MIRROR GRINDING YOU DON'T NEED TO BE AN ENGINEER TO MAKE YOUR OWN MIRROR

By Fred Rayworth

There is nothing more thrilling than looking through the eyepiece of a brand new telescope and seeing stars snap into focus, knowing that you made the mirror. The fact that you can save tons of money by making it might be outweighed by the hassle. But if you are a do-it-your-self person, like me, making it is all part of the thrill. In this little article, I will not tell you every detail on how to grind a mirror, but provide enough details to give you and idea of what you will be going through to make it. From there, you can decide if you want to give it a try and at the end I will list a few good authors of books that cover all the specifics of making a mirror.

The first thing you will need to do is figure out how big of a mirror you want to make. In the old days, beginners usually started out with a 4 ¼” to a 6”. With this size, you do not have to worry about special tools or pitch laps (which I'll explain in a minute). Seeing the advantages of aperture even as a high school sophomore, I picked an 8” mirror which is the upper end of working with a simple tool and pitch lap. The next thing you'll need to do is find a kit or assemble the materials from a supplier.

There are very few companies that provide mirror-grinding kits. I know of Newport Glassworks, Wilmann-Bell, and ASM Products (as of 2003). Each offers kits varying in range from 4 ¼” to 18”. Just keep in mind that the larger the mirror, the more the kit and the aluminizing process will cost. Now once you've purchased the kit, it is time to set up your shop.

Many mirrors have been ground and polished in apartments, kitchens, even in schoolrooms. For the average Joe, the best spot is a workshop or garage. When I made my 16” mirror, I hogged it out in the 4th bedroom of my house, much to the annoyance of my wife, especially when she was trying to watch TV. I did not have a garage in that house in Spain and it was just too cold to work outside. Once we moved to Turkey, I did the polishing in my carport, but had to move inside to test it. So what kind of space will you need? This all depends on the work stand and testing area.

By far the best and most popular grinding stand is a 55 gallon barrel filled with water. It provides a sturdy and solid base and has access all around. The caveat is that you must have an open space large enough for it to sit in the middle of because you are going to be spending many hours walking around it in circles and it will be very heavy and hard to move. Another factor is the mess you are going to make. The compounds used to grind and polish a mirror are messy and when mixed with water, create mud that can get everywhere. Finally, you must consider the noise level, particularly in the rough grinding stage. What you are basically doing is placing rough sand (or fine gravel) between two pieces of glass and grinding them together, a very noisy process. While you are working away in a telescope making frenzy, your family will be going nuts trying to hear anything else if you are working in the house like I did! Once you reach the fine grinding stages, the noise level goes down considerably. Besides, the barrel, you will need access to a large sink for washing off the mirror and tool after each stage of grinding. There is a possibility the sink may plug up from the mud you are washing down the drain. It would be a good idea to set some kind of a catch basin in the sink to trap the heavier sediment so it can be thrown away.

The next consideration is space for testing. To test a mirror, you must work at double the focal length of the mirror. For example, you decide to make a 10” mirror with an f/10 focal ratio. That means the mirror focal length will be 100”. To test this mirror you will need an open space that leaves at least 200” between the mirror and the tester. That is almost 17 feet! On top of that, the test rig should be inside and in an area away from air currents such as air conditioning and heating ducts.

So now you have the book and the kit, space to work in, and are ready to start. So what about the math? I'll have to admit I'm close to the dunce level at math and have a very hard time with anything above basic algebra. Well guess what? All the mathematical formulas have already been created for you and all you have to do is plug some numbers in place of the letters in the formula. You get these numbers from the focal length of the mirror and the size of the testing zone (well explained in each of the books). So all you really have to do is plug in the numbers into the formula and use a little addition, multiplication, division, and maybe subtraction. So if you are mathematically challenged like me, take heart, it is not that hard! I did my first mirror in high school at the age of fifteen and I understand math a lot more now than I did back then!

The final major piece of equipment you need is a testing device. A Focault tester is very easy to make. I used an oatmeal box, a clear Christmas light bulb, and a razor blade. The instructions for making this device are in every telescope making book and this simple device magnifies the surface of the mirror over 100,000 times! There are some really fancy Focault testers commercially available but you do not need all of that. A little ingenuity and you can make one with nothing but junk materials. The most precision part of the tester is the scale. You can use the pattern provided in many of the popular books and scratch it into a piece of brass, or like me, you can buy a 6” steel ruler that has markings in 1000ths.

You now have the kit, the work stand, the tester, and room to work. So now what? The object of making a mirror is to generate a concave curve in the surface of the mirror to direct light to a point where it can be magnified with an eyepiece. In other words, you need to grind a bowl shape into the mirror surface. On mirrors 8” and smaller, this is done with a tool on the bottom and the mirror on top. The mirror will usually be Pyrex 7740 glass while the tool will usually be either plate glass or ceramic and of the same size (for 10” and larger mirrors, the mirror will be on the bottom and the tool on top, but in this case, the tool will be smaller in diameter than the mirror and the grinding strokes will be different). The idea is to clip the tool onto the top of the work stand. Then it is time to wet the surface of the tool and sprinkle a layer of rough grinding compound on it (usually #60 or #80 Carborundum). Once this is done, you place the mirror on the tool face down, and start grinding. Grinding is rubbing the mirror back and forth across the tool. You do this for about eight strokes back and forth. Then you move 1/8th of the way around the barrel, and rotate the mirror 1/8th of the way the opposite direction then continue grinding for eight more strokes. You circle the barrel over and over again, wearing down the grinding compound. What happens is that eventually you wear a convex curve into the surface of the tool on the bottom and a concave curve into the surface of the mirror on top (on larger mirrors this action works a little different but the basic process is the same).

