Is a laser required?
No. A laser collimator is convenient, but it is not required. A simple collimation cap, a sight tube or a Cheshire eyepiece, plus a star test, can get a Newtonian or Dobsonian well aligned. Many observers prefer these methods because there is no beam to trust. The steps below describe the common approach. For background on terms and tools, see our telescope collimation guide.
Signs your telescope needs collimation
- Stars near the center of the field look like they have a flare or tail.
- A defocused star looks lopsided instead of showing even rings.
- Planetary detail looks soft even in steady air.
- The scope was recently bumped or transported.
Check cooling and seeing first, because they can look similar.
What you need instead of a laser
- A collimation cap (a cap with a small centered hole), or a sight tube, or a Cheshire.
- A screwdriver or hex key matching the secondary and primary adjusters.
- A bright, even background such as a white wall or a sheet of paper in good light.
- Optionally, a center mark on the primary mirror. Many mirrors come with one; if yours does not, check your maker’s guidance before marking the mirror yourself.
Before you start
Set the telescope on a stable surface, point it toward the bright background and take out the eyepiece. Rack the focuser to a position near the one you normally use. Note the adjustment screws before you touch them, and make small changes.
Step 1: center the focuser view and the secondary
Look through the cap. Without adjusting anything yet, the view should show the secondary mirror, the primary reflected in it and the reflection of the cap itself. If the secondary does not look centered in the focuser view, small movements of the secondary position along the tube axis may be needed, depending on your scope. Many scopes have a central bolt for this. Aim to have the secondary look round and centered.
Step 2: aim the secondary at the primary
Next, tilt the secondary using its three screws so the primary mirror appears centered in the secondary. The edges of the primary should look evenly spaced all around. Make tiny turns and change only one screw at a time. If a screw gets tight, back off another before continuing.
Step 3: adjust the primary mirror
Now the reflection of the secondary inside the primary, and the cap, should appear. Adjust the primary mirror’s collimation screws so the reflection of the cap, centered in the secondary’s reflection, lines up with the primary’s center mark. When the reflections line up, this stage is done. Check Step 2 once more, because it can shift slightly.
Step 4: fine tune with a star test
Collimating by day gets you close. The star test can finish the job.
- At night, let the telescope cool.
- Center a moderately bright star.
- Use fairly high magnification and defocus slightly.
- Look at the ring pattern. If the dark center is not centered in the rings, adjust the primary mirror screws a small amount and see whether it improves.
- Re-center the star between adjustments, since it will drift when the mirror tilts.
Stay with the star test until the pattern looks round and even on both sides of focus. Avoid doing this in turbulent air.
Collimating with no tools at all
When you have no tools, an experienced observer can use only the eye: look into the focuser and judge the reflections. This is less precise, because your eye moves around, but it can improve a badly misaligned scope until you obtain a cap. Treat it as a stopgap, not a replacement for a cap and a star test.
How often and common mistakes
Checking before a session is a common habit, especially after the scope has been moved. Mistakes to avoid:
- Tightening locking screws hard, which can warp a mirror.
- Adjusting several screws at once.
- Trusting a cap view on a scope with a poorly centered focuser.
- Skipping the star test.
If the secondary will not stay put, the problem may be the support rather than the screws. Our article on telescope spiders explains how the secondary is held.
FAQ
What is the best telescope collimation tool?
There is no single winner. A collimation cap is inexpensive and works well with a star test. A Cheshire, a laser or other aids add convenience.
How often do you need to collimate a telescope?
Check a Newtonian whenever it has been moved and adjust only if needed.
How to tell if your telescope needs collimation?
Use a defocused star. Uneven rings or a shifted dark center suggest it does.
How do I collimate my telescope if it is a refractor?
Many refractors are not user adjustable. See how to collimate a refractor telescope.