Telescope Focal Ratio Calculator Formula

Understand the math behind the telescope focal ratio calculator. Each variable explained with a worked example.

Use the Telescope Focal Ratio Calculator

Formulas Used

Focal Ratio

focal_ratio = focal_length / aperture

Variables

VariableDescriptionDefault
focal_lengthFocal Length(mm)1200
apertureAperture(mm)200

How It Works

Focal Ratio (f-Number)

f/# = Focal Length / Aperture

Lower f-numbers ("faster" scopes) collect light over a wider cone and are better for astrophotography of faint objects. Higher f-numbers ("slower") give longer focal lengths for planetary detail.

  • f/4 to f/5: fast, wide-field
  • f/8 to f/10: moderate, good all-round
  • f/12+: slow, high magnification

    • Worked Example

    1200 mm focal length, 200 mm aperture.

    focal_length = 1200aperture = 200
    1. 01f/# = 1200 / 200 = f/6
    2. 02This is a moderately fast scope, good for both visual and imaging.

    Ready to run the numbers?

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