How to Determine the Focal Length of Your Telescope Mirror

A magnifying glass shows a good example of focal length of a lens or 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.

Tips & Warnings

• This is the same process as burning a hole with a magnifying glass and the sun.

• The room you use will probably have to be dark to properly see the light reflecting onto the cardboard.

• You can replace the cardboard with any object that the light will show up clearly on.

• Mount the mirror securely on its end, so that it does not fall and break.

References

• Find A Scope: Glossary|Focal Length
• Astronomy Cafe: How Do you Determine the Focal Length of a Telescope Mirror?
• University of Florida: Basic Telescope Optics