Collimation is the process of aligning the mirrors in a Newtonian reflector so that the optical axis of the primary mirror, the secondary mirror, and the focuser are all perfectly aligned. A well-collimated telescope produces crisp, sharp stars and planetary detail. A poorly collimated one produces comet-like smeared stars and reduced contrast. Here’s how to get it right.
Why Reflectors Need Collimation
The mirrors in a Newtonian reflector can shift slightly with use and transport. Even new telescopes sometimes arrive slightly out of collimation from the factory. Regular users should check collimation at the start of each observing session — it takes less than five minutes once you’re familiar with the process, and the improvement in image quality is immediately apparent.
Tools You’ll Need
- A collimating eyepiece (a Cheshire eyepiece or laser collimator) — essential. A Cheshire costs $20–$30 and lasts a lifetime.
- A Phillips screwdriver (for the secondary mirror adjustment screws on some mounts)
- Allen keys (hex wrenches) — often provided with the telescope
- A secondary mirror tool or center cap with dot marker (often included)
The Two-Step Collimation Process
Newtonian collimation has two stages: adjusting the secondary mirror (flat, diagonal mirror near the focuser) and adjusting the primary mirror (large curved mirror at the bottom of the tube). Always do secondary first.
Step 1: Center the Secondary Mirror
Remove any eyepiece and look down the focuser tube without any tools. The secondary mirror (a small oval mirror on a spider vane) should appear centered in the focuser tube. If it’s significantly off-center, loosen the central secondary mirror screw (the bolt holding the secondary holder to the spider) just slightly and shift the entire secondary holder until it appears centered when viewed from the focuser. Re-tighten.
Step 2: Align the Secondary Mirror to Reflect the Primary
Insert the Cheshire eyepiece into the focuser. Look through it. You’ll see the primary mirror reflected in the secondary. Three circles should be concentric: the edge of the secondary mirror, the reflection of the primary mirror, and the reflection of the Cheshire’s aperture ring (or the reflection of your eye). If these are not concentric, adjust the three tilt screws (usually Allen bolts) around the secondary mirror holder. Tightening one screw while loosening the opposite one tilts the secondary. Work gradually until all three circles are concentric.
Step 3: Align the Primary Mirror
Now adjust the primary mirror. The primary mirror cell has three pairs of screws: locking screws and adjustment screws. Loosen the locking screws first (just enough to allow movement). Looking through the Cheshire, you’ll see the reflection of the Cheshire’s crosshairs in the primary mirror. These should be centered on the primary mirror’s center dot (a small sticky dot usually placed at the mirror’s center). Adjust the primary mirror’s three adjustment screws (while keeping the locking screws slightly loose) until the center dot reflection aligns with the center of the crosshair. Then re-tighten the locking screws.
Step 4: Star Test
The definitive test of collimation is the star test. Point the telescope at a moderately bright star and bring it to the center of the field. Slowly defocus the star — it should expand into a series of concentric rings (diffraction rings). If these rings are perfectly concentric, collimation is excellent. If they’re off-center (the rings cluster on one side), fine-tune the primary mirror adjustments while watching the defocused star until the rings become concentric.
Tips for Easier Collimation
- Work in daylight initially — you’ll see the components more clearly and won’t be rushing before it gets dark.
- Make small adjustments — the mirrors are sensitive. A quarter-turn of an adjustment screw is often enough.
- Always loosen locking screws before adjusting and re-tighten afterward — or you’ll strip the threads.
- Check collimation every session — it takes 5 minutes and immediately improves your views.
- A laser collimator (about $30) is faster but less accurate than a Cheshire for the secondary step. Use both if you want the best results.
How Often Should You Collimate?
Check collimation before every session if you transport your telescope. If your telescope lives in a permanent observatory or isn’t moved, monthly checks are usually sufficient. After any accidental knock or drop, always check collimation before observing. A few minutes of collimation before each session is a small investment for dramatically sharper views.
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