Telescope Making

A long time ago, when I was a kid living on a farm in Oregon, I wanted a telescope. I wanted a telescope because it was cool, because Jimmy had one, because I liked to look at the stars. Mostly I wanted a telescope because I thought they were cool. I didn't want to admit it but I liked to look at the stars and the moon. I liked to watch birds and the way the snow melted off the mountains in the spring. Some things are just better seen as snippets of detail from a distance.

Telescopes are expensive when you don't have any money, which was my situation. But telescopes have been around since 1608, and books have been written on how to make them. I got one of those books for Christmas when I was ten. I still have that book.

One day when I was 46 years old, I had some extra money and some extra time. I was cleaning up in my room and I found my telescope book and I decided, "I'm going to make a TELESCOPE!"

I am not the sharpest tool in the drawer but I set out to learn how to make a telescope. What I found along the way—the mistakes I made, the neat tricks I learned—I am going to share with you. You can make a telescope for around $25 in materials. You can make one for $50 that will amaze your friends and family, and show you things you didn't know existed. I will show you how to make a telescope and how to use it, and how to have some fun along the way. Let's hope it won't take you 36 years as it did me. It can be done in a few weeks. However, telescope-making can get messy. You can break a piece you've slaved over. Then you walk around in the worst mood you have ever been in. But in the end you have a telescope that people will look at, and through, and say, "You couldn't have made this." Yes, you did, and that is the best part of all.

Lets make a telescope.

Thinking About Telescopes

People have been making telescopes for 400 years, and it's no surprise they've built a vocabulary of specialized terms. In this section we'll take a quick look at some of the most important, and provide links to pages that explain them in more depth. Follow those now, or come back and follow them later when you feel the need for more detail.

Refracting Telescopes

The first telescopes were refracting telescopes. In a refracting telescope, light passes through two or more lenses. The right combination of lens shapes brings the light from a small area to focus on the retina of your eye, giving you a magnified image.

A basic problem with a refracting telescope is that different colors of light are bent by different amounts. The red light comes to a focus at a very slightly different point than the blue or green light does. The result is a fringe of rainbow color around bright objects. Camera designers tinker endlessly with lens materials and coatings to avoid this false color problem.

Small inexpensive refracting telescopes can often be found at surplus and used merchandise stores very cheaply. Amateurs can make refracting telescopes. You need a magnifying lens of long focal length, called the objective lens, paired with one of short focal length (the eyepiece). A usable objective lens can be salvaged from many types of equipment that use projection lenses and other optical components.

Refracting Telescope (english)

Reflecting Telescopes

The reflecting telescope is designed around mirrors, not lenses. The light does not pass through a lens and is not refracted. It is simply reflected. Since there is no refraction, all colors of light are affected equally, hence there is no problem of false color.

A reflecting telescope has its own problems. Its mirrors must be shaped precisely, within a fraction of the wavelength of light, in order to avoid distortion. (Historically, the main mirror of the Hubble Space Telescope was improperly curved, due to a failure of communications between the designer and manufacturer. A space shuttle trip had to be made to introduce a compensating lens in the system to clear up the image.)

It is not as hard to shape a perfect mirror as you might think. Our first project, the Newtonian Reflector, requires mirrors, and we will make them. It is really very easy to do because (as you will find), physics is on our side.

Reflecting telescope (english)

Technical Terms

We need a few technical terms. The first two are focal point and focal length.

Focal length for a lens or a mirror

The focal length of a lens, or of a curved mirror, is the distance at which it brings incoming light to a focal point, or simply focus. The curvature of the lens or mirror surface determines the focal length. The less the curvature, the longer the focal length.

The greater the focal length, the greater the effective magnification of the image. As a general rule, the greater the focal length, the less the impact of imperfections in the curvature. The less the focal length, the more critical it is to have a perfect curve.

Project 1: The Newtonian Reflector

The Newtonian Reflector Telescope is an extremely simple and efficient design. It consists of two mirrors in a tube. That's all!

The arrangement of mirrors and the path of light in a Newtonian reflector

The larger, curved mirror directs incoming light to a focus. The smaller, angled mirror shunts the focussed light out the side of the tube, where you can place your eye to see the image. Read the Wikipedia article on the Newtonian Reflector for its history.

Newtonian Telescope (english)

After we consider how you will use your telescope, we will get deeply into how we choose the exact shapes and sizes of these simple pieces. Then we'll build it!

How Will You Use Your Telescope?

How will you actually use your telescope? Most folks, me included, spent a lot of time dreaming about making the telescope without giving serious thought to how we might use it. I talked about looking at stars and planets—but I also talked about looking at birds and retreating snow fields. Only when I looked through the telescope did I discover some problems. One is, the basic Newtonian telescope makes everything look upside down! Looking at the stars? This is not an issue. Looking at birds or scenery? We have a problem. Fortunately you can buy an inverting eyepiece that will turn things right side up. A second issue with the Newtonian telescope is that it is by design quite powerful. This means you might be looking at a bird's feather detail, not the bird. Third, the telescope is much larger than a pair of binoculars and needs to be supported so you can aim it and hold it steady. I don't want you to stress over this, but you need to consider, what will you do with the telescope?

Telescope Design

The telescope design covers everything from the optics (in our case the two mirrors), the brackets that hold those optics and keep them aligned, and the tube assembly that contains it all. Design also includes the supporting framework that holds the tube and lets you swivel and point it. That piece is called the telescope mount.

So let's say you think that you mainly want to look at planets and stars in the sky. Longer focal length telescopes give higher magnification. That is just how it works: the longer the focal length, the more magnified the image at your eye. Bigger mirrors will gather more light, but two telescopes with different sized mirrors will give the same size image at the focal point, if they have the same focal length. Mirrors with a long focal length are somewhat easier to make and more forgiving of error. However, the longer the focal length, the longer the telescope, making it more difficult to move around and to store.

Shorter focal-length telescopes give less magnification, but a wider field of view. The image is a bit smaller but that is rarely an issue when looking through the eyepiece. The mirrors require a bit more work, a different technique, but they are as easy to make as the long ones. A shorter telescope is easier to move, easier to store, easier to set up for use.

There is no right answer here. The telescope you make should be one you are comfortable with and will use. I have made a lot of telescopes in the last few years. My first--I called it the Long Dog--had a truss design that took about five minutes and 10 bolts to set up. The truss poles were six feet long. It was heavy and long and you had had to stand on a ladder to reach the eyepiece, but it gave images that would take your breath away. I got one of my favorite comments while using that telescope. I showed a friend the planet Saturn. It was a perfect night, calm sky, dark, Saturn was high. He looked at the planet and said, "That is so amazing it doesn't look real." That is what we call resolution.

Regardless of the length and size of telescope you choose to make, you will find the information to build it here. I am going to do two mirrors and several mounting ideas for this, the first project of the book. I will grind and polish a shorter focal length mirror and a longer focal length mirror. Whatever you decide on I will cover it here and you will be successful with your first telescope.

