 246.5".  The quartz was positioned 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 shown below.  Thus, this constitutes a double-pass test in transmission for the quartz disk.  The light from the test device passes through the quartz, bounces off the spherical mirror, and then passes through it again on the way back to the tester/analyzer.

Quartz in front of sphere

Below at left is an image of the sphere under test without the quartz in front of it.  It shows a fairly good null.  Thus, any deviations caused by the quartz will appear as dark or light spots when it is placed in front of the sphere.  At right is an image of the quartz in front of the sphere, with the tester still positioned at the radius of curvature of the sphere.  It is noted that one side of the quartz darkens before the other, indicating that the quartz has refracted the light and light passing through it comes to focus at a different location than the sphere.  In this case, the radius of curvature was shortened by approximately 2" by the convex faces of the quartz forming a weak lens.

Sphere test Sphere with quartz

Below is a series of images taken as the knife edge is brought into the beam, showing some of the irregularities in the surface of the quartz disk.

Test TestTest
Test Test

By moving the knife edge toward the spherical mirror approximately 2", the knife edge is close to the radius of curvature of the quartz/sphere system, and the surface irregularities of the quartz may be more readily seen.  The image below shows this quite clearly.  Note the semicircular features that are either on the surface of the quartz or due to subtle variations in the properties of the glass throughout the disk.  Without first de-wedging the blank and then figuring both sides to at minimum a figure of revolution (flat is best, but a figure of revolution will do), I cannot say more about the source 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 the 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 mirror, 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 could not 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.

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