How to Collimate a Newtonian Dobsonian: Step-by-Step Guide

Quick answer

Collimate a Newtonian Dobsonian in three stages: center the secondary mirror under the focuser, aim the secondary so the laser or sight tube hits the primary's center mark, then adjust the primary until the return beam or Cheshire crosshair aligns. Work in a lit room first, finish with a star test, and make small turns—never force a screw.

What you need before you start

Gather a collimation cap or Cheshire eyepiece, a laser collimator if you have one, and the right tools for your scope. Most Dobsonians use a combination of Phillips and hex keys; check your manual for the exact sizes. A small bubble level helps if you are working on uneven ground. You do not need to remove the mirror cell or disassemble the tube for routine collimation.

Work indoors or in shade first. Temperature changes can shift the tube slightly, so let the telescope reach ambient temperature before the final check. If you are collimating a collapsible model, extend the truss tubes fully and lock them—collimation done with the tube collapsed will not hold when you extend it.

Step 1: Center the secondary mirror under the focuser

Insert a collimation cap or sight tube into the focuser and look through it. The secondary mirror should appear as a centered circle inside the focuser drawtube. If it is off to one side or rotated, adjust the secondary mirror's central screw or the three tilt screws on its holder. Make small adjustments and recheck after each turn.

Common mistake: overtightening the central screw, which can pinch the secondary and distort its shape. The mirror should be held snugly, not crushed. If the secondary is badly rotated, loosen the central screw slightly, rotate the holder by hand, then re-tighten just enough to hold position.

Step 2: Aim the secondary at the primary mirror

With the secondary centered, use a laser collimator or a Cheshire eyepiece to aim the secondary so the beam or crosshair lands on the primary mirror's center mark. Adjust the three tilt screws on the secondary holder—turn one screw a quarter turn at a time, then check. If you are using a laser, the dot should sit on the center spot; if you are using a Cheshire, the crosshairs should line up with the primary's center mark.

Common mistake: adjusting the primary mirror before the secondary is aimed correctly. The secondary controls where the beam hits the primary, so if it is off, no amount of primary adjustment will fix the view. Also check that your laser collimator itself is aligned; a misaligned laser will send you chasing a false error.

Step 3: Adjust the primary mirror

Now adjust the primary mirror's three collimation screws (usually at the back of the tube). If you are using a laser, the return beam should come back to the laser's target or aperture; if you are using a Cheshire, the primary's center mark should sit under the crosshairs. Turn one screw a small amount, then check the others—the screws interact, so you may need to go around two or three times.

Common mistake: loosening the locking screws too much or forgetting to re-tighten them. Many Dobsonians have locking screws that hold the primary in place after adjustment. Snug them gently after collimation; over-tightening can warp the mirror and ruin the alignment you just achieved.

Step 4: Star-test and fine-tune

Take the telescope outside and point it at a bright star. Use a medium-power eyepiece and defocus slightly. The star should appear as a series of concentric rings. If the rings are lopsided or the dark center is off to one side, make tiny adjustments to the primary mirror until the rings are symmetrical. This is the final check that matters for visual use.

Common mistake: star-testing before the telescope has cooled to ambient temperature. A warm mirror produces a distorted star image that looks like a collimation error. Wait 20–30 minutes for a 10-inch scope, longer if the temperature is dropping fast. Also avoid star-testing in poor seeing; wait for a steady night if possible.

How often should you collimate?

Solid-tube Dobsonians often hold collimation for months if handled gently. Collapsible models may need a touch-up each time you set up because the truss sections can shift. Check collimation after any bumpy car ride, after removing and reinstalling the mirror cell, or if you notice soft views at high magnification. A quick laser check takes less than a minute and saves a wasted observing session.

Collimation steps at a glance

1. Center the secondary under the focuser: Use a collimation cap or sight tube. Adjust the secondary's central screw or tilt screws until the mirror appears as a centered circle. Checkpoint: the secondary should look round and centered, not oval or offset.
2. Aim the secondary at the primary's center mark: Use a laser collimator or Cheshire eyepiece. Turn the secondary tilt screws a quarter turn at a time until the beam or crosshair hits the center spot. Checkpoint: the laser dot or crosshair should sit on the primary's center mark, not near it.
3. Adjust the primary mirror: Turn the primary collimation screws until the return beam aligns or the Cheshire crosshairs match the center mark. Work in small increments and recheck. Checkpoint: the return beam should land on the laser's target; re-tighten locking screws gently.
4. Star-test and fine-tune: Point at a bright star, defocus slightly, and adjust the primary until the rings are concentric. Let the scope cool first. Checkpoint: symmetrical rings mean collimation is good for visual use.

Product facts

• 10" Flextube Collapsible Dobsonian (S11720): 10" (254 mm) Dobsonian-style Newtonian with 1200 mm focal length (f/5), 2" Crayford-style focuser with 1.25" adapter, 4-element Plössl 25 mm and 10 mm 1.25" eyepieces, 8x50 right angle erect-image finderscope, and a solid rocker-mount with Teflon bearings and tension clutch. The f/5 focal ratio is relatively fast, so collimation tolerances are tighter than a longer-focal-ratio scope; the included 1.25" eyepieces work with common collimation tools, and the right-angle finder helps you center stars for the star test without straining.