Links to new terms and topics for this page:

• Newtonian Telescope (english)

• Truss Telescope Design (english)

The parts of the Telescope

Time to get into the details of design. The following will have some basic math and some pictures to help.

Radius of Curvature

Having considered the physical size of the telescope and the use to which I want to put it, I can now begin to consider the optics. The focal length depends on the choices made while thinking about the telescope. The focal length is set by the shape of the curve in the front of the main mirror. We make that curve by grinding the mirror. The amount of grinding and the depth of the curve are both directly affected by the choices made while thinking about the telescope.

It is the front surface of the mirror that forms the image, so it is the front surface that gets all of our attention. The back of the mirror may get quite beat up during this process. That is OK, as long as we don't crack it or put big chips in it.

The curve in the front of a telescope mirror is called the Radius of Curvature, abbreviated ROC, or sometimes just R. The front of the mirror is a smooth bowl-shaped depression. Imagine a ball, and if you have one handy, look at it while you read this. The ball has some radius, the distance from the center of the ball (an imaginary point inside where you can't see it, but it is there) to the inner surface of the ball. That is the ROC. It defines the ball's size, and the curve of the surface.

Imagine that the ball is reflective inside. Imagine that we place a source of light (a fancy way of saying put a light bulb or a flash light or something that emits light) at the center of the ball. All of the light would reflect back to the light source, at the center. However, if we move the light source away from the center, the point of reflection, or focal point, will also move away from the center in the opposite direction. When the source of light gets really far away from the center of the ball, at infinity, the focal point ends up being about half way between the center of the ball and the surface of the ball. (How far away is "infinity"? Well, figure at least 200 times the ROC.)

Now imagine that we cut off a small slice of the ball, or press the surface of the ball into some soft material, clay or moist dirt. Either way, we have a shallow, smooth, bowl-shaped depression. It would be curved just like the front surface of a telescope mirror.

The surface of the mirror has a uniform curvature that determines its focal length. The mirror is usually circular; it is a circular patch cut from that imaginary ball. It can be any size; ours will be a few inches across. (For comparison, the mirror of the Subaru telescope on Mauna Kea in Hawaii? 8.2 meters, 27 feet.) The radius of the mirror, one-half its diameter, is denoted as r.

• List of Large Telescopes (english)

The Primary Mirror

The depth of the bowl-shaped curve that is ground into the front of the mirror is called the sagitta.

There is a math formula to figure out how deep to make the sagitta to achieve a specific focal length.

$S={\frac {r^{2}}{2\times ROC}}$

The formula relates the ROC and r, which we know to be the radius of the mirror, to S, the sagitta. The formula reads, multiply the radius of the mirror times itself, then divide it by two times the Radius of Curvature. Huh? Well, we can measure the diameter of the mirror with a ruler or tape measure and divide it in half to get the radius of the mirror. If it is eight inches across, the radius is half of that or four inches. But how much is the ROC?

Well, remember how the focal point moves as the light source moves away from the center of the ball. When the light source is at infinity, or a long way away, the focal point is halfway between the center of the ball and the surface of the ball. So the focal length is one half the Radius of Curvature, one half the ROC.

This is when we have to get specific about the telescope: what focal length did you want? Well, how long of a telescope did you want? Lets say you wanted a four-foot telescope. The ROC is twice that length, or eight feet. (If you have made a telescope before, you are fuming right now, because you know the length of the telescope has little to do with the focal length of the mirror. I'll clear it up before I'm done.)

I asked, why am I figuring out the ROC when it is just another way of saying two times the focal length? Why not just use the focal length in the Sagitta formula? Two times the ROC is equal to four times the focal length, so why not use this?

$S={\frac {r^{2}}{4\times f}}$

Well, you can. For our purposes as amateurs we can use either and it will be good enough.

So! let's figure out a Sagitta for an eight inch mirror with a focal length of forty eight inches.

Sagitta calculation for a mirror eight inches in diameter and 48-inch focal length

$S={\frac {r^{2}}{4\times f}}={\frac {4^{2}}{4\times 48}}={\frac {16}{192}}=0.08333$

Not much, is it? 0.08333 inches, (and those threes go on forever). It is about the thickness of a (U.S.) penny. A ream of paper is pretty close to 2 inches thick. There are 500 sheets in a ream. 250 sheets times 0.08333 is about 21 sheets.

How accurate do you have to be? How close to a four foot telescope do you want? The focal length of the mirror will determine, to some extent, the length of the telescope tube, and for quick consideration you can assume the focal length of the mirror and the telescope tube are the same. But note that In practice the telescope tube will be as much as 25% longer than the actual focal length. If a five-foot telescope sounds too big, you can re-do the formula for a 3-foot focal length, or any other value you want.

We have spent several paragraphs on the primary mirror and I have only shared the basics. Many good books and web pages document the making of the primary mirror. Look at the links at the end of the book. Now we need to consider the second half of the optics, the secondary mirror.

The Secondary Mirror

In the diagram above (and the wikipedia page on Newtonian Telescopes) we see that the telescope gathers and focuses the light, bouncing it back up the tube. Then the optical path is turned 90 degrees and the focus and eyepiece are on the side of the tube. This is accomplished with a flat mirror set at a 45 degree angle in the tube. (It is possible in smaller telescopes to use a type of prism, but a mirror is simpler for use to make and works as well.)

• Roof Prism (english)

How big this secondary mirror should be, its shape, and where it should sit relative to the primary, are functions of the basic measurements of the telescope. In the previous section we used an 8 inch mirror with a 48 inch focal length as an example for our calculations. I will continue to use that example in this section.

Sizing the Tube

Before we can calculate the size and location of the secondary, we need to think about the telescope tube that supports the mirrors. How long should it be? How big in diameter? How far in will the primary mount?

The tube needs to be bigger than the primary mirror; by how much? A safe number is one inch clearance around the mirror. This works out to a tube that is two inches bigger in diameter than the primary mirror. For the 8 inch primary that is our example mirror, the tube would be 10 inches in diameter.

Tube length is determined by the focal length of the primary mirror, plus the space at the end for the primary mirror and the mirror cell—that is, the framework that supports the mirror and holds it perpendicular to the axis of the tube.

The typical primary mirror and cell form a stack about 3 to 4 inches in height. Add that to the focal length and you would have a tube that is long enough (remember, part of the focal length is turned sideways by the secondary mirror). A little extra length, say 3 or 4 more inches, gives a shield that works to block stray local light. Our example mirror has a focal length of 48 inches. Add 4 more for the mirror and cell, and 4 more for blocking light, and we have a tube 56 inches in length, and at least 10 inches in diameter.

Sizing the Secondary

So, how big shall we make the secondary mirror? Before we can answer we need to consider one more key dimension, the focal offset.

