Why does the secondary mirror need an offset at all?

The light cone from the primary converges, so the cross-section is wider on the side nearer the primary than on the side nearer the focuser. Offsetting the elliptical secondary away from the focuser and toward the primary centres the cone on the mirror and illuminates the focal plane evenly.

What is the formula for the offset?

The standard approximation is offset = secondary minor axis / (4 × focal ratio), where focal ratio is primary focal length divided by aperture. The offset is applied equally in two directions: away from the focuser and toward the primary mirror.

Do I offset in one direction or two?

Two. The full offset value is applied both away from the focuser and toward the primary mirror. Many people quote the single-axis figure, so always check whether a number means the per-axis offset or the diagonal total.

Is offset important for fast scopes?

Yes. Offset scales inversely with focal ratio, so an f/4 Newtonian needs roughly twice the offset of an f/8. For slow scopes above f/8 the offset is often small enough to ignore in practice.

Does the focuser-axis distance change the offset?

Not the offset magnitude itself, which depends on minor axis and focal ratio. That distance matters for confirming the secondary is large enough to fully illuminate the field, which this tool also reports as a sanity check.

Newtonian Telescope Collimation Offset Calculator

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.

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 tips

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: drop the same secondary into an f/4 tube and the offset doubles to 5 mm. If your secondary holder has a fixed central mount, you can build the offset in by shortening the spider vane on the focuser side and lengthening the one toward the primary.