Backyard Voyager
Synta EQ-6 Mount Review
How To Build A Telescope: A step-by-step guide to building your own 8-inch Dobsonian
by Olle Eriksson
This is guide about making your own 8" Dobsonian telescope. It's complete with photos, illustrations and accompanying text describing every step of the process. There are no finished plans available but almost everything is discussed in the text so that shouldn't be a problem.
The telescope presented here is the first I ever built myself. The ideas has since been used to produce several similar telescopes by other people. As far as I know all of them have been very pleased with the results. If you have questions, I'd be glad help you. Just send me an e-mail and I'll try to answer it as soon as I can.
1. Introduction
1.1. Introduction
I once found a very basic book at the library about amateur astronomy. It included a description of how you could build your own telescope with only two convex lenses and a cardboard tube. I called several opticians and asked if it was possible to buy uncut lenses. When I was told how much it would cost I let go of the whole idea.When I a couple of years later recovered my interest for astronomy I decided I would make a more serious attempt to build a telescope. This time I was also ready to put out with more money to get a telescope of higher quality.
I gathered information about telescope making projects both in the library and on the Internet. I found out that the best information is on the Internet. The problem is, no matter how much you search, you will not find any complete plans for telescopes. What most experienced telescope makers want is for you to be inspired and get ideas by looking at pictures of their telescopes. Then you can make your absolutely own telescope.
With the little knowledge I had I started building. Thanks to very helpful people of the Internet I could get help any time I needed.
The meaning of this report is to show how easy it is to build your own telescope. The first part explains how I built the telescope. The last part shows the result of the project and how the telescope works under the stars.
2. Construction
2. 1. Overview
In January 1997 I decided to build my own telescope. My requirements on the telescope weren't very many since I had no experience using telescopes. All I wanted was an instrument to look at the stars and planets with.First I thought about whether I should build a refractor or a reflector. After having looked into it I decided a reflector of the type Newton would be easiest to build. It can be used for both planet and deep sky observation, it is easier to find optics for and finally, it is very cheap.
From the beginning I had planned to make an equatorial mount for the telescope but then I was influenced by an amateur astronomer and telescope maker in Sweden and finally set for a dobsonian type mount. Dobsonian mounts are easy to build and doesn't require any materials difficult to access. A couple of plywood sheets are all you need while an equatorial mount requires heavy-duty metal pipes to be rigid.
Fig. Newtonian telescope.
Basically, you can say a Newtonian consists of a mount holding a tube, which itself holds a parabolic mirror, a secondary diagonal mirror and an eyepiece. For the mirrors to stay in place inside the tube, you build holders. These are the parts of the telescope that are the most difficult to build. The holder for the parabolic mirror is called a primary mirror-cell and the one for the secondary diagonal mirror is called a diagonal holder or secondary holder. The secondary holder together with the "legs" holding it to the tube is also sometimes called spider thanks to its similarities with the insect.
I decided to buy the mirror, secondary and the eyepiece. It would take too long to build them myself and I had no experience of this. To be able to adjust the eyepiece's height from the tube wall you use a focuser. You build these too but it's also too difficult for the beginner.
There are almost no suppliers of telescope making parts in Sweden where I live so I had to turn to a company in Denmark called Astro Mekanik.
I ordered the following parts:
- 8 inch f/6 parabolic mirror
- 1.57 inch secondary mirror
- 1.25 inch rack-and-pinion focuser
- 1.25 inch super plössl 26 mm eyepiece
- Pieces of Teflon (for the mount)
I decided to start building the primary mirror-cell and secondary holder first. I realized these were the parts that would be the most difficult to build and if I failed with these and lost my interest in the telescope I wouldn't have ordered mirrors for a lot of money.
A telescope also needs a tube. I had no idea where to find a suitable tube but I thought I'd find some sort of plastic pipe later. I planned to use plywood sheets of 0.4" and 0,6" thickness to the mount. I was going to buy them later.
Fig. The tube with it's holders.
2.2. The primary mirror-cell
Fig. Primary mirror-cell without the mirror.
The point with the primary mirror-cell is to hold the mirror in its place inside the tube and also to allow the observer to adjust its position a little. When all the optical parts have been assembled and fastened inside the tube you have to adjust them to make sure the light that hits the primary mirror really is reflected exactly towards the center of the secondary mirror and then exactly towards the center of the focuser and eyepiece. If the light is reflected away from this central axis along the optical path through the telescope, it could result in a totally black view through the eyepiece. To adjust the mirrors of the telescope like this is called collimating.
