Advice Making Telescope

Making a Reflector Telescope

by Ken Graun

1
It is not practical to make a reflector telescope anymore because it is difficult to find the supplies and it will cost you at LEAST twice the amount to purchase the same diameter scope. However, if you like to make things, it will be a unique challenge.

2
Generally, it is much easier, actually a lot easier, to make a reflector telescope than a refractor telescope. For a reflector telescope, you only have to create one optical surface. For a refractor, you will have to create four optical surfaces and have had experience creating optical surfaces—it is absolutely not something for beginners.

3
I have made three reflector telescopes, that is, made the mirrors (one 6-inch and two 4-inch), and all the various components that comprise a telescope. Over the years, I have bought about four reflector mirrors and the various parts to put together telescopes to my liking.

4
Making a telescope mirror is absolutely different from almost any other type of craftsman's skill. A person who is fairly good with their hands and has some sense of geometry can probably accomplish making the reflector mirror. However, because the process is so different, being able to get advice from someone who has endured the process will be extremely helpful. For my first completed mirror, I stalled at one point (it was the polishing stage) and I would have been permanently stuck if my best friend who had experience at making mirrors had not identified what I was doing wrong. Oh, the more skills and tools that you have with woodworking and metal, the easier it will be for you to make the telescope tube, mount, tripod and other components.

5
There is one organization, basically a club, that still promotes telescope making. It is Stellafane located in Springfield, Vermont. Membership into the club requires having made at least one objective, usually a reflector mirror. But, every year, they have an open-to-the-public conference—on club property at the end of July or beginning of August. Click on this sentence to visit their webpage on telescope making.

My first completed telescope, a 6-inch Newtonian Reflector on a simple pipe mount.
The white painted paper-type tube is a sonotube bought from Home Depot. Concrete is poured into sonotubes to make round pillars for foundation construction of buildings. The end-rings are necessary to keep the tube rigid. The focuser was purchased as well as the Telrad finder. The altitude-azimuth pipe mount and 3-legged stand were inspired by illustrations from Sam Brown’s book, All About Telescopes that was sold by Edmund Scientific Company, now defunct (there is a link to get a free PDF copy farther below). Over the years, I picked up the hobby of woodworking (and many tools), so making the various parts was not difficult.This telescope moves very easily and smoothly, and is easier to move than the Dobsonian mounts normally sold with telescopes of this size today.

My Long Tale of Making a Reflector Mirror

“I am slow because it took me 36 years to “make” a reflector telescope mirror.”

When I was in Junior High, about 9th grade (in the late 1800s), I started to make a 6-inch diameter reflector mirror. I stalled out at the polishing stage and put it aside as I got busy in high school and college and then life. I lost that partially completed original mirror, but I came back and completed a 6-inch mirror—but the gap was long—36 years.

–Ken Graun

Click on the image to see and download the 2-page PDF.

Sam Brown & Telescope Making

Sam Brown? Who was he?
Sam Brown (1906–1976) was a very knowledgable technical writer and illustrator who wrote/worked for various publications and businesses, including Popular Mechanics during the mid 1900s His explanations were accompanied by superb illustrations that simplified “difficult” or technical topics. He was very reclusive so little is known about his life but he may have had training as an engineer because his range of knowledge and ideas were extensive. He was certainly an amateur astronomer and responsible for writing an abundance about amateur astronomy and telescope making for Edmund Scientific Company (from 1942 to about 2000). During its day, especially the 1960s and 70s, Edmund had a fabulous catalogue listing optics, telescopes, telescope components, scientific instruments and other science-related novelties that all of us amateur astronomy nerds relished. Edmund Scientific Company had Sam Brown write about topics for amateur astronomers and building telescopes, especially listing and incorporating parts sold by Edmund. Edmund sold the various topics individually or as an inclusive 192 page book titled, All About Telescopes, which had a price of $3.00, at least when I bought a copy. This book is great for ideas on making a telescope, including making a reflector mirror.

GET A FREE COPY!
Click on the cover to take you to a webpage where you can legally download a PDF of
All About Telescopes. The listing for it is at the bottom of that webpage.

