How to Find the Focal Point on a Telescope

Finding the Focal Point

• 1

  Use your tape measure to measure the aperture of your telescope. The aperture is the diameter of the main light-collecting opening. You may need this number to determine the focal length. Decide whether you will be using inches or millimeters for your measurements and be consistent.

• 2

  Find the focal length of the telescope. The focal length of your telescope should be listed in its manual or printed on the telescope near the front lens. You can likely find this information online as well. If you cannot find it anywhere, use your tape measure to measure the distance between your telescope's main lens and first mirror or eyepiece lens that it strikes within the tube; this will be a good estimate of the focal length.

• 3

  Calculate or convert. You may see the focal length listed as "FL=1200mm." If you wish to convert it to inches, just divide the number by 25.4. You might find a focal ratio listed instead of focal length. This would be listed as "f" over a number, such as "f/6." To find the focal length, simply multiply the bottom number by the aperture of the telescope.

• 4

  Extend your tape measure enough to reflect the focal length of your telescope.

• 5

  Starting at the main lens of the telescope, lay the extended tape measure down the length of the tube. The focal point for parallel rays entering the telescope's lens will be in the center of the tube where the tape measure comes to an end.