The rough grinding continues until you make the concave curve in the mirror deep enough to match the focal length you want from the mirror. The deeper the curve, the shorter the focal length. If for instance, you have an 8” mirror and want to make it an f/7, you want a curve that will reflect light to a point 56” from the mirror. To get this rough curve, you use a template that approximates the correct curve depth. This template may be cut from a piece of thick cardboard (not corrugated). To get this curve, you tie a string to a nail on the floor. Then stretch the string to double the focal length you want, in this case 56 X 2 = 112” and tie a pen or pencil to it at that exact spot (this will also be the distance required when you get to the testing stage). Now move the pen back and forth to draw the curve on the thick cardboard. Now take a razor blade or other sharp cutting tool and cut the curve. That is your template. As you deepen the curve with grinding, you can place the template over the mirror and see how much deeper you need to grind the curve.

Once the curve is the correct depth, it is time to smooth the surface of the glass. This is done by using progressively finer grinding compounds. The first fine grinding stage erases the pits from the rough grinding. Once those pits are gone, the next finer stage is started, and so on until the pits are small enough to start polishing. When the frosted appearance of the glass is practically gone and no larger pits can be seen under a magnifying glass, it is time to go to the polishing stage.

Now things are about to get rather smelly. To polish the mirror, you are going to make a pitch lap. A pitch lap is a layer of tar stuck to the curved surface of the tool, and channels are either cut or molded into the surface. A camp stove may be used for this along with an old throw away cooking pan. Using proper safety precautions and fire safety, you melt the pitch material in the pan. Before doing this, the mirror is placed on the work stand face up and a rim made of cardboard or plastic is wrapped around the mirror to act as a dike to hold the pitch. If a facet mold pattern is used, place this on the mirror face then paint the whole thing with polishing compound mixed with water. This polishing compound will be embedded into the pitch lap. Now pour the melted pitch into the mold then place the tool face down onto it immediately before the pitch cools. Now let this sit and cool for a little while. When cool, remove the cardboard from around the mirror and the tool should slide off the mirror, the polish you painted on earlier acting as a release agent. Now remove the facet mold from the pitch face and you have a polishing tool.

Some people think polishing and figuring are the same thing but they are not. Many will combine the polishing into the figuring phase but they are two distinctly separate things. The process of polishing rubs the glass to the smoothest possible surface, free of any pits or dull areas so that the mirror looks clear when dry. As the polishing stages begin, there will be enough of a reflective surface to use your Focault tester and you can see how the mirror surface is coming along. For polishing, your main worry is to get the surface to the best polish without distorting the surface curve too much.

Figuring is the most demanding and delicate part of the entire process of making the mirror and is where you make or break the optical quality. To me, figuring is where 90% of the real work takes place, though it can be done relatively quickly if conditions are right and your skill is high. Once the mirror is polished, the Focault test really comes into play because this is where you shape the surface of the mirror to make all the light come together at the same place. In a normal sized mirror of less than focal ratio f/10 (10” and smaller), the mirror will not display a good image if the curve is a perfect sphere. The light at the center of the mirror will focus in a different spot than the light at the edge. This is called spherical aberration and in mirrors of greater than f/10 focal ratio, the differences in these focusing points are so small they will not affect the image on an average night. But with shorter focal lengths, below f/10, the error is enough to blur images and provide poor performance. Bringing the edge and center focusing points together is called correction and the shape of the surface of the mirror will need to be parabolic. The edge of the mirror is flattened slightly and the center raised to make the image focus correctly. You must use the tester to measure zones of the mirror, based on that formula I alluded to earlier, to produce the proper curve. Once you get the focal lengths of the edge of the mirror, the center, and the mid zones to match the correct curve for the formula, the mirror should perform well. Now testing the mirror will not in itself tell you the mirror is the correct curve. After all of this, the final test will of course, be to aim it at a star (or planet) and see if your test procedures and math were correct!

Now what all of that figuring boils down to is using polishing techniques to re-polish the mirror into the correct shape. You will parabolize the mirror using the same polishing tool you used for general polishing. On a mirror larger than 8”, this means using one or more smaller tool sizes with the mirror always on the bottom. This boils down to technique and varies from person to person. In either case, this means, a few minutes of polishing, cleaning the mirror, placing it on the rack, waiting for it to reach thermal equilibrium, then testing it. This is done over and over again until the curve matches the formula. The process is tedious and time consuming, but after all that work, seeing a star snap into focus with your finished product is a real rush!

Now the final step is to send the mirror off for the reflective coating. In almost all cases, it must be mailed somewhere else to get the aluminizing done. Just be very careful packing your optical work of art.

So now what do you think? Do you have the patience and desire to make a mirror? If you are like me, there is no bigger rush than to finally see the result after all that work!

A few good telescope making books to seek out are Making Your Own Telescope by Allyn J. Thompson, Standard Handbook For Telescope Making by Neal Howard, How to Make a Telescope by Jean Texereau, and The Dobsonian Telescope Book by David Kriege and Richard Berry. There are also pages and pages of articles on the web with many on technique versus instruction.

A few simple guidelines must be kept in mind if you are going to make a mirror. Cost of the kit (or materials), cost of the coating, the actual size of the finished scope, the time required to actually grind the mirror, and the space to make it.

Good luck and happy telescope making!