The Basic Steps to Making a Reflector Telescope Mirror

1. Read my 36-year Odyssey PDF above. Download All About Telescopes and read/study the 26 pages on making a reflector telescope mirror. You will have to study it to fully understand it, especially the part on testing and parabolizing the mirror. You will stand and walk around a barrel or pedestal for most of the entire process—not fast enough to get dizzy. You may spend anywhere from 12 to 24 or more hours going in circles.

2. ROUGH AND FINE GRINDING. To create a curve in the mirror, so that it will focus light, you “rub” two pieces of glass together in simple patterns with grit and water between to slowly create the curve. One of the pieces of glass is the tool and the other is the actual mirror. The mirror usually stays on top. The grit is like sand and you go from course to very fine using about 7 or so grades. The very fine grit is almost like baby powder. Maybe 12 hours.

3. POLISHING. After grinding, and the mirror’s surface will be very smooth, you then polish it to make it perfectly clear—like glass. This requires placing a layer of pitch, which is like a hard tar, on the tool and using a polishing compound to get the mirror to look just like glass. Maybe 4 to 8 hours.

4. TESTING & POLISHING TO A SPHERE. After the mirror is fully polished, you then have to test it with an instrument that you will most likely make. This testing provides information on the exact shape of the mirror. You want it to be spherical when you finish polishing. If it tests with another shape, you will then polish it some more to make it spherical (using specifics strokes).

5. TESTING & PARABOLIZING. Once the mirror is spherical, you then use the same polishing setup, coupled with specific strokes, to change the shape from spherical to parabolic. You have to test the mirror using the testing instrument to check for it being the correct shape. This is a fairly tedious process and if you “overshoot” you have to polish the mirror back to a spherical shape and then try again to make it a parabolid. If you are really lucky, 30 minutes but probably 2 to 6 hours.

6. ALUMINIZING. When you are satisfied with the shape of your mirror, you send it off to a business that can put an aluminized coating on it to make it into an honest-to-god mirror! You can make the rest of the telescope before sending the mirror off and this will be a good way to test the mirror on the Moon and bright planets.

7. THE TUBE, MOUNT, STAND, AND COMPONENTS. After the mirror is complete (or before), you have to make the rest of the telescope. Some parts, like the focuser, you can buy but the mount and stand will probably be homemade. The more woodworking and metal tools you have the easier it will be to complete your telescope. Over the years, I have seen individuals with machinist skills make incredible scopes and the same for those who know how to work with wood.

You will need some sort of stand to hold the mirror and tool at about waist level. Pictured is a stand that I made for a telescope—it came in handy for grinding. The basic movements are a back and forth shown by the red arrow, make six or seven of these, then you rotate the mirror a bit clockwise, the blue arrow and at the same time move your body around the stand counterclockwise a little, the yellow arrow. The grayish stuff is the grit, like sand, that will carve the spherical shape into the mirror. Many have used the top of a 55 gallon drum to grind+ their mirrors.

A thick pitch lap is made on the tool and channels are cut into it for the polishing compound to move/drain and to increase the polishing action. The pitch, which is like a hard tar, is gently warmed up on a stove until it is a liquid and poured onto the tool—the tool has a paper collar around it to get a level amount of pitch. Once the pitch cools, you cut groves or channels into it — I use a Dremel tool to cut the channels.

A completely polished and figured mirror ready to be sent off to be aluminized—so it will actually be a mirror.

There is no way to easily explain the testing process. It makes the most sense when you have to do it. But, you can study the section on Testing in All About Telescopes to give you a sense of the process. You will most likely have to make your own Foucault Tester. Mine is above and it is basically a jury-rigged setup. The white tube has a light in it and a small opening on the other side which shines the light to the mirror placed in front of it (the round circle in the middle of the picture with a line through it). The light from the mirror is reflected back to the weird black thing to the right of the light that has a razor blade taped to it. With your eye next to the razor blade, you see certain shapes (shadows) and measure them to determine if your mirror has the right shape.