Sunday, June 8, 2014

Star Honor Series: Part 6 - Building a Reflector Telescope

John Dobson introduced his wonderful homemade low-cost, non-equatorial plywood telescope mount for large homemade reflector telescopes in the 1950s.  Telescope mounts can be as expensive as the telescopes themselves, if not more so. The simple Dobsonian mount was able to handle even relatively large telescopes is wonderfully cheap and works really well.

Having a cheap mount allows amateur astronomers to spend their money on larger, more expensive, high-quality scopes rather than on the mount. The design below is for a 10 inch “Dob” that will fit on the mount described in part 1 of this series. You can collect the parts for the scope on eBay and other online sites if you are patient. This can be a long term project for your Pathfinder club.

Here's what you need:

Painted sonotubes.
  • 12-inch tube about eight feet long. You can get a sonotube at a concrete supply store. They are used as forms for making concrete pillars.
  • 10-inch telescope mirror
  • 10-inch mirror mounting cell
  • 10-inch Spider mount for the secondary mirror
  • Secondary mirror
  • 2 inch focuser
  • Eyepiece
  • 4 by 8 foot sheet of half inch plywood
  • 1-inch screws, 1 box
  • 2 old phonograph records
  • 1 half inch carriage bolt, 3 inches long with nut and flat washer
  • Felt strips

  • Drill and bits
  • 2 inch drill hole saw bit
  • Saber Saw
  • Screwdrivers
  • Scissors
  • Wood glue

The Scope:

This will be a quick run-through of the process. You can find a more detailed description of the project with pictures and dimensions of things at this link to a blog of mine on another site. Here's a quick description of how to do this so you can get an idea of what all is involved.

Step 1
Cross section of mirror mount
Mount the telescope mirror first. It should come with a mirror-mounting cell that allows you to fix the mirror at the lower end of the you to adjust the angle of the mirror to line it up with the secondary mirror and eyepiece.  Follow the instructions that came with the cell.  Drill holes in the lower end of the Sonotube to match the mounting screws and screw the mount into the end of the tube.  Here's a more detailed description of the process

Step 2
Figure out the focal length of the mirror.  It will be in the specifications that come with your mirror.  Subtract 6 inches and the length of the focuser from the total focal length of the mirror. Measure from the center of the mirror up the side of the Sonotube.  Mark the side of the tube at the distance you computed above. Measure four more inches up the tube and cut off the rest of the Sonotube.  Hold the tube upside down while cutting so that the dust falls away from the mirror. 

Step 3
Mirror spider mount
Drill a 2-inch hole in the side of the tube at the mark you made on the side of the tube (the adjusted focal length).  Screw the spider mount for the secondary mirror so that the mounted mirror is directly below the 2-inch hole you just drilled.

Step 4
Screw the focuser assembly directly over the 2-inch hole in the tube. You will have to collimate (align) the optics so that the focuser lines up with the center of the secondary mirror. There should be detailed instructions that come with your optics for collimating the focuser. Once you've screwed down the focuser, put a low power eyepiece in the end of the focuser. Point the tube at something distinctive during the daylight and make sure you can focus on the object to make a sharp image. You may have to adjust the primary mirror a bit, but if you got the measurements right, it shouldn't take much. You also may have to adjust the angle of the main mirror to make sure the image is projected directly off the secondary mirror and through the secondary.

Step 5
Next mount the finder scope a quarter of the way around the tube from where you mounted the focuser. You'll need to calibrate the finder scope during daylight so that the finder points at the same place in the sky that the telescope does. You can do this by pointing the scope at a small object a few hundred yards away and then lining up the finder scope to point it's crosshairs at the same object.  Your finder scope probably comes with instructions.

Now that you're done, the next thing we'll do is mount the tube in the Dobsonian Mount you built in Part 5 of this series

Refracting finder scope
Here are some cool links to some other telescope building resources.

  1. Mother Earth News: A Homemade Telescope 
  2. Larry Brown: Homemade Astronomy
  3. Scopemaking:  Plans for a Homemade Dobsonian Telescope
  4. Howdy Ya Dewit:  A Homemade Telescope: A Quick Run-Through

© 2014 by Tom King 

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