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How to Determine the Focal Length of Your Telescope Mirror

The focal point of a mirror is the location where the light rays that are reflecting off that mirror come to a point. This is the same for a lens. The focal length is the distance between the mirror or lens and the focal point. Telescopes have a mirror that bounces light and concentrates it at a focal point which is the point where the image you see is formed. You must remove the mirror of your telescope and do a short experiment to determine the focal length of your telescope yourself.

Things You'll Need

Telescope mirror
Mount
Flashlight
Piece of cardboard
Measuring tape

Instructions

- 1
  Remove the mirror from the telescope carefully and place it on a mount to hold it vertical. Place a flashlight on the other end of the room facing the mirror.
- 2
  Place the piece of cardboard between the flashlight and the mirror, slightly off to the side so that the light bounces off the mirror and onto the cardboard.
- 3
  Move the cardboard until the light focuses as sharply as possible, and then mark that location.
- 4
  Measure the distance between the mirror and the cardboard. Divide this distance by two, which is the focal length of the mirror.

Astronomy