September 1998 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - September 1998 Issue

The Telescope Nut
by Jeff Baldwin

Mirror Making

What have we covered so far?

We've ground the back flat, curve generated, coarse and fine ground, polished, and checked to see if the mirror is polished out.

Let's take a break here and answer some general questions about making mirrors.

Frequently Asked Questions

Q] What kind of glass should I use?

A] Most mirror makers use Pyrex brand borosilicate. It has average thermal expansion coefficients, average density, average ease in grinding and polishing, and is fairly cheap.

Some of the other glasses used are plate glass (lime), Cit-all, Cervit, Zerodur, BK-7, Black Vitrified Ceramic (bvc), quartz, and titanium silicate. Plate glass historically has been lousy for optics due to stress in the glass that causes it to have inconsistent shape with thermal changes. It is now better, and is frequently used for primaries, even more used for secondaries.

Cit-all, Cervit and Zerodur are low expansion glasses. When they change temperatures, they don't change shape much. That's because they have glass and ceramic mixed together, one gets bigger and the other gets smaller when temperatures change, the two working together keep a consistent size and shape. They cost more. BK-7 is a glass used for refractors, it is expensive, it is very hard to grind and polish, and it should not be used for mirrors. Quartz is expensive, but it is extremely uniform in crystalline structure keeping stress minimized. It is also very expensive and not necessarily better for reflective optics.

BVC is the new thing in glass today. It is manufactured in Quebec, Canada, and is cheap, polishes and grinds like Pyrex, is fairly well controlled in thermal expansion, and can be cut to your specifications. I plan on making my 40" mirrors out of bvc. Titanium silicate is a very hard, very well crystalline structured glass-like material, one that I have not used and can't comment much about.

Q] How thick does the blank need to be?

A] There are no longer rules for mirror thickness. The old rule was the mirror's diameter needed to be 6 times its thickness, a thickness ratio of 6:1. My 24" mirror has a thickness ratio of 15:1, while my friend Dan Bakken has a thickness ratio with his 41" of 20:1. The mirror I made for James was 12.5" in diameter and 0.7" thick, a thickness ratio of 18:1. This mirror needs to be held perfectly to keep from bending beyond optical tolerance, causing him to make a fantastic multiple pt. flotation cell. So, you can have any thickness ratio you want, but I would keep it fairly conservative.

Q] What is a beveled edge?

A] When the tool is grinding on the mirror, the mirror edge can break while under pressure. If the edge is beveled -- ie. ground at a 45° angle -- it won't break. I usually grind the bevel about 1/8" with a Makita hi-speed grinder. All the books tell you to use a hand carborundum stone. The glass doesn't care.

Q] Can the edge bevel be worn away?

A] Yes. If you notice that the edge bevel is going away, trim it so it is at least 1/8" at all times. It's ok to make it bigger, but if it gets too small the mirror can break at the edge.

Q] What is Turned-Down Edge?

A] Turned-Down Edge (t.d.e.), is created when the edge of the mirror is polished harder than the rest of the mirror. This probably happens because the soft pitch relaxes as it goes over the edge and then is pressed back when it comes back over it again. This constant extra force eventually pushes the glass down a couple of millionths of an inch, which is very far when dealing with the exactness of measuring the edge. The optical results are poorly focused stars, lessening of contrast, and visible spherical aberration.

Q] Do scratches matter?

A] A scratch is usually not going to alter the view. A few scratches probably can't be seen in the view by most observers. A zillion scratches will eventually lower the contrast of the view and cause chaotic diffraction. Most scratches are unpleasant to the mirror maker because he'll think that everybody will notice the scratch. It really is not an optical problem in most amateur telescopes.

Q] What is astigmatism, how do you detect it, and how do you fix it?

A] There are two types of astigmatism, radial astigmatism (most professional opticians refer to this as a normal part of an optic), and directional astigmatism (the kind we don't want). When I talk about astigmatism in a telescope mirror, I'm referring only to directional astigmatism.

Astigmatism is when the optic is not a surface of revolution. A spoon has astigmatism, while a bowl doesn't. A side cut out of a football has astigmatism, while any side cut from a basketball doesn't. When your mirror is astigmatic, one cross-section of your mirror has a longer radius of curvature than another cross-section. This makes the focus appear non-stellar. Just inside of focus, the point becomes a long guck, and just outside of focus, it is a long guck only rotated 90°. It can be detected either in the shop or in the telescope. In the shop we set up a false star at the center of curvature, allow it to reflect off of the mirror, and then focus a high power eyepiece on the image of the false star. It will either be round or astigmatic. When testing the mirror in the telescope, you point the scope at a star and see if you get a point or astigmatism. If you see astigmatism, it is either because the mirror is astigmatic, or because the test set-up is asymmetrical. If you rotate the mirror and then check it again, either the astigmatism will stay the same, rotate with the mirror, or go away. If it goes away, then either the mirror is now relaxed and okay or the system's astigmatism and the mirror's astigmatism are canceling each other out (weird). If the astigmatism rotates with the mirror, then the astigmatism is in the mirror. If the astigmatism stays the same, then it is probably in the system and you need to remedy it so you can retest the mirror.

If it turns out that the mirror is astigmatic, then the only fix is to go back to about 320 grit and work your way back to polishing. I have never successfully polished out an astigmatism, and I feel that it might not be possible, so going back to 320 grit and grinding your way back to polishing is your only option -- other than living with the astigmatism.

Notice on the left image the inside-of-focus star is long at 9:00 - 3:00, while with the right hand image the outside-of-focus star is long at 12:00 - 6:00, while the best focus has a non-stellar quality. Severe astigmatism is worse than this.

This brings up a major point. When you have finished grinding your mirror and you start polishing it, you need to check it for astigmatism early in the polishing process. Otherwise you will work for many hours, find out you have astigmatism, and have to go back to fine grinding again. You can check it after only about an hour into polishing. If the mirror is good, keep working. If it is astigmatic, stop and regrind.

I make my false stars by shining a flashlight into a ball bearing at the center of curvature, then placing a focuser at the center of curvature right next to the ball. There are other ways. I plan on trying to make a fiber optic light feed to shine on the ball to get a smaller, less obstructing and less blinding test.

Q] What's next?

A] Test to see if the mirror is spherical, then discuss and begin figuring (parabolizing).

Clear Glass...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.