A Newtonian’s tilted secondary mirror must sit slightly off the geometric centre of the tube so the converging light cone lands evenly on the focal plane. This calculator computes that secondary mirror offset from your primary focal length, aperture, and secondary minor axis.
Why the offset is needed
The primary mirror forms a converging cone of light that narrows as it travels toward focus. When that cone crosses the 45-degree flat secondary, the cross-section of the cone is not circular — it is elliptical, and the near edge (closer to the primary) is wider than the far edge (closer to focus). If the secondary sits exactly at the geometric centre of the tube, the wider near-side of the cone spills off the edge of the mirror, illuminating one side of the focal plane more brightly than the other. Shifting the secondary toward the primary and away from the focuser centres the cone on the mirror and produces even illumination.
How it works
Because the light cone from the primary narrows toward focus, the cone is wider on the primary side of the 45-degree secondary than on the focuser side. To re-centre the cone on the elliptical mirror, the secondary is shifted by:
focal ratio = primary focal length / aperture
offset = secondary minor axis / (4 × focal ratio)That single offset value is applied in two directions at once: away from the
focuser and toward the primary mirror, each by the same amount. The total
diagonal displacement of the mirror centre is offset × √2.
Example and worked calculation
A 200 mm f/5 Newtonian (1000 mm focal length) with a 50 mm minor-axis secondary
gives a focal ratio of 5 and an offset of 50 / (4 × 5) = 2.5 mm per axis, or about
3.5 mm along the diagonal. Faster scopes need more offset: the same secondary
in an f/4 tube gives 50 / (4 × 4) = 3.1 mm per axis.
| f/ratio | 50 mm secondary offset (per axis) |
|---|---|
| f/4 | 3.1 mm |
| f/5 | 2.5 mm |
| f/6 | 2.1 mm |
| f/8 | 1.6 mm |
Slow scopes (f/8 and above) have such a small offset that it is often within the construction tolerance of the spider and can be ignored in practice. Fast scopes (f/4 to f/5) need the offset to be accurate or off-axis coma is worsened.
Practical collimation tips
If your secondary holder is adjustable, set the offset before collimating the primary. The two operations are independent: offset centres the cone on the mirror, while tilt points the cone toward the eyepiece focal point. Applying both correctly means the diffraction rings in a defocused star image will be concentric, which is the standard test of good collimation. A Cheshire eyepiece or laser collimator is the simplest way to verify the tilt step without star-testing.