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How to Measure a Secondary Telescope Mirror

The primary mirror of a telescope focuses light down to a single point. It gathers the incoming light and sharpens it at the focal plane, so you can see the image magnified by the eyepiece. Some types of telescopes have eyepieces on the side of the tube as opposed to the end. The center portion of the telescope contains the focuser, which projects the clear image to the eyepiece. In order for the cone of light to reach the side of the tube and the eyepiece, you must have a secondary or diagonal mirror to divert it. A crucial step in designing a telescope with a side eyepiece is determining the size of the secondary mirror.

Things You'll Need

Paper tape
Moon image
Flashlight
Measuring tape
Wood piece
Calculator

Instructions

Focal Plane
- 1
  Remove the eyepiece from the side of your tube.
- 2
  Tape a piece of frosted tape, like medical paper tape, over the end of the focuser.
- 3
  Place an image of the Moon, taken from a book or printed from the Internet, over the eyepiece hole and use a light source to project the Moon onto the tape.
- 4
  Keep moving the focuser in and out until the moon image is sharp. Now measure the length from the tape to the center of the tube. This is the focal plane.
Focal Ratio
- 5
  Slide a piece of wood down the tube, from the front, until if makes contact with the primary mirror. Mark the wood where it crosses the middle of the focuser hole.
- 6
  Measure the distance between the end of the wood and the mark. Add this number to the focal plane (the number from step one). So if the focal plane was 9 inches and your wood measurement is 39 inches, then the focal length is 48 inches.
- 7
  Determine the focal ratio. Take the focal length and divide it by the diameter of your primary mirror. If you focal length is 48 inches and the mirror diameter is 8 inches then the focal ratio is 6.
- 8
  Divide the focal plane measurement by the focal ratio. If the focal plane measured 9 inches and the focal ratio is 6, the minimum diagonal size of the secondary mirror is 1.5 inches.
- 9
  Select a secondary mirror that is slightly larger than the number calculated. So if the minimum diameter is 1.5 inches, go with a 2-inch secondary mirror. This will allow for full illumination.

Astronomy