A formula for figuring the size of the secondary is as follows.

$d={\frac {Os\times D}{f}}$

In this formula,

• Os is the focal offset,
• D is the Diameter of the primary mirror
• f is focal length of the primary mirror
• d is the minor axis diameter of the secondary mirror

We multiply the diameter of the primary mirror by the focal offset and divide by the focal length of the primary mirror. This gives us a minimum minor axis. A second rule of thumb is, the secondary should be no bigger than 20 percent of the primary mirror diameter. Given an 8-inch primary, that is 1.6 inches. Purists haggle over these numbers forever. My opinion and experience are, a secondary that is a bit too big is preferable to one that is a bit too small.

Lets look at that formula in operation. I used these values:

$d={\frac {Os\times D}{fl}}={\frac {8\times 8}{48}}={\frac {64}{48}}=1.333$

The formula yields a size of 1.33 inches. This is a minimum size. You could increase it by 25% and have a better secondary fit. 1.33 x 1.25 = 1.66 inches—which just happens to be at the 20% mark. Pretty cool, huh?

That's it! That is how, beginning with the diameter and focal length of the primary mirror, we decide on the tube diameter and length, and the size of the secondary mirror.

The formulas are important enough that I have included them here on a graphic that will make a 3 x 5 card. Print it out!

Sagitta Calculation / Depth of the Curve

$S={\frac {r^{2}}{4f}}$

S = Sagitta

r = Radius of the primary mirror

f = focal length of the primary mirror

Secondary Mirror Size / Minor Axis Formula

$r={\frac {Os\times D}{f}}$

r = minor radius of the secondary mirror

Os = Offset

D = Diameter of the primary mirror

f = focal length of the primary mirror

These formulas and the considerations that lead to them will guide your second telescope and every telescope you make after that first one. But next you need to teach your hands how to grind and polish a mirror, and that is best learned on simple inexpensive stuff.

Making the Optics

The Mirror Materials

Lets talk about the materials needed to make a mirror. Strictly speaking what we want in a telescope mirror is a hard polishable material. One that we can shape with a simple mechanical process to the optical curve that we need. In the very earliest days of telescope making the material of choice was Speculum Metal. This was an alloy of Copper, Tin and Arsenic that was hard and nearly white. Somewhere along the way someone came up with a process to coat glass with a reflective material and the modern period of mirror making began. If you have a piece of metal that you think will polish out, I say try it. It was done before you can do it now,

Glass was originally basic soda lime glass. Sort of what we would call window glass today. Softer and easier to work than the metal, it could be polished out perfectly. However, glass expands and contracts with heat and this can make the transition period a nasty time to try and view the stars. The glass will move enough to mar the figure of the mirror, it's ability to direct all of the light to one point, and you are basically stuck until the mirror equalizes with the environment. This can take about an hour on small mirrors. In the search for glass that would not distort in this way many different blends of glass have been tried. Pyrex is harder and does not expand as much as the plate glass does. Fused Silica, and other exotic processes yield a glass like material that is harder and will yield a mirror that hardly changes at all. But! You can make a mirror out of any material that will take and hold the shape of the mirror's curve and many materials have been used. You can make a mirror out of various rocks. Cryptocrystalline Silica will work. I have a mirror made out a rock called Chert. It works fine. Volcanic glass or Obsidian has been used. The point is, if you can grind it and polish it you can make a mirror out of it.

For the first telescope mirror I am going to use a very common material. The ubiquitous Pyrex pie pan. Now, if you happen to have a piece of glass or something that you want to work with, use it. If you find a more traditional piece of glass while shopping for your pie pan, don't feel obligated to go with a pie pan. Use what you can get that is within your budget. Any piece of glass will work. Drink coasters, those big glass disks they put under decorative candles, glass serving platters. They are all glass, they are all round, they will all yield a mirror. Big glass ashtrays. You learn to look in the glass isle of the local thrift store with an open mind and three criteria; Roundish, smooth curve to the bottom, or flat and thick enough. If the bottom has a curve already the glass can be thinner than if you have to grind the curve in. And how much curve? And what is smooth?

I will answer the second question first. Take the glass item and hold the concave part up against the front edge of the shelf. The shelf should be pretty straight but a little dimple won't hurt anything. The gap formed by the glass and the shelf is our sagitta. There shouldn't be any big bumps or dips in the curved surface of the glass when compared to the straight edge of the shelf. Don't worry about any embossed words, they grind right off. The outside bottom, which is what we will work with, should have a pretty smoothly concave curve.

How deep is that curve? Your average coin, penny, nickel, dime etc. are all about 1/16th of an inch, more or less. Close enough for right now anyway. Hold the glass against the shelf edge and see of a nickel will slide through the gap created by the bottom curve and the self edge. Will two pennies stacked on top of each other make it through? Use the middle of the curve when you test, that is the deepest part of the curve. Let’s say one penny will go through but not two pennies. That curve is just a bit less that 1/8th of an inch. Your average pie pan is about seven inches across the bottom, a little more or a little less, but about seven inches. So the mirror it will create is about seven inches in diameter. You have two parts of the saggitta formula, what is the focal length of that pie pan bottom?

$S={\frac {r^{2}}{4fl}}$

Our saggita is about 1/8th of an inch, or .125 in decimal notation. The diameter of the mirror will be about seven inches, half that is the radius, r. Half of 7 is 3.5 in decimal notation. So we can express what we have and what we don't know as ..

$.125={\frac {3.5^{2}}{4fl}}$

So if we divide 3.5 squared by .125 we should get 4 times the focal length of the mirror....

hmmmm 3.5 X 3.5 = 12.25. Divide 12.25 by .125 and we get ... 98. So four times the focal length is 98 inches. 98 is pretty close to 100 inches, so the focal length is about 25 inches.

There are a lot of "abouts" in the above calculation because we don't know the exact depth of the curve. It is more than one penny deep but not two pennies deep. Two pennies is about 1/8th of an inch, depending on how old and worn the penny is. Did you get the penny though the middle or just to one side? Is there a hump in the middle? On and on and on the guessing goes and it isn't worth sweating about. Most all pie pans that have a smooth regular curve on the bottom have a curve about that deep. They all yield a mirror that is about seven inches in diameter, that will have a focal length of about 35 inches when they are finished. Trust me here, I've done a bunch of them. There are some tricks we can use to make a longer focal length if you want to. But right now we are getting our mirror material and we should stay on track. Maybe you see a big old ashtray that is about six inches across and pretty round with a generally smooth curve on the bottom. Testing with the penny at the edge of the shelf you find that a penny will just slide between the shelf edge and the glass...

Sagitta?? 1/16th of an inch, .0625 in decimal notation. Diameter is about 6 inches so radius is about 3 inches. So 3 X 3 is 9 inches, divided by .0625 is 144 inches which is four times the focal length. 144 divided by 4 is 36. So that ash tray will yield a mirror that is about 6 inches in diameter with a focal length of about 36 inches.

