Sagitta to Focal Length Calculator

Did you buy a partially ground mirror at a swap meet and want to know it's focal length? Or perhaps you have been grinding your own piece of glass and want to know the current focal length. Just input the Sagitta (depth of curve) and Mirror Diameter, and this page will calculate the Focal Length, Focal Ratio and Radius of Curvature.

If you want to calculate many other telescope and mirror parameters, use our Telescope Formulas and Design Comparator page.

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Parameter	Value	Units
Sagitta (s):
Mirror Diameter (D):

Inputs	Value
Sagitta (s) spherical depth of curve at center of mirror	0.047 in
Mirror Diameter (D) optical diameter of mirror	6 in
Mirror Parameters	Value
Mirror Radius (r) = D / 2 mirror size; don't confuse with mirror ROC below	3 in
Radius of Curvature (R) = r² / 2s setup Foucault tester at Radius of Curvature	95.74 in
Focal Length (F) = R / 2 where the mirror will form the image	47.9 in
Focal Ratio (f) = F / D lower f gives wider views	8

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