For the easiest possible collimation, the primary mirror-cell is constructed with screws that does all of the work. All you have to do is turn them in the right direction and to the correct amount until the mirrors are aligned.
Fig. A drawing of my primary mirror-cell design.
Most primary mirror-cells consist of two plates. The mirror lies on the upper plate and the lower plate is connected to the tube walls. Between the two plates are the collimation screws holding them together. They are organized in pairs around the plate, 120 degrees apart. To change the distance between the plates you simply turn one screw and loosen the other screw in the same pair and the plate holding the mirror will be either lifted or lowered on one side.
When you move the telescope from a warm place, for example inside your house if that is where you store the telescope when you are not using it, to the chilly night air, dew can form on the tube and the optics. The changes in air temperature makes the air unstable which results in the light bending differently, causing image distortion and less light reaching the eyepiece. To minimize this effect it is favorable to make the circulation of the air inside the tube as good as possible.
Because the primary mirror-cell takes up a big part of the place in the tube, the circulation of air from the backside is limited. With a larger distance between the tube wall and the mirror-cell, the circulation is increased. My primary mirror is 8 inches in diameter and I used a tube with an inner diameter of 10 inches. That means I have 1 inch of free space on both sides of the mirror, which is most often acceptable.
Fig. The bottom of the primary mirror-cell with its collimating screws.
Another factor to keep in mind is the rate at which the mirror is cooled down when taken outside. Glass, like many other materials are distorted at temperature changes. The best result comes when the mirror is cooled down. By making a hole in the middle of the primary mirror-cell's plates, air can circulate behind the mirror and decrease the time it takes to cool down.
Mike Lindner has built a very nice 6-point mirror cell that you can find at http://www.starastronomy.org/TelescopeMaking/MirrorCell/index.html .
2.3. The secondary holder
The secondary holder is what holds the secondary diagonal mirror almost at the front inside of the tube. Since the secondary mirror sits in the path of the light an importance factor to consider is the size of the secondary. Always keep it as small as possible. Generally, it is said that the diameter of the secondary mirror should be about 20-25% of the primary mirror's diameter. But there are formulas if you want to find out the exact optimal size.
d = F * q / 57,3 + D * L de / F
d stands for the diameter of the secondary. Since it is elliptical it refers to its minor axis. F is the focal length of the primary mirror and D is its diameter. q is the fully illuminated field in degrees. For most purposes a field of 1/2° is enough, the same angle as the diameter of the moon forms. L de stands for the distance between the center of the secondary to the eyepiece. The unit for all amounts is inches if not otherwise noted.
In my case the formula looks like this.
d = 48 * 0,5 / 57,3 + 8 * 8 / 48 = 1,75"
To get the best possible image you will need a secondary with the minor axis at least 1.75" in diameter. Since secondary mirrors are manufactured with certain dimensions you need to choose the one with the next closest diameter. A larger one blocks more light so it preferred to choose the next smaller one. You can't use absolutely all of the light gathered by the primary mirror but the difference is often not even noticeable. I choose a 1.57" secondary.
The support of the secondary holder is what holds it up in the tube. This is also where you collimate the secondary mirror. Because the mirror has to be placed in the middle of the tube, thin vanes go from the tube to the secondary holder. There are many ways to put the vanes but in most cases the whole support will look like a spider seen from above. That is why the secondary holder with its support is often called the telescope's spider.
Fig. A drawing of my secondary holder design.
Just as with the primary mirror-cell you want to be able to adjust the secondary mirror's position, or collimate it. The idea behind the secondary holder is the same as with the primary mirror-cell. Two blocks: one holding the mirror and the other hanging on to the tube wall (through the spider vanes). Between the two blocks are collimation screws. The difference here is that I have used only one screw to pull the two locks together and three screws to push them away. The pull screw is placed at the center of the two blocks and the tree push screws are placed in a cirle around the center, each separated 120 degrees apart.
Here is a map with different spider types. The circles are supposed to illustrate the tube seen from above and the boxes placed just over each circle are the focuser and eyepiece.
Fig. Different spider types
There are many different spider types. Everything from those using one vane to those using four vanes. It is most common to use straight vanes but it is also possible to bend them. The optical difference doesn't differ very much. The downside of using a one-vane spider is that it will not be very rigid but it is maybe easier to build and it causes less optical distortion.
