Newtonian telescope calculator

How it works

Enter your primary mirror and the fully illuminated field you want: the calculator gives the size and offset of the secondary mirror, traces the rays through the tube and shows how the illumination drops towards the edge of the field.

The path of the light

Cross-section of the tube in the plane of the focuser, to scale. Move the slider to change the field point: lost rays are drawn pale where they stop.

Illumination curve

Focal-plane illumination against the distance from the axis, relative to the whole primary. The plateau is the fully illuminated field, less the obstruction. Click the curve to move the field point there.

The classic formulas of Newtonian design. The illumination curve is computed by following the rays through the same geometry.