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How to Design a Newtonian Telescope

In a Newtonian telescope, the image enters an open end of the tube, travels the length, bounces off a curved mirror, travels back up toward the open end, bounces off a secondary mirror at 90 degrees and travels through a focuser and eyepiece. All you need to plan the design is the specifications of your telescope optics. After that, it’s simply a matter of taking measurements and placing the optics in the right configuration.

Things You'll Need

Sonotube 8 feet long (available at a concrete supply store)
Telescope mirror
Mirror mounting cell
Spider mount for the secondary mirror
Secondary mirror
Focuser
Eyepiece
Finder Scope

Instructions

Placing the Optical Elements
- 1
  Add two inches to the width of the telescope mirror you purchased. Obtain a Sonotube from a concrete supply store two inches wider in diameter than the mirror to accommodate the mirror-mounting cell. Sonotubes are cardboard forms used to make concrete pillars. They make lovely telescope tubes.
- 2
  Find the primary mirror’s focal length in the mirror specifications. The focal length tells you where you need to put the eyepiece. Measure the length of the focuser assembly.
- 3
  Add this number to half the diameter of the Sonotube. For instance if the focuser is four inches deep and the Sonotube is eight inches in diameter, add four inches for the focuser and four inches for the tube. This gives you the distance from the eyepiece to the center of the secondary mirror.
- 4
  Subtract the distance from the eyepiece to the secondary mirror from the focal length of the primary mirror. This gives you the distance from the primary to the secondary mirror.
- 5
  Install the cell mirror mount according to its instructions. Measure from the center of the primary mirror the distance you figured from the primary to secondary mirror. Mark this distance on the side of the Sonotube. Measure four more inches above the mark and cut off the Sonotube.
- 6
  Cut a hole the size of the opening in the focuser at the mark on the side of the tube and mount the focuser over the top of the hole.
- 7
  Mount the secondary mirror on the spider mount and locate the screw holes for the spider mount legs on the Sonotube. Place the legs so that the secondary mirror is positioned directly under the focuser and screw the legs into the Sonotube. When you look into the focuser, the image of your eye will be seen at the center of the hole reflected in the secondary mirror. Place the eyepiece in the focuser mount.
- 8
  Screw the finder scope a quarter way around the tube from the focuser.
- 9
  Follow the instructions in your optics’ instruction sheets to align the optical elements of the telescope and align the finder scope.

Astronomy