When I was to choose spider type I was ready to make a four-vane spider but after a while I realized one problem that would mean. It would be hard to come up with a good way of fastening the spider vanes to the body of the secondary holder without unless I welded them.
Fig. My spider design
I got the idea for a three-vane spider (no 7 in the figure) from an amateur astronomer and telescope maker in Sweden. He had used a long vane from one side of the tube to the other and on this he had glued the body of the secondary holder. 90 degrees against this vane he had glued another shorter vane going from the secondary holder to the side of the tube facing away from the focuser. The advantage with this design as I see it is that you can use glue to fasten the vanes to the body of the secondary holder and don't need to use screws or anything else that will result in scattered light and unnecessary obstruction of the light.
As spider vanes I used 0.04" thick metal saw blades. Try to keep them as thin as possible to avoid too much obstruction of light. 0.04" may sound little but it creates a very rigid spider when assembled. Since the metal in the saw blades was hardened I couldn't saw them to the right length but it was easy to just break them.
Also, have a look at Bruce Weertman's homepage where he uses my secondary holder design on his 8" travel scope (Acc. november 2002).
2.4. The tube
As a telescope tube I used one of those cardboard tube that is normally used as concrete forms. It chose one with an inner diameter of 10" and a wall thickness of 0.2". This meant I had 1" of free space on each side of the primary mirror. This kind of tube actually quite rigid and very light.
Fig. The dimensions of the tube.
At this stage, all you want to do is to determine the length of the tube, so you shouldn't shouldn't worry about any small margins here. Start by deciding how far from the front end of the tube the secondary mirror will be and how much tube you want behind the primary mirror. In doing that you have to consider how your spider and mirror-cell are constructed. Remember we are taking about distances between the face of each mirror here. I chose about 8" from the secondary to the front end of the tube and about 4" behind the primary mirror.
The reason the distances in the figure above are not rounded off to complete integers is that I work with the unit mm and I have simply converted them to inches. To see this, notice that 7.87" ~ 8" and 3.94" ~ 4".
Now, to know how long the tube needs to be between the primary and secondary mirror, you need to know the distance between the secondary mirror and the eyepiece since the eyepiece should always be placed in the focal point of the primary mirror. Take half the tube diameter, the tube's wall thickness, the height of the focuser when fully racked in and finally half an inch or so extra focus travel and add it together. Substract that sum from the focal length of the primary mirror and you end up with the required tube length between the primary and secondary mirror. In the figure above, this is Note that this length is between the *face* of each mirror so you will have to adjust the distance before you drill the holes for the spider and mirror-cell in the tube.
You will not drill the holes for the primary mirror and its mirror cell at this point, only the holes for the spider. That means the distances are not crucial. You will always be able to adjust the distance from the eyepiece to the primary mirrror later when you insert the primary mirror into the tube. Actually, if you mis-calculate by as much as an inch, all that happens is that the tube becomes an inch shorter at one end of the tube and an inch longer at the other. As long as the tube doesn't get too short at the front end (near the secondary mirror) and allow for stray light to enter the focuser from the back side, it is ok.
I cut the tube to 52" but waited to drill the wholes for the mirror holders and the focuser.
To avoid light from being reflected by the inner sides of the tube it is a good idea to paint the inside of the tube with the most flat black paint you can find or add so called baffles. Baffles are thin rings of some material that are fastened along the inside of the tube, about 6" from each other. These are probably the best way to prevent stray light but it can be difficult to obtain rings of perfect diameter. For that reason I choose to use the black paint which absorbs the light instead of blocking it out.
When you are observing with the telescope, dew forms very easily on the tube. If the tube is made of metal, this is no problem, but when it is made of paper, it will get decayed. Painting the outside of the tube with enamel will make a perfect protection against dew and fluids (enamel is used to paint the parts of boats that are underwater). There are both one- and two-component enamels. The two-component version is a better protection against fluids but since the one-component version is so much easier to coat it became my choice. I painted the tube with three layers of enamel because it took two layers for the paper to suck up the first enamel and the third layer formed a nice strong surface.
Choice of color is optional but I choose black because of several reasons. If you look through the eyepiece with both eyes open, the part of the tube you indirectly see with the "wrong" eye should be as dark as possible to keep your eyes adapted to the dark. Another reason is that dirt doesn't show up as much on a dark surface as it does on a white surface.
