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.