The focal ratio of a telescope, written as f followed by a number, is simply the focal length divided by the aperture. A fast scope like f/4 gathers light onto a small area quickly and is ideal for wide field imaging of faint nebulae, while a slow scope like f/12 gives a large image scale that suits the Moon and planets. This calculator finds the focal ratio from your focal length and aperture, then reports whether the system is fast, medium or slow for imaging. You can also work backward to find the focal length a given aperture needs to reach a target f-ratio, useful when picking a focal reducer or Barlow. Everything runs in your browser instantly.
The focal ratio of a telescope, written as f followed by a number, is simply the focal length divided by the aperture. A fast scope like f/4 gathers light onto a small area quickly and is ideal for wide field imaging of faint nebulae, while a slow scope like f/12 gives a large image scale that suits the Moon and planets.
Yes. Telescope Focal Ratio Calculator is completely free, with no sign-up and no usage limits.
Yes. Telescope Focal Ratio Calculator runs in any modern web browser. There is nothing to download or install.
Yes. Telescope Focal Ratio Calculator runs entirely on your device in your browser, so nothing you enter is uploaded to a server.