Just to be sure I knew what I was talking about I spent the last few weeks wandering around in various thrift stores, doing the things I talked about here. Yes, people will look at you strangely. Well just remember, Einstein was once considered a weird old man too. If they ask what you are doing tell them, or make up a good lie. I found five odd bits of glass that I bought for and average of $1.99 each. Though the two glass platters cost me $2.99 each. But! They will yield a nine and one half-inch mirror, which is a respectable mirror.

I got three pie pans, two glass platters and one white glass plate with an 8-inch bottom having a 1/8th inch deep curve. What is the focal length of that white plate?

Glass color does not matter. The glass can be any color, even rainbow. We will coat it when we are done, so who cares! I also found a 7-inch glass disk just a tad shy of one inch thick. I have no idea what it was for. It looks like some kind of fancy trivet or maybe a candle base. It is flat, so I will have to grind the curve in before I smooth it out. That is OK, it is plenty thick enough.

While you are out shopping buy at least one pie plate, we need it to make the tool. Go for cheap here, we are going to break it up into pieces so the cheaper the better. Now go find your mirror.

Other Parts You Need

There are other parts to the telescope and you should be looking for those parts while you look for your primary mirror. You will need a secondary mirror to bend the light at 90 degrees and bounce the image to the eyepiece. You will need a mount to hold the primary and the secondary mirror. You need a tube for the scope. A focuser assembly. A eyepiece and finally a mount for the telescope so you can point it where you want it. To use the telescope there are some other things like a finder and telescope cover that are handy to have, but the parts listed above are essential. To actually grind the mirror you will need a tool and some grit, as well as some polish.

The secondary mirror is usually an oval shape. It is set at a 45 degree angle when it is mounted in the telescope tube and this makes the mirror look like a circle when viewed through the eyepiece or drawtube of the focuser. More important than the shape of the secondary is the flatness of the surface. It must be optically flat and that is very flat indeed. You can polish a secondary mirror to be flat and I will cover how to do that later on, but there are some sources of glass and mirrors that may answer the need without all the effort of making one. There are lots of cheap sources for front surface mirrors. Some old Polaroid cameras have a front surface mirror in them and can be found for pennies on the dollar at thrift shops. Old photocopy machines have very flat front surface mirrors in them. Even if you cannot find the mirror in an old photocopy machine, the flat glass on the top, as well as the flat glass on the top of old flat bed scanners, are a good source for the material to grind your own secondary mirror. Watch out for tempered glass, we cannot use tempered glass as it shatters when ground. If you get a pair of polarizing sunglasses, (sometimes there is a bin of these at the thrift store and you can borrow a pair to check a piece of glass), and hold the glass in question up to the light the sunglasses will reveal a pattern of dark spots in a piece of polarized glass. John Dobson always recommended the eyepieces from old binoculars for the eyepiece of a scope. Well, in that old binocular there are at least four prisms, all of which are optically good and may be big enough for your secondary mirror. Sometimes you can find just the body of an old binocular and the prisms will still be in there. Learn to share! Get a friend who also wants to make a telescope and go in halvies on some of these parts. Overhead projectors have mirrors in them, as do some slide projectors and movie projectors.

There are sources on the internet for old optical parts. I have listed a couple at the end of the book, but searching for optical parts, used optical parts or even surplus optical parts can yield a lot of good usable cheap stuff. You might also check local swap meets. See if there is a astronomy group in your area. Old amateurs telescope makers have lots of this stuff laying around. You get involved in this hobby and then the stuff just appears. In a year or two you have to shovel out a load or you drown in the stuff. The best thing to do though is just keep your eyes open and your needs list at hand. You'll be surprised what you can find. Lastly, get to know your local glass shop owner. Cut off bits of thicker window glass can be ground flat and used for a secondary mirror. You can start with a simple rectangular mirror now and upgrade to a better shaped secondary later when you find one. It's your telescope, use it and rework it at will. In a way it is kind of like that first, bicycle or car or whatever. You get it and get it going and make it better over time.

We will make the mounts for the mirrors as well as the focuser assembly from scrap and scratch. Keep an eye out for popcycle sticks, tinkertoys, big roundish plastic or wood things like bucket or barrel lids. Little springs, maybe an inch or so long. Motorcycle clutch springs are great and are usually thrown away when they are replaced. Glue, tape, cardboard watch for cardboard. Everything from cereal boxes to refridgerator boxes. White plastic buckets like restaurants get food in. One or two of those are enough. Toilet paper tubes, paper towel tubes, the tubes from the inside of carpet or vinyl flooring rolls. Christmas paper roll tubes. Plastic plumbing pipes for sink drains. These are usually 1.5 inches or 1.25 inches inside diameter. The 1.25 inch ones are better but either will work. Old plastic vacume cleaner pipes. You can skip ahead to the chapters on tube design and focuser design to get some idea on how these parts will be used, but keep an eye open for them. Much of this is considered trash or recycling and to recycle it into a telescope is, I think, the highest purpose.

Grit is a material that is harder than the glass the mirror is made from. It is used to grind the surface of the mirror to make it optically smooth and perfect. Think of the grit on sand paper, that is what we are after. You can use fine sand or black sand but you will use a bunch. The harder and more irregular the grit the less you will use and the less time you will spend grinding. Lapidary suppliers are a source of grit and polish. Check your local phone book and yellow pages. Some industrial suppliers have sand blasting grit and that can be used. I have purchased or salvaged worn out and broken grinding wheels and crushed them up to make grit to grind with. You want loose grit as it cuts by rolling around between the surfaces of the mirror and tool. With that said, sand paper is not a good source of grit. Old timers have used iron filings and washed sand. If you search the internet you will find suppliers for grit and polish. I have listed a couple of sources below. You will need some 120, some 220, (I get this as a blend of 120/220), some 300 to 400 grit, some aluminum oxide or micron grit and some cerium oxide for polish. The 120 and 220 are for coarse grinding to remove irregularities form the pie plate bottom. The 300 - 400 and aluminum oxide are to smooth it out for polishing. And the cerium oxide is for polishing. You will need a pound or better of the coarse and half a pound of each of the finer grades. 8 oz of the cerium oxide should be enough.

Links to new terms and topics for this page:

• Speculum_metal (english)
• Glass (english)
• Chert (english)
• Obsidian (english)
• Pyrex (english)
• Decimal_notation (english)
• Einstein (english)
• John_Dobson (english)

Making the Tool

To make the mirror for a telescope we grind an optically perfect surface onto the material chosen to make the mirror. Simple, huh? It actually is pretty simple to do. The physics of the interaction between the mirror and the grinding tool assures us that one will develop a hole while the other develops a hump. Lets look at that for just a bit.

