Telescope Collimation: What It Is and When You Need It

You've set up your telescope on a clear night, pointed it at a bright star, and the image looks like a fuzzy blob with a bright spot off to one side. Or maybe the image is just never quite as sharp as you feel it should be, no matter how carefully you focus.

The likely culprit is collimation — or rather, the lack of it.

Collimation is the alignment of all the optical elements in a telescope along a common optical axis. In a well-collimated telescope, every mirror and lens is positioned so that light travels through the system in exactly the way the designer intended, converging to a precise focal point.

When a telescope is out of collimation, the optical elements are misaligned. Light doesn't converge properly, and the result is a degraded image — soft, asymmetric, or showing characteristic patterns like comet-shaped stars or off-centre diffraction rings.

Newtonian reflectors (including Dobsonians) require the most frequent collimation. They have a primary mirror at the bottom of the tube and a small secondary mirror near the top. Both mirrors need to be precisely aligned with each other and with the focuser. These telescopes can go out of collimation from transport, temperature changes, or simply from use.

Catadioptric telescopes (Schmidt-Cassegrain, Maksutov-Cassegrain) are more stable but still require occasional collimation, particularly after transport or if the telescope has been dropped or knocked.

Refractors (lens-based telescopes) are the most stable and rarely need collimation under normal use. When they do go out of alignment, it usually requires professional attention.

The classic test is the star test. Point your telescope at a bright star and defocus slightly — both inside and outside focus. In a well-collimated telescope, the defocused star image should appear as a series of concentric rings, perfectly centred.

If the rings are off-centre, or if the pattern looks different inside focus versus outside focus, your telescope needs collimation.

Other signs:

• Stars appear comet-shaped or have a "tail" pointing in a consistent direction
• The image is noticeably sharper on one side of the field than the other
• You can never quite achieve a sharp focus, even on nights of good seeing
• The secondary mirror shadow is not centred in the primary mirror when you look down the focuser

For Newtonian reflectors, collimation is a skill that most owners can learn. The process involves:

1. Centring the secondary mirror under the focuser
2. Tilting the secondary mirror so it reflects the primary mirror centred in the focuser
3. Adjusting the primary mirror so its reflection is centred in the secondary

This is done using the adjustment screws on the secondary and primary mirror cells, and a collimation tool (a Cheshire eyepiece or laser collimator makes the process much easier).

The process sounds complex but becomes straightforward with practice. Many Newtonian owners collimate their telescope before every observing session.

There are situations where professional collimation is the right call:

After a significant impact. If your telescope was dropped or knocked hard, the mirror cells themselves may have shifted or been damaged. A technician can assess whether the cells need adjustment or repair before attempting collimation.

Refractors and catadioptrics. Collimating these telescope types requires accessing internal adjustments that are not user-serviceable on most models. Attempting it without the right tools and knowledge risks making the problem worse.

When basic collimation doesn't hold. If you collimate your Newtonian and it goes out of alignment quickly, there may be a problem with the mirror cell, the secondary holder, or the focuser board. These are mechanical issues that need diagnosis.

When the star test shows persistent problems. If your telescope passes a basic collimation check but the star test still shows problems, there may be issues with mirror quality, mirror support, or other optical elements that require professional assessment.

Beyond collimation, a few habits will keep your telescope performing well:

• Let it cool down. Bring your telescope outside 30–60 minutes before observing. Thermal currents inside a warm tube degrade the image.
• Store it properly. Keep the dust caps on when not in use. Store in a dry environment to prevent moisture from affecting the mirrors and coatings.
• Clean mirrors carefully. Mirror cleaning is a delicate process. If your primary mirror has accumulated dust or debris, professional cleaning is safer than DIY attempts.

If your telescope isn't performing as it should, submit a repair inquiry and describe what you're seeing. We'll assess it and advise on the best course of action.

The Binoculars Repair Lab provides precision optics repair for binoculars, telescopes, microscopes, and sport optics across the Lower Mainland and by mail-in service across Canada.