You can change the mirror of your telescope

Did you buy a new mirror and want to know it's focal length?

Just input the Sagitta (depth of curve) and Mirror Diameter, click the "Calculate the Focal Length" button.

The calculator will calculate the Focal Length, Focal Ratio and Radius of Curvature.

Heart of Man Web Pages

Parameter Value
[1] Sagitta (s): mm mm
[2] Mirror Diameter (D): mm mm

Mirror Parameters Value
Mirror Parameters will be displayed here. mm


  1. ^ Sagitta:
    Spherical depth of curve at center of mirror.
  2. ^ Mirror Diameter (D):
    Optical diameter of mirror.
  3. ^ Mirror size:
    Don't confuse with mirror Radius of Curvature below.
  4. ^ Radius of Curvature (R):
    setup Foucault tester at Radius of Curvature.
  5. ^ Focal Length (F):
    Where the mirror will form the image.
  6. ^ Focal Ratio (f):
    Lower f gives wider views.