When grinding two surfaces of the same size, the surface that is on top will begin to hollow out or develop a bowl shape. The surface on the bottom will wear at the edge and develop a high center. It does this because of the stroke, the stroke length and the pressure we apply with our hands. The longer the stroke the deeper the hole. Shorter strokes flatten the curve. We will get into this a bit more later on but right now I want you notice that you are not just grinding the mirror, you are also grinding the tool. The action of the grind shapes both and this is exactly what we want to have happen. As the grind progresses mirror and tool become a perfect match. This interaction gives you one of several indicators as to how the grind is going. The sound of the grind, the feel of the interaction between mirror and tool, the bubble pattern that develops between the two surfaces. These are all indicators and you should be paying attention to them as you grind. The tool will become the base for the polishing lap. How well you shape the tool will affect how easy or hard it is to create the lap.

Tradition is to get two disks of glass the same diameter and grind. The top one becomes the mirror, the bottom one becomes the tool. That doesn't work with pie plates for a number of reasons. Fortunately the tool does not have to be an optical surface, it just needs to be a tool. The surface must be durable enough to survive the grind but must be soft enough to accept shaping by the grind. Tools are easy to make and that is what I recommend for the first mirror.

The simplest way to make a tool is to cast one out of some material. Plaster is cheap and will work to make a tool. You must be sure to seal it against water as the water will soften the plaster and cause it to fail. Concrete is more durable and as easy to work. Ceramic clay can be shaped and fired and used for a tool. You can use wood for the tool if you can match the curve of the glass pretty close. The reason that all of these various materials will work is because they will only form the base of the tool. Once the base is ready we glue the grinding surface to the base. The grinding surface is what gets shaped and this too can be made of many things.

Probably the easiest material to use for the grinding surface is broken glass. Ceramic tiles can be used as can washers, pennies, flattened marbles, decorative stones. Each will respond to the grind, each will grind in a particular way. Any of them are a valid choice.

My favorite tool is cast plaster or concrete with broken glass glued on for the grinding surface. That is the one I am going to discuss in the greatest detail but it is not the only way to make a tool. Your finished tool should match the curve of the pie plate pretty closely and should allow you to attach a grinding surface. It should be roughly the same size as the pie plate bottom but a bit smaller is not an issue. Bigger will flatten the curve and increase the focal length. Substantially smaller, say half the diameter, will work but will require you to be very vigilant while grinding.

Recently I was talking to a friend of mine and he was complaining to me that the pitch for polishing the mirror, (I will get into this more later), was bubbling up on his cast tool. The obvious culprit was moisture in the casting and the tiles he had used to make the tool. He had cast the tiles into the tool instead of gluing them on later. The tiles absorb moisture, as will the cast part of the tool, and a way to combat this problem is to soak the tool before you grind on it. Grinding is very wet so this is not an issue while grinding. However when you polish you must stick pitch to the tool and the moisture will make that adhesion fail. That is why I use glue and glass. The glue provides a barrier that prevents moisture from getting to the pitch adhesion zone and the glass does not absorb water.

BUT! you do not have to make a tool. A ring is a section of any sphere larger that the ring. You can prove this to yourself by putting a ring against a larger sphere. The open end of a can, the mouth of a jar, the end of a pipe, these are all rings. Get a ring and hold it against a ball, a light bulb, the inside of the pie plate. You will find that the ring sits easily against the sphere. The reason I bring this up is that you can use a ring for your tool. At least in the coarser stages. So you could do a lot of the initial grinding with an old soup can, and that is a very valid thing to do. You will still need a larger tool for later but to get started you could use a ring.

So! if you are buying a pie plate to use for the mirror and if you decide to make a tool right now, get two and break one up for glass for the tool. You need a tool base, cast or shaped, carved or whatever and some glue to attach the grinding surface onto the base. I like 5 minute or 15 minute epoxy glue. The cost is reasonable and it will glue just about anything to anything.

Time to make the tool.

First things first. Prepare your grinding surface material. If you are using glass pieces make sure they are all in one place and ready to be glued on. Now is the time to break up the other pie plate if that is the source of your glass. The bottom is the same basic shape as your mirror and should be an easy match for the tool. Put it in a paper bag and gently whack it with a hammer or a big rock. You want pieces about and inch square more or less. Irregular shapes are fine. Break long pieces in half. Wear gloves and eye protection. If you are using washers or pennies make sure you have enough to cover the bottom of the pie plate. Flattened marbles or tiles, the same thing applies. The grinding surface material should all be the same thickness, no mixing of obvious thick and thin pieces! If you are using a ring to grind you are done. Skip down to the grinding section in making the mirror.

You can take the flat material you are going to use for a tool base, say a disk of wood or the bottom of a pan, or even that big round candle base, and make it fit a bit closer. Get a piece of plastic sheeting, heavy plastic wrap will work, as will plastic bags, and cut a piece that will cover the tool base with some overhang. Remember! the tool base should be about the same diameter as the pie plate bottom. Get one of those tubes of epoxy glue and squeeze it all out into the middle of the base. Mix it well and then cover it with the plastic sheeting. Now take the pie plate and press it down onto the glob of glue using the plastic sheeting to keep it off of the mirror. This will cause the glue to be flattened and squeezed out over the surface of the base. Rock and slide the pie plate around on the plastic to spread the glue under it so that it covers the surface of the base and takes on the curve of the pie plate. I squoosh it around until the base is covered and then I center the pie plate and press down pretty hard to make the hump shape and then pick the pie plate straight up. Leave the plastic in place and let the glue harden. Once the glue has hardened you can peel off the plastic and you will have hump on the tool that matches the pie plate curve pretty close. Next you mix up some more epoxy, enough to spread a thin layer over the hump, and glue on the grinding surface. You can have spaces between the pieces that make the grinding surface. Just get as much on there as you can and make sure it is well glued down. Also! remember to place the pieces so there are no high edges sticking up. You are done. Skip down to making the mirror and read about grinding.

You can cast a tool using concrete or plaster or some other castable media of some kind. I use 2 inch wide tape to form a dam around the pie plate at the bottom where the sides join the bottom. You want a tool base that is at least one inch thick. Spread a separator, cooking oil works well, on the glass and the inside of the tape. You want a good coating on all surfaces but not flooding. Place the pie plate on a hard level surface, bottom up, tape around the base. Now mix up the casting media and pour it into the mold created by the tape ring on the pie plate. Let the casting harden and then peal off the tape. You should be able to separate the casting from the pie plate with a bit of pressure.. If it does not want to come apart tap the side of the casting with a light mallet of some type, a short piece of wood works well, all the way around the perimeter and push on the casting while bracing the pie place against something. If you got a good coating of oil on the glass the casting will pop off. The casting will have a raised edge due to the curve of the surface on the pie plate where the side joins the bottom. You will need to remove that raised edge. You can grind it off on the curb or the sidewalk. An old wood rasp will wear it off with a bit of work. If the material is too hard to grind you can chip it off with a hammer but you need to get that edge off. Now clean up the top surface of the tool where you will attach the grinding surface. Mix up some epoxy glue and spread it over the surface of the tool like frosting. I just squeeze out the epoxy onto the surface of the tool base and mix and spread it. Press your grinding surface material into the epoxy and let it harden. You are done, time to start grinding the mirror.

