The Joy of Mirror Making

Mel Bartels

Introduction

The telescope and the toolmaker.

Though our species isn't the only toolmaker on the planet, we have taken toolmaking to new heights. Stone tools go back 3.3 million years. Pounding, scraping, knapping, grinding combined with fire, heat, melting leads us today to grinding telescope mirrors from glass.

Making a telescope mirror is one of the most satisfying sublime joys you will experience. It's also arguably the most accurate surface made by man or machine. If the mirror is expanded to the size of a football field, then the mirror’s surface will be smooth to 1/1000 inch or 1/30 millimeter! Our experience begins with the varied sensations through the hands and sounds during grinding and carried through to the cerebral challenge of parabolizing the mirror to perfectly reflect light. Our experience both ends and begins again with the mirror in a telescope to contemplate the mysteries of the universe and the meaning of life.

Here is my 13 inch [33cm] f/3.0 meniscus plate glass mirror, 1 inch [2.5cm] thick, sagitta of 0.27 inches [7mm], ready for aluminum coating - Enjoying pinpoint star images at the eyepiece (Oregon Star Party, 2011)
 

Compare mirror making to rock climbing. 

MOYERS: What drew you to climbing?
HOUSTON: It's a beautiful thing to do. You're surrounded by beauty. No matter whether it's a storm, or a sunny day, or clouds, or not, the mountains are simply beautiful. I just liked climbing. I like the feel of the rocks.
MOYERS: The feel of the rocks?
HOUSTON: Rock feels good, yes.
MOYERS: How? I mean, you're talking to somebody who doesn't climb.
HOUSTON: Well, rock climbing; you have a sense of the rock. Almost as though it were a living thing under your hands and you learn to explore... I've never been a great climber. I'm just a competent climber and I know my limits. But I love getting out and doing it. [PBS, http://www.pbs.org/now/transcript/transcript350_full.html]

The Goal

This means that the mirror should focus the light as sharply as possible. We want the stars to be pinpoints in the eyepiece at our telescope’s highest powers, tight dots in our digital images and we want our planetary and lunar views to be richly detailed.

You’ll test your telescope mirror at the eyepiece, finding any areas that are slightly high to polish down. You’ll also test your mirror for smoothness using a simple tester made from inexpensive easy to obtain materials.

I’m purposefully side stepping discussions of the Rayleigh Criteria et al. Don’t be sucked into the online whirlpools swirling around wave ratings, secondary sizes, contrast, super polishing compounds, exotic testers and so on – the list is endless. Too many amateur astronomers have too much time on their hands waiting for clear skies, too many blather on about what they don’t know, and very few have any real observing experience in good seeing conditions with a variety of telescopes *and* have taken the time to separate the various factors that impact telescope performance.

Suffice to say that if you concentrate on producing a smooth mirror that focuses sharply and properly baffle your telescope for best contrast then you’ll have a telescope that you can enjoy every night and will pass all the tests. A smooth mirror all the way to the edge: Ronchigrams of 13 inch [33cm] f/3.0 mirror, 100 lines per inch [4 lines per mm] grating
 

Reaching the Goal

To finish a project doing only the work necessary at each stage, it is useful to imagine the finished product and determine what needs to be done. This betas an earlier stage that we can similarly treat. Eventually we reach the beginning of the project, with a clear path of dependencies or what needs to be done in order to accomplish the stages.

The four milestones in reverse for completing a mirror are: parabolizing, polishing, fine grinding, and curve generation. Each stage depends on the previous stage being completed correctly.

Here are the four steps in sequence

  1. Generate the curve (rough grinding)
  2. Smooth the curve (fine grinding)
  3. Polish the surface (polishing)
  4. Figure the mirror (parabolizing)
No matter how complicated and how many ins and outs you consider, never forget that mirror making boils down to these four essential steps. It need not be that hard! It can get a lot of fun and be deeply satisfying!

Parabolizing

In order to make an indistinguishable from perfect star image, the mirror surface must be accurate to a small fraction of the wavelength of visible light. The stage of adjusting the mirror surface to a paraboloidal shape by preferential polishing is called parabolizing. To begin this phase, the mirror surface should be smooth and spherical.

Polishing

To achieve this preparatory to parabolizing stage, the mirror is polished to a shape that is smooth and spherical. The rate of glass removal during polishing is exceedingly small. It could take fifty years of non-stop polishing to polish a flat piece of glass to within a wavelength of light of the desired mirror profile. We need much strong action!  Using silicon carbide grit, the curve can be achieved in hours, albeit with heavy damage to the mirror face by the grit particles.

Polishing with a pliable material like pitch (first used by Isaac Newton three hundred years ago) results in a smoothly polished surface, accurate to a wavelength of light or better, that is ready to begin parabolizing. The act of polishing is both a mechanical and a chemical process.

Oversized laps and turned edges

Mirrors are notorious for turned edges during polishing. Flash polishing after each stage in fine grinding shows an even edge, so the turned edge must occur during polishing. I’ve found that an oversized pitch lap controls turned edge. Oversized ratios can be up to 6:5, the so called magic oversize ratio that automatically maintains a spherical shape at desired radius of curvature. Oversized laps have a long history. They were in use almost from the start of glass mirrors and pitch tools. Brashear used them over a hundred years ago and professionals today use them (see Strong's Procedures in Experimental Physics).

