What collimation means
Collimation is the process of aligning a telescope’s optical parts so that they share one optical axis. In a reflector, that means the mirrors; in a refractor, the lens elements. When the parts line up, light from a star is brought to a single sharp focus at the eyepiece. When they do not, images soften and stars distort.
You may see the phrase “collimation telescope” in searches. It is not a special kind of telescope. It simply refers to a telescope that has been, or needs to be, collimated.
Why collimation matters
A telescope that is out of collimation can still show bright objects, but it rarely shows them at their best. Poor alignment can cause:
- Soft or smeared star images, especially at higher magnification.
- Coma-like flaring, where stars look like small comets, that is stronger than the scope’s design should produce.
- Lost contrast on planets and the Moon, where fine detail depends on a clean star image.
Collimation costs nothing once you have a basic tool, and it is often the cheapest way to improve what you see.
Which telescopes need collimating
| Telescope type | Typically user-adjustable? | Notes |
|---|---|---|
| Newtonian reflector / Dobsonian | Yes | Primary and secondary mirrors both have adjustment screws. Most commonly in need of attention. |
| Schmidt-Cassegrain / Maksutov | Usually the secondary only | Often adjusted with small screws on the secondary holder, following the maker’s instructions. |
| Refractor | Sometimes | Many are factory set. Some have push-pull screws on the objective cell. |
Newtonians and Dobsonians drift out of alignment most often because their mirrors sit in adjustable cells that can shift when the scope is moved or transported. Always check the manual for your specific model.
Signs your telescope is out of collimation
- In-focus stars look like they have a tail or a flare on one side near the center of the view.
- A defocused star looks lopsided instead of showing evenly spaced concentric rings.
- The Moon and planets seem less sharp than the aperture and conditions would lead you to expect.
- The scope has recently been transported, bumped or had its optics removed.
Poor seeing, unfinished cooling and dirty optics can look similar, so it is worth ruling those out before adjusting anything.
Collimation tools
- Collimation cap: a cap with a small center hole that fits in the focuser. Cheap and a good first tool. See our walkthrough on how to collimate a telescope without a laser.
- Sight tube or Cheshire eyepiece: helps you centre the secondary and see the primary’s center mark. A Cheshire is also used for some refractors.
- Laser collimator: projects a beam along the focuser axis. Convenient, but its own alignment must be trustworthy, and some users prefer to check it first.
- Barlowed laser or autocollimator: extra aids that some observers use for fine tuning.
- The star test: not a tool you buy, but the final check on a real star.
There is no single best tool for everyone. Many observers combine a simple cap or Cheshire for the mechanical steps with a star test for the final adjustment.
The basic Newtonian procedure
The details vary by telescope, so use this as an outline and follow your manual where it differs.
- Point the telescope at a bright, evenly lit surface and let it settle.
- With a collimation cap or sight tube in the focuser, adjust the secondary mirror so it looks centered in the focuser view and round.
- Tilt the secondary so the primary mirror, reflected in it, appears centered.
- Adjust the primary mirror’s tilt screws until the reflection of the secondary and your eye lines up with the center mark or ring.
- Recheck, since each adjustment can nudge another.
- Finish with a star test.
Make small adjustments, usually a fraction of a turn at a time, and change only one screw at a time so you can see its effect.
Checking the result with a star test
Pick a moderately bright star, center it, and use fairly high magnification. Defocus slightly on both sides of focus. A well collimated telescope shows a round pattern of rings with the central shadow in the middle. If the shadow looks shifted to one side, the optics probably need a little more adjustment. Steady air helps, as turbulent seeing can blur the rings.
How often to collimate
There is no fixed schedule. A common habit is to check a Newtonian before an observing session, especially after transport, and make adjustments only when the check shows they are needed. A scope that stays put in an observatory may hold alignment for long periods. Sealed or fixed designs may rarely need attention.
Collimating refractors and the role of the spider
Refractors are different, because their alignment is usually set at the factory. If yours has adjustment screws on the lens cell, our guide on how to collimate a refractor telescope walks through the checks. In a Newtonian, the secondary sits on a support called the spider, and its condition can affect alignment; read about telescope spiders and their vanes for more.
Common mistakes
- Adjusting before ruling out cooling, seeing or dirty optics.
- Turning several screws at once, then losing track of what changed.
- Over-tightening locking screws, which can strain a mirror.
- Trusting a laser without checking that the laser itself is aligned.
- Chasing perfection on a night of poor seeing.
FAQ
What does it mean to collimate a laser?
A laser collimator is only useful if its beam runs along the center of its own barrel. Some users check this by rotating the laser in a V-block or on a flat surface and seeing whether the dot wanders, then adjusting it according to the maker’s instructions. This is separate from collimating the telescope.
How hard is it to collimate a telescope?
Most people find the first attempt slow and later attempts quick. It is mainly a matter of patience and making small, careful changes.
How do you adjust a telescope?
In a Newtonian, you turn the collimation screws on the secondary and primary mirror cells. Other telescope types vary, so check your manual.