2.5. Installing the mirrors
There are a few different ways to install the mirrors in the holders. The most commonly used nowadays is to use silicon adhesive to simply glue the mirrors into the holders. That was what I did. Silicon adhesive is often used when gluing together the glass sides of an aquarium and it is very strong. En fallen secondary is bad, but 400 liters of water in the living room is worse.I started by making three small pieces of wood that I glued around the sides of the primary mirror-cell so that they stood up about an inch above the upper plate. I also drilled a hole through each of them (see figure). Then I placed three 0.08" nails on the upper plate with the meaning of keeping the primary mirror separated from the plate while being glued.
Fig. Gluing the primary mirror.
I put three blobs of silicon on the upper plate and then lowered the primary mirror onto the nails. When the silicon had dried after about a day I could pull out the three nails and the mirror nicely rested on the silicon about 0.08" above the mirror-cell. This margin is useful to prevent the mirror from getting strained if undergoing deformations at temperature changes.
When gluing the secondary mirror I did the same, only I used 0.04 nails and less silicon.
2.6. Assembling the tube
Fig. The tube assembled and painted.
When it is time to insert the mirrors inside the tube, start by drilling holes for the spider and fasten it with the secondary mirror attached and mount the focuser to the outside of the tube. Then insert an eyepiece in the focuser and turn the focuser knob until the eyepiece is about half an inch or so from fully racked in. This time it is much more crucial with correct distances since it is now time to mount the mirror in it's final position.
At this point it is good to be two people. Lay the tube down on a chair or something and point it towards a distant horizon. Let one person look into the eyepiece at the same time as the other inserts the primary mirror in its mirror-cell from the backside of the tube and slowly pushes it further and further into the tube. The person looking into the eyepiece will now see a fuzzy picture of the horizon if the tube is pointed correctly. As the other person moves the primary mirror further into the tube, the picture in the eyepiece will eventually get sharper and sharper and when it is as sharp as you can get it, put a mark on the tube indicating the current position of the mirror-cell. Now take the primary mirror out of the tube, drill holes for the mirror-cell and finally assemble everything.
Try to use an eypiece with as low magnification as you can find when doing the procedure above, as low-magnification eyepieces normally require more focuser in-travel than a high-magnification eyepiece. Doing this, you will be quite certain that no eyepiece combination require more focuser in-travel than your telescope allow.
2.7.The mount
Now was the telescope tube with all its optics ready. The only thing I had left to make was the mount. I choose to build a dobsonian mount, which basically consist of three main parts: a tube box holding the telescope tube, a cradle and the base plate. These models try to explain how the mount works.
Fig. The dobsonian mount.
The vertical motion of the telescope tube comes from the tube box's altitude bearings (the rings on its side) gliding against the cradle and the horizontal motion comes from the cradle rotating around a screw going through both the cradle and the base plate. Both the altitude bearings and the bottom side of the cradle glides against Teflon pieces giving a perfect friction when aiming the telescope. The altitude bearings on my mount are two rings taken cut from a PVC plastic pipe of 10" in diameter.
Fig. The altitude bearings gliding against teflon pieces.
The sides of the tube box should be short enough to make the tube box just a little smaller than the outer diameter of the tube. If it is you can fasten the tube in the tube box just by screwing together the sides of the tube box a little harder. Do this after you are sure the gravitational point of the tube is exactly at the center of the tube box.
Fig. The cradle and the base plate almost completed.
I cut out all of the sides for the mount from a plywood sheet. Then I assembled the rest of the mount except the sheets of plywood for the sides of the cradle on which the altitude bearings would glide. By measuring the distance from the center of the altitude bearings to the back of the telescope tube I knew the minimum height of the mount. I added a few inches and fastened the two remaining sides with screws at the correct height.
Finally I added the Teflon pieces and then the mount was ready.
3. Using the telescope
3.1. "First light"
The moment you look through a new telescope is often called "first light". I had my "first light" on the evening the 5th September 1997. I couldn't wait until the mount was finished so when the tube was completed I carried it out to the backyard and put it down on a wood sheet in the grass. I inserted an eyepiece in the focuser and looked through it. It was quite clear that night and the stars were visible. I was actually amazed at how many stars there were that I had never seen before.
I aimed the tube toward bright planet Jupiter in the southeast. When I first looked through the eyepiece I didn't see anything but after moving around a little the planet's disk appeared in the field of view. It was larger than I though it would be along a line was Jupiter's four brightest moons. On the planet were features as cloud bands visible.