Oversized 14 inch [36cm] pitch lap for 13 inch mirror; note the micro-facetting in place of channels - Parabolizing the13 inch [33cm] f/3.0 mirror with extremely long 'mirror on top' strokes
 

Fine grinding

A series of ever smaller grits are employed in order to repair the damage caused by rough grinding, ending with aluminum oxide which leaves much smaller pits and few fractures compared to the silicon carbide. This stage is called fine grinding. I like to use three grit sizes, 220 silicon carbide, 500 silicon carbide, and 9 micron aluminum oxide. Grit of a particular size comes with a wide distribution of particle sizes. Typical are 20% of particles that are twice the stated size. Comparing particle sizes of 400 grit with 500 grit, the size ratio looks to be 4:5. But when looking at the 20% particle distribution, it is a nearly identical 9:10 ratio. Consequently it’s wasteful to run through a long series of grit sizes, as commonly practiced: 220, 300, 400, 500, 600, 25 micron, 12 micron, 9 micron, and 5 micron aluminum oxides. The third and final grit that I use is 9 micron aluminum oxide. Ending with 9 micron instead of 5 or 3 micron reduces the chance of sticking on large blanks and controls scratching. Comparing 9 micron to 5 micron looks to be a nearly two times reduction in glass pit depth, but looking at the 20% particle distribution, the reduction is only one-third.

I use plaster tools cast to the curved mirror with unglazed ceramic tiles glued to the face. Stroking the tool on top of the mirror, I rotate the mirror 30-45 degrees every fifteen minutes. This prevents astigmatism from occurring via print through from the mirror's backside. The frontside of the mirror can flex more over areas where the mirror's backside is thinner. Flexing downward during polishing can result in less glass removal, resulting in a bump when the polishing tool is removed. Mirror on top can also be used to avoid astigmatism, since the tool supports the mirror, but the grit seems to fall down between the tiles requiring more grit and wets to complete.

6 inch [15cm] tiled tool

Rough Grinding

Unless the mirror comes pre-generated, the initial curve will have to be ground into the mirror. A ring tool of half the diameter of the mirror used on top of the mirror’s flat face with the coarsest grit will rapidly grind a spherical curve into the mirror.

Grinding a 6 inch [15cm] mirror to F/2.8 using a ring tool.

References

- Jeff Baldwin's telescope making pages http://www.jeffbaldwin.org/atm.htm
- Bell's The Telescope
- Richard Berry’s Build Your Own Telescope
- Richard Berry and David Kriege’s The Dobsonian Telescope
- John Brashear's The Production of Optical Surfaces from Summarized Proceedings and a Directory of Members, 1871, http://tinyurl.com/pn3crhl
- Sam Brown’s All About Telescopes
- William J. Cook’s The Best of Amateur Telescope Making Journal
- John Dobson’s How and Why to Make a User-Friendly Sidewalk Telescope
- Myron Emerson’s Amateur Telescope Mirror Making
- GAP 47's machines summary
- David Harbour’s Understanding Foucault
- Albert Highne’s Portable Newtonian Telescopes
- Neale E. Howard’s Standard Handbook for Telescope Making
- Albert G. Ingall’s Amateur Telescope Making, Volumes 1-3
- Henry King’s The History of the Telescope
- Karine and Jean-Marc Lecleire’s A Manual for Amateur Telescope Makers
- Allyn J. Thompson’s Making Your Own Telescope
- Allan Mackintosh’s Advanced Telescope Making Techniques – Optics, Advanced Telescope Making Techniques – Mechanical
- Daniel Malacara’s Optical Shop Testing
- George McHardie's Preparation of Mirrors for Astronomical Telescopes
- Robert Miller and Kenneth Wilson’s Making and Enjoying Telescopes
- James Muirden’s Beginner’s Guide to Astronomical Telescope Making
- Donald Osterbrock’s Ritchey, Hale, and Big American Telescopes
- Henry Paul’s Telescopes for Skygazing
- Robert Piekiel’s Testing and Evaluating the Optics of Schmidt-Cassegrain Telescopes, Making Schmidt-Cassegrain Telescope Optics, ATM’s Guide to Setting up a Home Optics Shop, Tips for Making Optical Flats
- Norman Rember’s Making a Refractor Telescope
- Sherman Shultz's The Macalaster Four-Goal System of Mirror Making and the Ronchi Test, Telescope Making #9
- John Strong’s Procedures in Experimental Physics
- Scientific American’s The Amateur Astronomer
- H.R.Suiter's Star Testing Astronomical Telescopes
- Telescope Making magazine (no longer published)
- Jean Texereau’s How to Make a Telescope
- Stephen J. Tonkin’s Amateur Telescope Making
- John Walley’s Your Telescope, a Construction Manual
- Wilkins and Moore's How to Make and Use a Telescope
- Stellafane Amateur Telescope Making pages http://stellafane.org/stellafane-main/tm/atm/ (comprehensive collection of links to web articles)
- Advanced mirror makers who are also experienced observers

(end of introduction)

For more see
Rough Grinding
Fine Grinding
Polishing
Parabolizing
Star Testing