Intro: Wooden Telescope Part 2: Tube and Mount
A lot of you enjoyed reading the story of how I built the primary mirror of my telescope. This Instructable will show you how I designed and built the tube and the mount.
The overall appearance of the telescope is a merge of ideas taken between various forums, telescope building websites and renowned opticians sites.
I did not aim for something ultra-transportable or light weight. Instead, I designed it with one idea in mind: "It will probably stay in the living room most of the time so make it look nice". I decided to build it entirely out of wood. Some of the advantages are that the closed tube protects the optics from dust and the higher mass makes it more stable in the breeze. The drawback is a heavier instrument and slower thermal exchanges.
Step 1: Choose a Design
The Optical Path
The good thing about telescopes is that the design is almost entirely up to you. There are only a few rules to keep in mind:
- The curvature of your primary mirror dictates the length of the tube.
- Choose a focuser before building
- Decide if the instrument will be used for visual observation or astrophotography
In my case, it was easy to obtain the curvature of my mirror since I made it myself. If you bought the primary mirror, it will probably come with some info such as the diameter and the focal ratio. To get the focal, multiply the diameter by the focal ratio (often called F/D):
Focal = Diameter x Focal Ratio
In my case f = 7.93 x 4.75 = 37.67 inches. This is the distance from the mirror where a sharp image is produced. However, you can't put your head in front of the mirror, that would block the light coming from the stars. That's why we will use a secondary mirror (also called elliptical) oriented at 45 degree, reflecting the light to the side.
The distance between this mirror and your eye will depend on the size of your focuser. If you chose a low profile focuser, this distance will be minimal and you will need a smaller mirror. However, if you chose a taller focuser, this distance will be longer and the elliptical will need to be larger, thus reducing the amount of light being reflected on the primary mirror.
The last thing you need to decide is whether you want to use this telescope for visual observation or astrophotography. For visual, a Dobson mount (alt-azimutal) and a small elliptical are all you need. For photography, you will need a precise mount to cancel the earth rotation, a 2" focuser and an oversized elliptical to prevent vignetting on the images.
Luckily, there is a convenient website that lets you play with these criteria and optimize your design: Newt for the Web. You can load the file attached to view my own configuration.
Like I said, once you've chosen the optical path configuration, the rest is entirely up to you. One of the websites I was using during the mirror construction is Mel Bartels website. He is a master mirror builder and has produced awesome rich field telescopes. I really liked the hexagonal design of his 10.5 in @ F2.7.
As I started to look for hexagonal telescopes, I found Normand Fullum's instruments. They are a mix between a scientific tool and a piece of art. I borrowed the 16 sided tube idea from him.
Another source of inspiration is this walnut octagonal scope from which I borrowed the tube rings design, the baffles and primary mirror cell.
For the secondary mirror holder, I used Gary Seronik's curved vane design.
Lastly, I designed my mount using the Stellafane website.
Step 2: Materials and Tools
You can use a lot of various materials to build a telescope. I will only list here the ones I used to build mine.
- Primary mirror. Mine is a hand built 8" diameter with a 4.75 focal ratio.
- Secondary mirror. Building one is not really worth it. It would require a really flat reference which is hard to achieve. I ordered mine from Antares Optics. It has a 1.83" small axis and is rated 1/18 wave peak to valley. It cost me $99.
- Focuser. I bought a Moonlite Crayford 2" dual speed with 1.5" travel. It has a low profile of 1.45". This one cost $265. I know some of you will scream at the expense but trying to focus precisely on a low aperture scope is quite hard and frustrating with a first entry focuser. Also this focuser won't shift under the weight of a DSLR.
- Wood. I used 2x4 red cedar boards. They need to be a few inches longer than your primary mirror focal length. I used 40" boards. Refer to the dimensions given by "Newt for the web".
- Plywood. Even though I wanted to use hardwood everywhere, I decided to use some plywood for the base of the rocker. I also used some 1/4" plywood to create the baffles.
- Various hardware store items such as a stainless steel ruler, some countersunk screws, nylon screws, silicon glue, teflon pads, springs, etc
Here's a list of the tools I used:
- Table saw for the tube boards
- Bird's mouth joinery router bits from Busy Bee and a router table.
- Hand plane to adjust the tube boards.
- Various grits of sand paper.
- Band saw / jig saw for the rocker.
- Hole saw for the focuser hole.
- Laser cutter to cut the baffles, mirror cell and tube rings
Step 3: Preliminary Assembly
As I am not following any plans, I had to do a preliminary assembly to validate the design. I started by cutting the baffles out of 1/4" birch plywood. I used a laser cutter because I figured I would not be able to cut a thin piece like that with a jigsaw without breaking it.
The next step is to cut the 16 boards of the tube. Mine where 2" wide but you would have to scale it to your own design. Using the bird's mouth joinery bit, route one side of each board. This will create a groove that will lock onto the adjacent board, preventing it from moving.
To assemble the tube, use two bands of masking tape to hold all the boards together and roll it all around the baffles. You can use only one baffle at each end since it is only a temporary assembly.
Step 4: Baffles and Boards
Now that you've made sure that all the boards fit together and have the right dimension, you can start glueing the baffles to one of the board. You will find the measurements and positions using the Newt for the Web Software.
Glue the boards on every other faces of the baffles. This will ensure a better regularity of the tube. You can then adjust the other boards to fit perfectly in between using a hand plane and some sand paper.
