Newtonian Secondary Mirror Size Calculator
Enter the primary mirror, focal length, secondary-to-focal-plane distance, and target field to size a Newtonian diagonal and check a candidate secondary.
Required secondary minor axis (mm)50
- Telescope focal ratio
- 6
- Minimum on-axis secondary (mm)
- 33.33
- Candidate size margin (mm)
- 0
- Candidate fully illuminated field (mm)
- 20
- Illumination at requested field edge
- 100%
- Edge light loss (magnitudes)
- 0
- Linear central obstruction
- 25%
- Obstructed primary area
- 6.25%
- Secondary offset per axis (mm)
- 1.82
- Fully illuminated angular field (degrees)
- 0.95
Required size is a geometric minimum before commercial-size steps, bevels, holder lips, and collimation allowance.
How to use this calculator
- Enter the usable primary diameter and the actual primary focal length.
- Measure the secondary-to-focal-plane distance from the secondary center to the focal plane.
- Enter the sensor diagonal, eyepiece field stop, or focal-plane diameter you want fully illuminated.
- Enter a candidate secondary minor axis to see obstruction, field illumination, and offset.
How the secondary size is calculated
A Newtonian secondary has to catch the converging cone from the primary mirror and send it out to the focuser. The bare on-axis size is only the diameter of that cone at the secondary. A real visual or imaging field also needs room for light bundles whose focus lands away from the optical axis.
The calculator uses consistent millimeter inputs. Let D be primary diameter, f be primary focal length, L be secondary-to-focal-plane distance, w be desired fully illuminated field diameter, and s be the candidate secondary minor axis.
focal ratio = f / D
on-axis cone = D * L / f
required secondary = on-axis cone + w * (f - L) / f
candidate fully illuminated field = max(0, (s - on-axis cone) * f / (f - L))
What moves the result most
The intercept distance matters a lot. A taller focuser, larger tube, or large camera spacing pushes the focal plane farther from the secondary, so the light cone is wider where the secondary intercepts it. Faster focal ratios also grow the on-axis cone quickly. The requested fully illuminated field then adds a separate allowance for sensor diagonal or eyepiece field stop.
The candidate check uses Mel Bartels' overlapping-aperture method at one-half of the requested field diameter. In that method the shifted field bundle sees only part of the primary when the projected secondary aperture no longer fully covers it. The returned edge illumination is that visible fraction of the primary, and edge light loss is -2.5 * log10(illumination) magnitudes. If there is no overlap at the requested field edge, the magnitude loss is unavailable rather than infinite.
Obstruction and offset
Linear central obstruction is s / D. Obstructed primary area is its square, so a 25% linear obstruction blocks 6.25% of the primary area before spider vanes and the holder are considered. The offset result follows Bartels' geometric construction; use the same magnitude away from the focuser and toward the primary for a fully offset installation.
What this calculator leaves out
This is paraxial Newtonian geometry. It does not model spider vanes, holder lips, mirror bevels, focuser drawtube vignetting, coma correctors, filter wheels, alignment error, or loss of contrast from diffraction. Commercial secondary sizes may quote physical glass rather than clear reflective minor axis, so builders usually choose the next suitable size after adding practical allowance.
Worked example
For a 200 mm f/6 primary with a 1200 mm focal length and a 200 mm secondary-to-focal-plane distance, the on-axis cone at the secondary is 200 * 200 / 1200 = 33.33 mm. Asking for a 20 mm fully illuminated field adds 20 * (1200 - 200) / 1200 = 16.67 mm, so the required minor axis is 50.00 mm.
A 50 mm candidate has zero size margin in that example. It fully illuminates a 20.00 mm focal-plane diameter, gives 100% illumination at the requested field edge, has a 25.00% linear obstruction, blocks 6.25% of the primary area, and has a Bartels offset of about 1.82 mm per axis.
Common questions
How do I measure the secondary-to-focal-plane distance?
Measure from the center of the secondary mirror to the focal plane where the eyepiece field stop or camera sensor comes to focus. In practice this includes the tube radius, focuser height above the tube, and any extra spacing needed by a camera, filter wheel, or coma corrector.
Should the illuminated field match my sensor diagonal or eyepiece field stop?
Use the size of the field you care about. For imaging, the sensor diagonal is a clear starting point if you want full illumination in the corners. For visual use, the field-stop diameter of the widest eyepiece is often more relevant, and accepting some edge falloff is common.
How much larger than the calculated minimum should the secondary be?
The calculated size is a geometric minimum. Many builders choose the next available commercial size after allowing for bevels, holder clips, edge quality, measurement uncertainty, and collimation tolerance. The candidate margin shows how much room a proposed mirror has before those practical allowances.
What central obstruction is acceptable for planetary observing?
There is no single cutoff, but smaller obstructions generally preserve more low-contrast detail. Planetary Newtonians are often kept modest by using a small illuminated field and a low-profile focuser. The tradeoff is less fully illuminated area for wide eyepieces or large sensors.
Why is the bare on-axis secondary too small for wide-field observing?
The bare on-axis value only lights the exact center of the focal plane. Away from the center, the converging bundle is shifted and part of the primary can be clipped by the secondary. A wide eyepiece or camera sensor needs extra minor-axis size to keep the field edge illuminated.
How much edge illumination is adequate for visual use?
Visual observers often tolerate gradual falloff because the eye is not very sensitive to mild edge dimming. Imaging is less forgiving because flats can correct brightness falloff only when the signal remains strong enough. Use the edge illumination and magnitude loss together instead of relying on obstruction alone.