Newtonian Telescope Collimation Calculator
Calculate the secondary mirror offset for a Newtonian reflector telescope to achieve correct collimation.
Ensure accurate optical alignment for sharp views.
Collimation is the alignment of a telescope’s optical components. In a Newtonian reflector, the secondary mirror must be precisely positioned so it reflects light exactly down the center of the focuser tube.
Why the secondary mirror needs an offset:
Due to the angle of the secondary mirror (45°) and its position within the tube, the reflection of the primary mirror is not centered on the secondary if the mirror is placed exactly at center. A small lateral offset corrects for this, ensuring the full aperture of the primary is visible through the focuser.
Offset formula:
Offset = (Secondary Minor Axis ² ) ÷ (4 × Focal Length of Primary)
A simpler approximation:
Offset ≈ Secondary Minor Axis ÷ (4 × f-ratio)
Where:
- Secondary minor axis = the shorter diameter of the secondary mirror (the elliptical mirror’s narrow dimension)
- f-ratio = focal length ÷ primary mirror diameter
Example:
A 200mm f/6 scope with a 50mm secondary minor axis:
Offset = 50 ÷ (4 × 6) = 50 ÷ 24 = 2.08mm
This seems tiny, but at high magnification it makes a measurable difference in illumination uniformity.
Typical secondary mirror sizes:
| Primary Aperture | f/ratio | Common Secondary Minor Axis |
|---|---|---|
| 114mm | f/8 | 28–33mm |
| 150mm | f/5 | 40–50mm |
| 200mm | f/6 | 50–63mm |
| 254mm | f/5 | 63–75mm |
| 305mm | f/4.7 | 75–90mm |
Collimation steps for a Newtonian:
- Center-spot the primary: Apply a small ring (Cheshire cap or center-spot sticker) at the exact center of the primary mirror
- Align the secondary: Looking down the focuser, center the reflection of the primary (the dark circle) in the secondary
- Align the primary: Using a collimation cap or laser, center the reflection of the center spot in the focuser
- Star test: At high magnification on a star, the diffraction rings should be concentric and centered
Signs of poor collimation:
- Stars show a “comet” shape or flaring at the edge of the field
- Diffraction rings are off-center around a bright star
- One side of the image is sharper than the other
- Double stars won’t resolve properly
Collimation frequency:
Collimate before every serious observing session (transporting a scope knocks it out). Good news: with practice, collimation takes under 5 minutes.
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This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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