How to Build a Dobsonian Telescope at Home

Because of the simplicity of the Dobsonian’s reflector design and altazimuth mount, it is possible to build one yourself at home. While doing so may not necessarily save you significant amounts of money, the satisfaction of enjoying the nighttime sky with a viewing instrument you crafted yourself make building your own well worth the effort. Following is a basic overview of the steps to follow.

Choose your aperture size and design.

Before you begin building, you will need to choose a few basic features that control the other design details: Your aperture size (the diameter of your primary mirror) and your focal length.

Once you select these features, you should be able to input them into a design program that will calculate the dimensions you need. One of the most popular of these programs is Newt-Web, which will create a Dobsonian design that meets your specifications. Alternatively, many builders have found it useful to consult the designs in books such as Plans for Building a Sidewalk Telescope by John Dobson himself, or Build Your Own Telescope by Richard Berry, though these designs require you to scale them yourself to match the aperture size and focal length you desire for your scope.

Buy the parts and tools.

Once you have your design, you should purchase the parts and tools you will need. When Dobson first created the telescope, he located most of the material he needed inexpensively or for free in the form of castaway construction material. Today, builders report that the material can be harder and more expensive to find. However, through careful research and a couple of trips to a well-stocked hardware store, you should be able to find what you need. Following is a basic list of the supplies you will most likely need:

  • Primary mirror
  • Secondary mirror
  • Teflon
  • Telescope tube (otherwise known as a Sonotube. This part is one of the more difficult pieces to source.). Alternative to telescope tube: Plywood to build your own telescope tube
  • Spider
  • Mirror cell
  • Focuser
  • Ebony star strips
  • Baltic birch sheets
  • Telrad base
  • Birch veneer
  • Stain
  • Washer, nuts, and bolts for the focuser
  • Medical cotton
  • Silicone adhesive
  • Woodworking hand tools
  • Electric drill
  • Miscellaneous items as required by your design/building process

Construct your optical tube.

The optical tube design for a Dobsonian is very simple, consisting of a number of baffles (octagonal pieces of wood placed within the tube to support it and to direct the light through the lenses and mirrors).

As with many aspects of building your own Dobsonian, you can choose whether to build the tube yourself or to purchase it. If you choose to build the tube yourself, you will need to cut the correct number of baffles, and attach them to a long piece of plywood that is the correct length of the tube. You will then need to complete the tube by cutting and securing pieces of plywood around the baffles to create the tube. Your design program can help you to determine how many baffles you need and how long your optical tube should be.

The simplest method, however, is simply to purchase a sonotube, which is a long length of plywood tubing that can serve as the optical tube. At that point, you simply need to cut the tube to the required length.

Mount the focuser to the optical tube.

The focuser is the part of the telescope that focuses the light as it comes in the scope. It is mounted in the top of the optical tube, and is the first piece of the tube which you must mount. There are several steps involved:

  1. Measure the diameter of the focuser base.
  2. Your design program should give you the information you need to know where to drill the hole.
  3. Drill a hole slightly larger than the focuser base.
  4. Center the focuser in the hole.
  5. Mark where the bolts will go.
  6. Remove the focuser and drill the holes for the bolts.
  7. Replace the focuser and bolt it onto the tube.

Choose whether to build or buy your mirrors and lenses.

Your next step will be to decide whether to build your buy your own mirrors and lenses for the scope. To be as true to Dobson’s telescope-building experience as possible, you should grind your own mirrors and lenses. However, the process is relatively involved, and many builders opt to purchase their mirrors in order to save time and hassle.

Mount the primary mirror to the mirror cell.

Once you have either created or purchased your mirrors, you must place the primary mirror in the mirror cell, which will hold it steady within the optical tube. At this stage, gentleness is key to preventing optical distortion. Even slight bending of the mirror can create optical distortions.

If you are using a mirror cell that uses retaining clips, you should tighten the clips just enough to prevent movement of more than 1/16th of inch on either side. However, the clips should do nothing more than barely touch the mirror in order to prevent optical distortion.

If you are using a homemade mirror cell, you should be able to gently secure the mirror to the cell with the use of silicone. Because the homemade cell does not use retainer clips, it lowers the risk of optical distortion from overly tightened clips.

Mount the mirror cell to the optical tube.

The mirror cell can then be secured to the front end of the optical tube by using half-inch long plywood wedges secured around the tub end. These strips provide a mounting upon which you can place your mirror cell, which is attached to the mounting by silicone adhesive pads.

To this mirror you should also add collimation bolts. These bolts allow you to adjust the mirror as needed when collimating the scope. They also more securely attach the mirror cell to the telescope, holding the primary mirror securely in place.

Mount the secondary mirror to the spider.

The last two internal pieces to your Dobsonian will be the spider (the holder for the secondary mirror) and the secondary mirror. Again, purchasing a spider and mirror instead of making them is the more convenient option.

The secondary mirror, also known as a diagonal mirror, mounts to the spider using either a holder with a shell and a rim or using adhesive mounting. To mount the mirror to the spider using the shell and rim set up, you need to carefully insert the mirror into the shell and use medical cotton to gently press the mirror into the rim. As with the primary mirror, gentleness is paramount to prevent bending the mirror and suffering optical distortions.

If you are using an adhesive mount, you can attach the secondary mirror simply by applying silicone adhesive to the holder, applying toothpicks to add space to between the holder and the mirror, placing the mirror on top, and allowing the whole thing to dry. After a day of drying, you simply need to remove the toothpicks and your mirror will be securely attached to the spider.

Mount the spider to the optical tube.

The spider, with the secondary mirror in place, will need to be mounted in the optical tube directly below the focuser. Following are the steps to take:

  1. Set the holder adjustment gap to ½ inch.
  2. Measure the shortest distance between the mirror surface and the back of the holder.
  3. Measure the longest distance between the mirror surface and back of the holder.
  4. Add the values from steps 3 and 4 and divide by 2.
  5. Measure the distance from the back of the holder to the mounting hole on the spider.
  6. Add the values from steps 4 and 5.
  7. Measure the distance from the front of the tube to the center of the focuser.
  8. Subtract the value from step 6 from the value from step 7 to determine how far to mount the spider from the front of the optical tube.
  9. Drill your holes and mount the spider at the determined location.

Construct your mount.

The mount itself requires only a few hours of basic carpentry work. The base consists of three parts:
Telescope base
This base is what the optical tube holder rests upon. You can build a basic, four-sided box upon which the mount the holder.
Holder for the optical tube
The holder for the optical tube also requires basic carpentry skills. It should be just large enough to snugly fit the telescope tube and can include a rocker and diameter bearings to make it adjustable.
Ground board
The ground board is a round piece of plywood that stabilizes the mount on the ground.

In order to assemble the scope, you attach the base to the ground board with Teflon pads. You then bolt the holder onto the top of the base, and place your telescope within the holder. You should then collimate and test your scope before you begin enjoying your own, homemade Dobsonian telescope.