How to Calculate the Focal Length on a Telescope
- 1). Look in the manual for your telescope. If the manual is missing, consult the manufacturer's website.
- 2). Find the number for the f/ratio (or f/number).
- 3). Find the number for the aperture of the lens.
- 4). Multiply the f/ratio by the aperture of the lens. This will give you focal length.
- 1). Look in the manual for your telescope. If the manual is missing, consult the manufacturer's website.
- 2). Find the magnification of the telescope.
- 3). Find the number for the focal length of the eyepiece.
- 4). Multiply the focal length of the eyepiece by the magnification. This will give you the focal length of the telescope.