Making the Primary Mirror

The Grind

Everyone talks about grinding mirrors, but very few do. Grinding takes patience, dedication and a desire for perfection. I have all three but I have never made a perfect mirror. Some people have. Mel Bartels has made several perfect mirrors, as has Jerry Oltion of TrackBall fame. Don't let perfect scare you, average works as well.

The pie plate has the hardest part of the grinding process already finished, the basic curve. That saggitta that I have hammered on all the way to this point. All you need to do is smooth the glass. Take the pie plate and point the bottom at a bright light and refect that light onto a surface. What you will probably see is some focusing but it will be very scattered, kind of an amorphous blob of light. That is because even though the surface is curved it is very irregular covered with high and low zones. You will grind the high zones down till they equal the low zones and then polish the glass. It is really very easy to do, physics is our friend here. When we are done the surface will be optically smooth.

Grinding can be done with the tool on top, TOT, or the mirror on top, MOT. The position of the mirror and the length of the stroke will change the curve of the glass. Long strokes deepen the curve, short strokes flatten the curve. MOT will be more apt to deepen the curve while TOT will have a tendency to flatten the curve. These are general statements about the interaction of the mirror and the tool but the interaction is there and you should be aware of it. Long strokes deepen, short strokes flatten. A maintenance stroke, one that doesn't really change the curve but just works the glass is a 1/3rd diameter, center over center stroke. Center over center means just that, the center of the tool passes over the center of the mirror or fairly close to it. The 1/3rd part is a stroke with a total length of 1/3rd the diameter of the mirror. If you were to measure that stroke you would measure it at the center. In practice we guage the stroke by how far we over hang the edge of the bottom thing, tool or mirror. I say bottom thing because either the tool or the mirror can be on the bottom. Since the total stroke should be 1/3rd the diameter of the blank, (the pie plate), you overhang by half that at each end of the stroke. Since our pie plate is about 7 and 3/8ths inches across on the bottom. 1/3rd that diameter is about 2 and 1/2 inches so you will overhang on each end of the stroke by 1 and 1/4 inches. Push away from you, the thing on top should overhang by 1 and 1/4 inches when you stop. Pull back towards youself till you have that same overhang and stop. That is one stroke. In practice the strokes vary greatly, maybe as little as and inch of overhang, maybe as much as two inches. Don't sweat it. That irregularity will produce a very good surface, maybe a perfect one. Just go for an average of an inch and a quarter and grind. "Less talk, more work." John Dobson

A basic grinding board setup

The basic grinding zone is a surface to put the mirror and tool on, some towels for cushion and cleanup, water and grit. If you are working inside you should have a bucket of water to rinse the mirror and tool in. If you are outside you can spray them off with the hose. John Dobson likes a board placed over two buckets. He can sit on one end and grind on the other. I like the washing machine because it is the right height for me and it is heavy enough that it won't move when I grind. Some folks use a bench or table for the surface. The main concerns for me is that it be durable, easy to clean and it should not move or shift when I grind on top of it. I should also be comfortable as the work of grinding is taxing enough with out adding body contortions to it. When I use the washing machine top I have a board that sits on top of the machine and the grinding goes on, on top of that board. A board is a good thing in that it allows me to pick up the whole mess and move it to storage when I am not grinding. I also put two stops on the board to keep the mirror or tool from moving around.

You will need your tool, some medium course grit, 120 is good, and some water. Since we are going to maul the surface with tool and grit you can start with the pie plate on the bottom and the tool on top, this will use the grit efficiently and there is plenty enough grinding ahead to correct errors. Splash a little water into the curve of the pie plate, maybe a table spoon or two, and add about a teaspoon of grit. A little hump about an inch in diameter and half and inch high. You can put your grit into a shaker some sort so you can just shake it on like salt, or dip in on with a spoon. Put the tool on top and push away and pull back. Nice soft gravely sound and you have done one stroke. You are going to work around the circumference of the mirror making strokes. You should rotate the tool a bit too. The classic instruction is to take a step to the left and rotate the tool to the right. Take another stroke. As you work the two surfaces together the gravely sound will diminish. When the noise is very soft you have finished one wet. A wet runs from the first stroke to next charge of fresh grit. Push the tool away and lift it off of the mirror in one movement. Add a bit of water, some more grit, put the tool down gently and grind some more. After three or four wets rinse the tool and the mirror with water. Then put them back on the board, TOT, and start grinding again.

As the grit cuts the glass it will wear down. When it is fresh the grit is dark gray, almost black in color, and it sparkles. When it is used up it is very pale, almost white, and looks like mud. The color change is due to the introduction of glass dust into the water/grit mix. If there is a coating of mud on the glass the grit won't cut well, you need to rinse.

As you cut away the high spots zones of ground glass spread across the surface. This is a good thing, it shows where the high and low spots are. It also gives us a ready made grinding guage to check the grind. Initially the grind will go slow and your tool will be rough to push, catching and jerking as you do your strokes. This is because the tool and glass of the pie plate do not match well. As you grind the two surfaces together they will wear and the strokes will become smoother, more consistent. As you grind the clear glass will give way to frosted glass. That frosted surface is smooth and soon enough you will polish it. In all the pie plates I have done there are always two final little spots, oposite each other, that seem to take forever. First one will disappear, then the other one will go away and you will have a perfectly smooth surface. The reason it takes so long is you are removing glass from the whole surface to get down to the bottom of those two zones. As deep as it looks it is a very small percentage of the overall thickness of the glass bottom. A trick I use to start my next grit size is when the last clear spot is about the size of a dime, about 1/2 inch in diameter, I switch to the medium grit. For me that is 400 or 500 grit. I grind with that grit until the last clear spot is gone. Now I'm ready for the fine grind.

Keep your strokes full diameter, resist the urge to scrub one spot. That will make a hole that will take alot more work to get rid of. You do not need to press down hard, just a light pressure or if the tool is heavy enough just use the weight of the tool and push it back and forth. Nice long strokes, 1/3rd center over center. Rotate the tool, rotate the pie plate the opposite direction or walk around so you are on a different diameter. One stroke a second is plenty fast enough, one every two seconds is better. We all have an inborn rhythm and if yours is too fast then time yourself. I really get to moving sometimes and to slow myself down I will chant. One onethousand as I push away, two onethousand as I pull towards myself. There is no need to be precise in your movements, just consistent. The randomness of your strokes and constant rotation will create a perfect optical surface. Just to give you an idea of where you are and what you are doing; at the end of your medium grind, if you have been consistent, the two surfaces will match to within one tenthousandth of an inch. That means they will be a perfect pair to withing 0.0001 of an inch. Good stuff, but by the time you polish it out you will be working to within a millionth of an inch and in figuring you will be moving fractions of a wavelength of light. "Grinding is caveman work. Work hard, eat well sleep well." John Dobson.

