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How to Make a Reflecting Telescope Focal Point

If you want your reflecting telescope to work at its optimum level, you need to determine the focal length of the telescope's concave mirror. A concave mirror's reflecting surface curves inward like the inside of a bowl. Every concave mirror has a focal length, defined as half of the mirror's radius of curvature (the distance from the mirror's surface to the center of the circle the mirror would make if it were a full sphere). The reflected rays of light all meet (focus) at the focal point. You can make the mirror's focal point land in the correct spot by studying the mirror and its parameters.

Things You'll Need

Concave mirror
Light source
Accurate measuring tape or ruler
Small, flat piece of cardboard

Instructions

Steps to Find the Focal Point
- 1
  Place your light source and concave mirror upright on a flat surface. Make sure the light source can hit the surface of the concave mirror directly.
- 2
  Lay the measuring tape on the flat surface, starting at the concave mirror.
- 3
  Place the flat piece of cardboard vertically along the tape measure and hold it there. Make sure the cardboard is perpendicular to the flat surface.
- 4
  Shine the light source onto the concave mirror so that the reflected light hits your vertical cardboard.
- 5
  Move the cardboard along the tape measure, toward and away from the mirror. Watch to see the intensity of the reflected light. When the intensity is the strongest, your cardboard is at the focal point.
- 6
  Record the distance between the mirror and the cardboard. This is your focal point.

Astronomy