Description
Collimation refers to the alignment of a telescope's optical elements. When it is incorrect (typically on a Newton whose primary or secondary is misaligned), the optical axis no longer passes through the center of the sensor.
The result: stars take on a comma or comet shape that resembles coma, with one critical difference. Here, the asymmetry is uniform across the entire field: all the commas point in the same direction, and even the central star is deformed.
This is a correctable optical defect at the telescope, and one of the most important to fix before a session: no processing can rescue a failed collimation, and it degrades sharpness across the entire image, not just in the corners.
Its cousin, coma, deforms stars radially while leaving the center clean. Not to be confused with a simple focus miss or sensor tilt.
Visual signature
Stars draw small commas or comets all oriented in the same direction, regardless of their position in the field.
The unmistakable sign: the star at the center of the image is also deformed. On a correctly collimated optic, the center is always sharp; a central comma directly betrays a misalignment.
When defocusing on a bright star (collimation test), the pattern is not a concentric ring but a off-center donut, with the secondary shadow shifted to one side.
The asymmetry is homogeneous from edge to edge, without marked worsening toward the corners, which immediately distinguishes it from defects that worsen at the periphery.
Differential diagnosis
Not to be confused with coma: coma leaves the center clean and deforms stars radially (each comma pointing toward its nearest corner), whereas a collimation error deforms the entire field, center included, in a single direction.
To be distinguished from astigmatism: astigmatism stretches stars into perpendicular lines on either side of focus, without the asymmetric comma shape of a collimation error.
To be separated from sensor tilt: tilt makes one edge sharp and the opposite edge soft (sharpness gradient), whereas collimation produces a directional deformation that is homogeneous across the field.
Not to be mistaken for a missed focus: a missed focus gives soft, round stars with no preferred direction.
Not to be confused with abnormal diffraction spikes: on a Newton, a shifted collimation moves the crossing point of diffraction spikes on bright stars. If only spikes are involved (asymmetry, doubling) without a general comma pattern, see that dedicated page.
Probable causes
- Primary or secondary mirror misalignment on a Newton
- Transport and handling of the tube, which disrupts alignment
- Secondary not centered under the focuser
- Collimation screws loose or mechanical play in the mirror cell
- Collimation never checked before the imaging session
- Optical train (corrector, camera) introducing its own misalignment
Course of action
- Collimate before every session (laser, Cheshire, or star test)
- Adjust the secondary first (centering under the focuser), then the primary
- Fine-tune on a defocused star: concentric rings indicate correct collimation
- Tighten and secure collimation screws, eliminate mechanical play
- Check collimation after every transport of the tube
- Verify the alignment of the optical train (corrector, rings, camera)
- Add a coma corrector once collimation is perfect
The Doc's advice
Collimation on a Newton is the basic step too many people skip: a tube that rides in the trunk decollimate, full stop. Before every session, a glance at your collimator (laser or Cheshire) takes two minutes and saves an entire night. The classic trap is thinking you have coma when you just need to redo collimation. The foolproof test is the central star: if it is a comma, the problem is alignment, not the optics. And adjust the secondary first (centering under the focuser), then the primary on a defocused star, in that order, otherwise you will go in circles.
Think you can see this defect in your image?
Run a diagnosisFrequently asked questions
How do you distinguish a collimation error from coma?
The decisive test is the central star. On a correctly collimated optic, the center is always sharp; if the central star is a comma, it is a collimation error, not coma. Coma leaves the center clean and deforms stars radially (toward the corners), whereas collimation deforms the entire field in one and the same direction. Practically: redo collimation first, it takes just a few minutes; if the commas disappear, the diagnosis is confirmed. If they persist only in the corners with a clean center, it is optical coma.
How often should you collimate your telescope?
It depends on the instrument type. A Newton decollimate easily and should ideally be checked before every session, especially after transport by car. A Schmidt-Cassegrain is more stable and holds collimation for several outings, but still deserves regular checks. A refractor is collimated at the factory and normally never needs adjustment, except after a significant impact. Practical rule: if your tube has traveled, take two minutes to check before imaging.
Can a collimation error be corrected in post-processing?
Not really. An optical misalignment degrades sharpness across the entire field, and no processing can recreate lost detail. Tools like BlurXTerminator can recenter star shapes and partially mask the defect, but at the cost of resolution loss and with a risk of artifacts if collimation is badly off. Collimation is a hardware adjustment taking a few minutes: it is always more productive to do it correctly at acquisition than to try to recover afterwards.
Which tool is better for collimating: laser or Cheshire?
The two are complementary. A laser collimator is fast and practical for aligning secondary then primary, but it must itself be well adjusted to be reliable. The Cheshire (or sight tube) needs no battery and gives very reliable secondary alignment, at the cost of a bit more method. Whatever alignment tool you use, the final judge is always the star test: a defocused star must show perfectly concentric rings. Many imagers use the laser to rough-in and the star test to validate.