The fine grind is more of the same but we use a different agent. We will switch to Aluminum Oxide. This looks like flour or powdered sugar but it is a very hard, very fine abrasive. If you cannot fine Aluminum Oxide, Tripoli will work but it cuts slower. Aluminum Oxide comes in several grades but for our purposes 25 micron or 12 micron is what we need. If you use 25 micron extend your wets, make them longer, and use plenty of water. Add a table spoon or two every few minutes. If you use 12 micron it will wear out quicker so you will change it more often. Rinse both the tool and the pie plate between each wet. The aluminum oxide will make a nice froth at the edge of the tool. The stroke will be very smooth, almost silent. Stay slow, resist the urge to speed up. Don't press down hard on the tool or the pie plate, gentle pressure. You will gain nothing and may cause later issues. If you are using a full size tool you can work MOT every other wet. If you are using a ring stay with TOT but you need to start thinking about a full size tool to build a lap on. You should grind with the Aluminum Oxide until the surface looks smooth and matte like. You should be able to read a newspaper through the glass, not the headlines but the regular print. Take the pie plate out into the sun or use a very bright light and look at the ground surface. Look at it from the edge, rotate it and look again. If you have a small magnifying glass look at the surface with that. What you are looking for is sparkles. If a big bright sparkle of light shows up in the middle of matte surface you should grind more. Grind as long as you need to in order to remove the sparkles. Those are pits and you can grind them out but you cannot polish them out. A wet of fine grinding may last several minutes, maybe as much as ten minutes, maybe a couple a hundred strokes. It is hard to say how long the fine grind will last as it depends on you and your technique but I would expect to use at least an hour, maybe two or three.

    Making the Secondary Mirror

The Telescope Parts

    The Primary Mirror Cell

The Secondary Mirror Mount

The secondary mirror mount must hold the secondary mirror at the proper position in the tube and in the optical train. It must allow for minor adjustments of both of these positions and retain the final position without slipping. The mount is actually made of two parts, a mirror mount and a tube position mount. There are just a few considerations to think about ..

• Telescope tube size and shape,
• Secondary mirror size and weight,
• Material available,
• What you want to try and do.

I will start with the mirror mount.

The mirror must be held without tension so that the surface is not distorted, which would impact the image. Hard adhesive products contract as they cure and squeeze the glass, which in turn distorts the surface. Mechanical attachments, clips, rings, etc. must allow the mirror to float, because the differing rates of expansion of the different materials can squeeze or bind the glass. When you decide what mirror mount to use you should also consider how that mount will be attached to the tube position part of the assembly and how it will be adjusted to get proper collimation of the optical train.

One of the oldest secondary mirror mounts is a simple mechanical mount. A short piece of tubing, a bit bigger in it's diameter than the minor axis of the secondary, is shaped by cutting one end at a 45 degree angle. Small L shaped clips are made and screwed or glued to the outside of the tube to hold the mirror in. The mirror is placed in the tube and cotton balls are loosly packed in behind it to hold it against the clips. The tube is then capped on the open end to hold the cotton balls in. See the drawing below ..

The idea here is that the tube takes all of the abuse of mounting and the mirror just floats inside the tube. The mirror is given it's basic orientation, (held at the proper angle), by cutting the end of the tube at 45 degrees and holding the mirror at that end with cotton balls. The mirror and the tube have about the same cross sectional area in the light path so additional obstruction is minimal.

This is a very good mount although it can be a bit difficult to make if you are short on tools. The tubing used can be plastic or metal or what have you, since you use a tube that is a little oversized and it really doesn't squeeze the mirror. You could make one of these out of a toilet paper tube pretty easily and it would function well. The clips could be of thin wood, (popcycle sticks), and could be glued on. The 45 degree angle on the front end should be as accurate as you can get it, but the optical alignment is accomplished with the tube position part of the mount and an angle that is a bit off is not a big deal. This mount is very good for the large secondary mirrors found in larger scopes.

Another mount is made from a simple piece of wood or plastic or even metal. Take a dowel, or even a square or rectangular piece of material, that has ends smaller than the secondary mirror, and cut one end at a 45 degree angle. Now you can glue the mirror to the 45 degree face and you are done. Remember, I said hard adhesives will squeeze the glass as they cure so use a soft adhesive to attach the mirror. Silicon rubber, also called room temperature vulcanizing rubber, RTV for short, will hold the mirror but will not squeeze it as it cures. It also resists temperature changes so expansion or contraction are not a factor. It is a spongy material and will buffer the mirror from shocks and changes in the mount. You should use three small blobs to form the bond, not one big smooshed out blob in the middle. Use some toothpicks or small wood skewers to hold the mirror away from the mount so the glass and the mount are not in direct contact but are held together by the rubber.

Another adheasive you can use for this mount is double sided foam core tape. Get the thickest core you can get. I use this stuff eveywhere on the telescope to mount things, so a roll is a handy thing to have. You must make sure the mount material is very smooth on the cut end, so sand it off very smooth and coat it with layer of nail polish. Now just cut a single piece of tape, put it in the middle of the mirror and stick it on the end of the mount. The foam core provides the same sort of buffering stand off that we got with air and rubber and it is much easier to use.

The danger in an adhesive based mount is not getting the mirror glued on squarely. The mirror must have it's minor axis perpendicular to the eyepiece for the minimum amount light obstruction. The simplest way to make sure it is properly lined up on the mount is to make some index marks on the mirror and the mount prior to gluing.

Get a sharp pencil and a ruler. Lay the secondary mirror face down on a piece of paper. Eyeball in the center, at the edge, of each end of the major axis. Get as close as you can but don't lose sleep over it. Make a little mark there with the pencil. Draw the diameter line for the major axis of the mirror, on the back of the mirror, using the two marks you made to orient the ruler. Go edge to edge. Now measure the major axis and divide it in half. Make a mark at the center of the major axis line and use the ruler to draw the diameter line for minor axis through that mark. Look at the drawing below. Get as close as you can and try to make the intersection of the lines at the middle of the mirror perpendicular to each other. Now take the piece of material you are using for a mount and draw a straight line one each side of the wood, as close the the middle of the side as you can. When you glue or tape the secondary to the mount, line up the lines on the mirror with the lines on the mount and you will be close enough.

These simple adhesive mounts will work well for smaller mirrors, the kinds of mirrors we will be using to make a pie pan telescope. Larger mirrors require a better mount and some form of mechanical mount would be best for those mirrors. There is a hybrid of these two forms that can be used for big mirrors.