The experience still didn't fully fulfill my expectations. It was difficult to focus the image and it was not as clear as I though it would be. I aimed the telescope towards the Andromeda galaxy. It can be found in the Northern Hemisphere and is visible as a faint fuzzy patch of light for the naked eye. This object didn't look as good as I had hoped either.
3.2. Collimation
When I had completed the telescope I found out why the telescope hadn't worked as I had hoped. There were several reasons: the view had been shake because I hadn't used the mount and the telescope hadn't been collimated.Collimating means aligning the mirrors so that they reflect the light along the central line through the telescope. If they are not aligned, the light will get lost somewhere and not reach the eyepiece. Collimation is done by turning the collimation screws on the primary mirror-cell and the secondary holder.
The following procedure is one way of collimating a telescope:
1. Center the diagonal in the focuser
2. Center the primary mirror in the secondary
3. Center the diagonal in the primary mirror
There is much more informatioin about collimation and how to collimate telescopes if you follow these links:
Collimating Newtonian Optics - by Mel Bartels ( http://www.efn.org/~mbartels/tm/collimat.html )
FAQ about Collimating a Newtonian telescope - by Nils Olof Carlin ( http://zebu.uoregon.edu/~mbartels/kolli/kolli.html )
3.3. Under the stars
A double cluster in the constellation of Perseus. Photo by Olle Eriksson.
The next clear night I brought the telescope out. Jupiter was visible in southeast and the moon was about to set in southwest. I aimed the telescope at Jupiter and looked through the eyepiece. This time the planet looked much better and the contrast was higher. I could clearly se the cloud bands along it's surface and the four moons were sharp pin points.
I also took a look at the moon close to the horizon. When it entered the field of view I jumped up in surprise. It was the most incredible view I had ever seen. Especially the contrast and high detail level was literally unbelievable.
I aimed the telescope toward the Andromeda galaxy and it was still very faint. As soon as I moved the telescope away from the nearest light bulbs it stood out much better.
Suddenly I realized Saturn was visible in the west and when I turned around I found it right away. I aligned the telescope and had an unforgettable view when I first caught the planet disk. I was shocked at how clear its rings were and that you could see features on the surface.
It's important to understand that the view seen "live" through a telescope can't be compared to the beautiful color photographs in astronomy magazines. In the eyepiece everything is in black and white except the brightest planets. Galaxies and nebulas look like faint fuzzy patches of light. To bring out the beautiful colors and sharp details in galaxy spiral arms you need to let them be exposed on photographic film during long times.
What drive amateur astronomers around the world to spend hour after hour behind the eyepiece is the knowledge that you see everything "live" and the extraordinary high detail level the human eye yields that photographs can't achieve by far. It's a special feeling to observe the manifold of a star cluster, search details on planets and navigate between galaxies and nebulas.
4. Summary
4.1. Summary
Fig. The final telescope.
The total cost of this project ended at a little over $550. It's about what a commercial telescope of this kind costs, only the shipping charges excluded. So for me, I got away a little cheaper. Another big advantage with making your own telescope is the experience you gain and the insight into the workings of a telescope, which is really invaluable. If you are only a little handy and have access to a working place I definitely advice you to make your own telescope if you are thinking about buying one. But the reason in front of all is that it is so much more fun to explore all of the things in the sky when you know that you have made the instrument yourself.
4.2. Costs and suppliers of material
A lot of people send me e-mails asking for how much the telescope cost to build so here is the complete list. The cost of the optics probably varies a lot dependning on where you live and where you can find the nearest supplier of telescope optics.8" primary mirror - $185
40 mm secondary mirror - $28
1.25" Rack-and-Pinion focuser - $37
1.25" Super Plössl 26 mm eyepiece - $54
Teflon sheets - $5
Telescope tube - $14
Plywood - $26
Paint - $20
Screws, bolts etc - $20
This information was written down when I built the telescope so the numbers should be fairly accurate, but there are also a lot of other expenses like new tools etc which are not included here.
I also get e-mails from people asking for where you can find suppliers of mirrors and other telescope accessories. There are a few ATM resource lists on the internet which list known suppliers. Have a look at the article ATM Resources in Scandinavia on my homepage for some addresses and links to the other lists.
4.3. The telescope rebuilt
The telescope worked just fine as it was but my requirements changed when I moved to an appartment two years later. At that time I rebuilt the telescope to make it lighter and more portable. Read all about the new telescope in the following article.My second telescope
If you have any questions regarding this article or something telescope making related, feel free the e-mail me.