Step 5: Smooth Out the Tube
Now that the tube is all glued, you need to trim the boards to make the surface smoother. You can use a hand plane for this and some sand paper. I used 120, 220, 400 and 600 to get the wood as smooth as possible.
If you notice that some boards are not perfectly joined together, make some wood paste using wood glue and wood dust. Mix the two together and apply it in the cracks. Let it dry and sand off the excess.
Step 6: The Focuser Opening
To place the focuser, the first thing you need to do is calculate its position. Use Newt for the Web to find the distance between the optical axis of the focuser and the end of the tube.
Once you've found that distance, use a hole saw slightly larger than the focuser tube and drill on the center of one side. Place the focuser, draw the position of the screws with a pencil and remove the focuser. You should now be able to drill the 4 holes at each corner. This is where you will use screws to strongly attach the focuser onto the tube.
You can see that my Moonlite focuser was slightly larger than the sides of the tube. I added 2 wedges on both adjacent boards to create a flat area.
Step 7: The Mirror Cell
I built the mirror cell out of 3 layers of 1/4" birch plywood glued together. The only reason I did this is because I did not have any 3/4" plywood at home.
I used the design available on the Stellafane website. It is a 3 points cell which is enough for my full thickness mirror. If your mirror is larger or if its thickness is lower than 1:6 its diameter, consider using a cell with more support (9 points for example). The PLOP software is a good tool to calculate the glass deformation for different cell configurations.
The cell is painted black to avoid unwanted reflections inside the tube.
Step 8: The Elliptical Support
I used Gary Seronik's curved vane design to support the secondary mirror. It is a simple design but surprisingly strong enough to keep its position even when the tube gets moved in various positions.
The support of the mirror is a piece of wood slightly smaller than the small axis of the mirror. I used a block of birch and made a 1.75" dowel on a lathe. You will need to drill the center to screw a threaded rod later on.
Step 9: Assemble the Secondary Mirror
The curved vane is made from a bent 12" stainless steel ruler. The dowel is flattened on one side and bolted in the middle of the ruler. The rest of the dowel is cut at 45 degree angle and the mirror is glued on it using silicon glue. Leave a gap between the wood and the glass to prevent tensions. Let the glue dry for at least 24 hours.
Paint the dowel and all shiny parts with a black satin paint to prevent unwanted reflections.
Step 10: Test the Optical Elements
This is the time to test your telescope. If you followed the distances and dimensions given by Newt for the Web, you shouldn't have any bad surprise. One of the disadvantages of a closed tube is that you can't easily make it shorter or longer. If you can't focus on a star using an eyepiece, you will have to move your mirror cell closer or further. This is why screwing the mirror cell is usually the last step.
To collimate your instrument, start by orienting the elliptical mirror towards the focuser. Make sure it is centered. If the mirror is too high/low, turn the mirror on its support until it looks centered. Tilt the mirror until you can see the primary mirror in the center.
Then, adjust the 3 screws of your primary mirror cell so that you can see your own eye in the center. You should now have a collimated instrument.
Step 11: The Tube Rings
I did not want to screw the altitude bearings directly on the tube so I looked for a system that would clamp around the telescope but allow movements, so that I could shift the center of gravity of the tube.
I got inspired by the design on this page. I made the rings out of plywood (3 sheets of 1/4" birch) and the altitude bearings out of walnut. I used wood glue and dowels to hold everything in place. The bearings are assembled using countersunk machine screws.
The contact between the rings and the tube is secured by cork pads.
Step 12: The Rocker Box
The rocker was designed using Stellafane's recommendations. The bearings are 1.2 times larger than the mirror. The recommended value is usually between 1.2 and 1.8. Oversized bearings help keeping the movements fluid.
The rocker is built out of walnut and maple. I used teflon pads for fluid movements. The altitude bearings are covered with kitchen counter-top laminate (Wilson Art's Ebony Star is considered the best surface).
The sides of the rocker are mounted on a 1" thick plywood circular base. Handles are cut on each side for easy transportation.
Step 13: The Azimuth Bearing
To be able to rotate the instrument from left to right, we need to add a vertical axis. This is called the azimuth bearing.
The base is made of 1/2" birch plywood mounted on 3 hockey pucks (reduces vibrations). There is a central rod and 3 teflon pads above the feet.
The bottom of the rocker is covered with a sheet of kitchen counter-top laminate and drilled in its center. A brass sleeve is mounted to reduce friction.
Step 14: The Finished Telescope
You can now assemble all the parts. You will need to find the center of gravity of your tube to place the rings properly. To do that, just lay the tube + rings on a table and shift the tube until it does not fall towards the front or the back.
You will need an eyepiece. The lower the focal length, the higher the magnification. To calculate the magnification use this formula:
magnification = telescope focal length / eyepiece focal length
My 11 mm Explore scientific 82 degree eyepiece gives me an 86x magnification.
Another thing to consider is the field of view. A wide angle eyepiece will generally be heavier and more expensive but it will give you that "floating in the sky" feeling. A narrow angle eyepiece will show you the sky as if looking through a tube.
Unless you enjoy looking at random parts of the sky, you will probably need a finder. There are a lot of different systems such as red dot, telrad, finding scope, green laser pointer, etc.
To prevent the dust from accumulating on your primary mirror, you will need a cap at the front end of the tube. A simple piece of plywood with a handle will do just fine. If you feel adventurous, you can make a more fancy one.