Collimation has a reputation problem. Somewhere along the line it picked up a mystique, as if aligning two mirrors were a rite of passage reserved for people who own laser levels and speak in equations. It isn't. A Newtonian is a simple machine, and the whole adjustment takes about as long as making a decent cup of coffee — once you understand what you're looking at and stop fiddling with all three screws at the same time.
What follows is the routine I use on a 200 mm f/6 Dobsonian and a 150 mm f/5 on an equatorial mount. It works indoors in daylight with a Cheshire eyepiece, then finishes under the stars with a defocused star to catch the last few percent. Ten minutes, honest tools, no drama.
What You're Actually Adjusting
A Newtonian sends light from the primary mirror up the tube to a small flat secondary, which folds it sideways into the focuser. For the image to be sharp, both mirrors have to share one optical axis that runs from the primary's centre, through the secondary, and straight down the middle of the focuser.
Two adjustments do all the work. The secondary's tilt screws aim the flat mirror; the primary's screws tilt the big mirror at the back. Rotation and position of the secondary along the tube axis also matter, but once set they rarely move unless something has been knocked.
Which mirror does what
In practice the secondary gets the primary's centre mark into view, and the primary gets the final, precise alignment. That division of labour is the whole trick. Fight it, and you'll spend forty minutes chasing your own elbow.
Before You Start: The Kit and the Setting
You need a Cheshire eyepiece — a tube with crosshairs and an angled reflective face — or a simple collimation cap if that's what came with the scope. A Cheshire is worth owning: unlike a laser, it cannot itself be out of alignment, and it shows the whole geometry at a glance.
- The Cheshire eyepiece (or cap), seated fully and snug in the focuser.
- A small screwdriver or the Allen key that came with the telescope.
- A bright, plain surface to point at — an overcast sky, a white wall, a sheet of card held a metre from the tube.
- A torch to light the Cheshire's side window if the room is dim.
- Ten minutes without a queue of people waiting to look through the scope.
Set the tube roughly horizontal so nothing falls onto the mirror, and never point it anywhere near the Sun. Do the work at room temperature, or better, outdoors where the scope will be used — a warm tube drifting into cold night air moves things slightly.
Step One: Square the Secondary
Look into the Cheshire. You should see the secondary's elliptical face, the primary mirror reflected inside it, and your own pupil reflected in the primary's centre. Three nested circles, roughly concentric.
First, check the secondary looks round rather than oval. If it's visibly elongated, loosen the central screw a whisker and rotate the holder until the ellipse appears circular, then snug it again. Now check it sits centred under the focuser, with even gaps around it; if it's badly off, the spider vanes or the holder need repositioning, which is a five-minute job with a ruler, not a mirror-tilt job.
Then tilt it. Turn the three tilt screws — loosen two slightly, tighten one — until the primary's centre mark sits under the Cheshire crosshairs. Small movements. A quarter turn is a big adjustment. Once the centre mark is close to the crosshair intersection, stop and leave the secondary alone.
Step Two: Dial In the Primary
Now the fine alignment, done entirely with the screws at the back of the mirror cell. On most cells there are three adjustment screws and three lock screws; a few use the same screws for both, with springs holding tension. Find out which is which before you start turning.
- Loosen the lock screws a half turn so the mirror can move.
- Look into the Cheshire and find the primary's centre ring.
- Turn one adjustment screw at a time, in small increments, watching which way the centre mark travels.
- Bring the centre mark onto the crosshair intersection, then check the reflection of your own pupil is also centred.
- Tighten the lock screws gently, alternating between them so the mirror doesn't shift.
- Recheck through the Cheshire — locking often nudges things a fraction. Repeat once if needed.
That's the daytime half finished, usually in under six minutes. If the weather is stubborn, this is genuinely enough for low and medium powers. High power on planets and double stars deserves the star test.
Step Three: The Star Test
Wait for darkness, let the mirror cool — a 200 mm primary wants the better part of an hour — and pick a star high in the sky. Polaris is ideal in the northern hemisphere because it barely drifts, but any reasonably bright star will do if your mount tracks.
Use a magnification of roughly 25 times per inch of aperture, or as much as the seeing allows. Centre the star, then defocus slightly until you see a bullseye: a bright ring pattern with the dark shadow of the secondary in the middle.
Reading the pattern
A well-collimated scope shows concentric rings, evenly bright, with the dark centre shadow sitting dead in the middle. If the shadow leans toward one edge, or the rings bunch on one side like ripples against a wall, the primary needs a final nudge.
Turn one adjustment screw a tiny amount, watch which way the shadow moves, and either continue or reverse. Re-centre the star after every tweak — the error itself pushes the star off axis. Then defocus the other way: the offset should flip to the opposite side. If it doesn't, you're probably looking at tube currents or poor seeing rather than misalignment.
Mistakes That Quietly Cost You Sharpness
- Overtightening. Screws should be snug, not torqued. Excess pressure pinches the mirror and spoils the figure.
- A sloppy Cheshire. If it wobbles in the focuser, the reading is meaningless. Seat it fully and tighten the thumbscrew on the barrel, not the drawtube.
- Adjusting two screws at once. Change one thing, look, then decide.
- Confusing lock screws with adjustment screws. Check the manual once; it saves a lot of confusion later.
- Judging a warm telescope. A mirror at a different temperature to the night air will look miscollimated when it isn't.
Make It a Habit
A quick Cheshire check before each session takes thirty seconds and tells you whether the drive to the dark site has disturbed anything. Do the full routine every few months, after any bump, and before a planet opposition or a star party where you'll be pushing magnification.
The reward is not abstract. A properly aligned 150 mm reflector will show the Cassini division and clean lunar craterlets where a sloppy 250 mm shows mush. Ten minutes, one tool, three screws — and the optics you already paid for finally get to do their job.
Photo: shameersrk / Pixabay