Make an oval that matches the shape of your secondary mirror. Glue and screw that oval to the mount face, or weld it on if you are using metal. The oval should be made of good solid material. You should line it up just as you lined up the secondary in the above proceedure using the index lines. Now glue your secondary mirror to the oval using several small blobs of rubber or several pieces of tape. The mirror is well attached to the oval by the adhesive and the oval is mechanically attached to the mount assembly. You could also add a couple of small clips to the oval to help support the secondary mirror. Remember, the mount should not pinch or squeeze the mirror.

The Tube Position Mount

Now that the secondary mirror mount has been made or chosen, we need to work out placing it in the telescope tube. The secondary mirror should be mounted in the very middle of the tube to catch the focused image from the primary mirror and turn it towards the eyepiece and focuser. This process is decided more by measuring than anything, but there are some tricks we can use and there are many ways to position the secondary in the tube.

The first step is to measure the telescope tube diameter inside and outside. You can do the math and size the mount accordingly, but I like to draw a full size diagram and work from that. I take a piece of paper at least three or four inches bigger than the outside tube diameter. Draw lines across the paper from corner to corner using a straight edge. Where the lines intersect is the middle of the paper. Now I use a compass to draw a circle as big as the inside diameter of the tube, or a square if I am using a square tube. Now I can play with various ideas for mounting the secondary mirror in the tube. I can get real measurements and understand what I am going to be working on. Lets talk about some mounts.

The simplest tube mount is a single arm holding the secondary mirror at the right place in the tube. The arm is usually attached to the tube near the focuser hole or on the opposite side. This overcomes one distortion from gravity. If you mounted it to the side wall the arm could flex downward from gravity and make it hard to collimate the telescope.

Single arms are quick and elegant but they have a fatal flaw. They vibrate. That one arm with the weight of the secondary mirror at the end is rather like a pendulum and bumping the tube will make it vibrate for a while. If you use a thicker material and one that dampens movement, for instance a soft wood, you can minimize the vibration, but it will always be there. My telescope, "The Long Dog", had a single arm secondary mirror mount. It could be irritating but I did a lot of viewing with that telescope and I never swapped it out for something else.

Attaching the single arm to the secondary mirror mount can also be a challenge. If you use a piece of all thread metal rod you could bore a hole through the mount and through the tube wall and mount and center the secondary with nuts. That is why I used a single arm on "The Long Dog" as it gave me an infinit adjustment to place the secondary in the tube. Final collimation was accomplished by bending the threaded rod to position the secondary mirror. If you want to glue the arm on you need to be sure the mirror is in exactly the right place and will stay there while the glue dries.

A variation of the single arm is a design using thin parallel strips, closely mounted, and flexed together to form the single arm. This is basically a triangular truss and cancels all vibration. Collimation is achieved by moving the secondary mount on the truss. The friction of the material holds the alignment, or you can glue it up and fix the position pemanently. See the drawing below for details. You can make this assembly out of popcycle sticks, clothes pins or just thin scraps of wood. It could also be made from stiff plastic. This is the mount I use in my pie pan telescopes. It is also in my rock scope.

Really the only consideration is that the material used be flexable enough to not break when put into flexation by the secondary mirror mount. You should cut out the slot on the mirror mount prior to mounting the mirror on it. The slot should only be as wide as three or four pieces of the strip material you are using. There is no need to go wider at the top than 3/4 of an inch. A shorter set of arms would be better served by an even narrower top end, say 1/2 inch or so. The best thing to do is hold the assembly in your fingers and squeeze the ends of the strips to see how much tension there is and adjust the separation accordingly. You want enough tension to hold the secondary in place.

Three arm mounts are an old and reliable standard. The arms are cut to the length necessary to hold the secondary in the middle of the tube. They are then mounted equidistant around the secondary mirror mount so that by their very length they center the mirror in the telescope tube.

John Dobson has used this design forever it seems. Once he has the assembly put together he puts it in the telescope tube, collimates the scope and then glues it in place. He recommends and I agree that the arms should be wide and thin with the thin edge placed towards the tube opening, not round or square. This gives a better strength to the tube mount.

You can do this mount quite easily in wood. Make the mirror mount out of a piece of wood cut to the shape of an equilateral triangle. Think of a long prism. Now cut the 45 degree end for the mirror to mount on. Make the arms and glue and screw or nail them to the end opposit where the mirror will attach. Make sure they are perpendicular to the mirror mount. I cut the arms long and size them for length by centering the assembly on my drawing and marking the arms there. Once it is assembled and the glue has dried attach the secondary mirror and put the assembly in the telescope tube. John Dobson does this mount using a dowell. He cuts a groove in the side of the dowell at 120 degree intervals and then glues the arm into place in the groove.

A variation on the three arm is a single arm that goes all the way across the telescope tube and a second short arm that runs from the mirror mount to the wall of the tube. These are a great design as they are simple to do. They are also good for square mirror mounts.

You can add another arm and do a four arm mount. These are very stable and work well in round or square tubes The more arms you use the more complex the mounting becomes. But! It all comes down to the same batch of considerations.

• Telescope tube size and shape,
• Secondary mirror size and weight,
• Material available,
• What you want to try and do.

Make a decision based on the above considerations. Make a complete mirror mount first, including arms and then attach the secondary mirror. Finally, mount it in the telescope tube.

    The Telescope Tube
    The Focuser Assembly

The Telescope Mount

    The Altitude or Vertical Bearings
    The Azimuth or Horizontal Bearings
    The Bearing Box
    The Ground Board
    The Legs

External Links

• The Surplus Shed, Optical Parts
• Astromart Telescope Making Page
• American Science and Surplus, Optical Parts
• Got Grit sells grinding materials. They have a good reputation.
• Facets Lapidary, These folks are just down the road from me, I buy my stuff there.

Further reading

• "Build your own telescope" shows how to set up an extremely low-cost demonstration of how telescopes work.
• The Cookbook Camera (CCD "Cookbook Camera" Project) was a great resource in 1996 for people who wanted to add a CCD to their telescope. Is "Wikibooks: Telescope Making" the best place to bring it up-to-date?
• adapting a low-cost web camera to a telescope
• adapting a low-cost web camera to a 35 mm film camera lens to make a telescope
• "The Trackball telescope"; OR "The Trackball telescope" describes yet another style of telescope mount that uses a sphere resting against a driven axle to track the stars.
• "Mel Bartels Mirror Making Page" One of the most respected and quoted amateur mirror and telescope makers.
• "Telescope Makers Web Ring Home Page" This is the home page where you can list your web site and show off your creation;This is the list of websites in the ring.
• "The Amateur Telescope Makers list server" You can join the list or just read the postings and the archives. A huge source of information.
• "The ATM Site: Resources and Techniques for Amateur Telescope Makers" Great ATM sites have come and gone but this site has preserved some of the best. Lots of articles to read.