Collimating a Newtonian Reflector: A Visual Step-by-Step Guide

What collimation actually does

Light enters the open end of the tube, travels to the primary mirror at the bottom, returns up the tube to the small flat secondary mirror, and is turned sideways into the focuser. Three things have to line up for a sharp image:

• The focuser axis must point at the centre of the secondary mirror.
• The secondary mirror must point at the centre of the primary mirror.
• The primary mirror must point back at the secondary mirror, so the reflected light lands in the focuser.

Each of these has its own adjustment. Collimation is simply the habit of checking them in that order. If you skip ahead to the primary screws while the secondary is badly turned, you will chase the error around without ever reaching a clean result.

Why small reflectors are sensitive

Short focal ratios, often around f/5 to f/8 in this aperture range, tolerate less error as the ratio gets faster. A 114 mm f/8 instrument forgives more than a 150 mm f/5 one. Even so, every Newtonian at this size benefits from a check before a serious session, especially if the telescope has been carried in a car.

Tools you need

You can do the whole job with a short list of items:

• A collimation cap, also called a sight tube: a plastic cap with a small centred hole that fits into the focuser in place of an eyepiece.
• A Cheshire eyepiece or a combined sight tube and Cheshire. This has a bright, angled opening that lights up the view down the tube.
• A laser collimator is optional. If you use one, treat it as a rough aid and finish with a star test, because a laser that is itself misaligned will mislead you.
• A small cross-head screwdriver, a hex key set, or whatever fits your adjustment screws. Many small reflectors have thumbscrews or knurled knobs on the primary cell.
• A centre mark on the primary mirror: a ring sticker or a small paper reinforcement ring, placed at the exact centre of the mirror. Many mirrors already have one. If yours does not, a mirror-marking ring made for the purpose is a permanent and safe fix, and it does not affect the image.
• A dim room light and a plain white sheet or card to hold behind the focuser, so the reflections are easy to read.

Do not touch the mirror surfaces. Fingerprints leave marks that are hard to remove, and a collimation job needs only the edges of the cell and the adjusting screws.

Prepare the telescope

1. Choose a stable place. Set the telescope on a table or on its mount in a room with even light. You will tilt your head to look into the focuser, so make sure the tube does not move when you press on it.
2. Level the tube roughly. Point the tube horizontally, or close to it, so the primary mirror is not sliding in its cell. A small tilt either way is fine.
3. Rack the focuser in. Draw the drawtube fully inward. This puts the optical path at its simplest and keeps the secondary easy to see.
4. Remove the eyepiece and fit your collimation cap or Cheshire in its place.
5. Look inside the tube. Note where the secondary sits, where the three primary screws are, and whether there are locking screws on the primary cell. Loosen locks slightly before you turn the main screws, and snug them again afterward.

Step 1: Centre the secondary under the focuser

Look through the sight tube at the secondary mirror. Without changing the position of your eye, check that the whole reflection of the primary is visible inside the oval of the secondary.

The aim here is to make the secondary appear centred in the focuser's view. If it looks offset along the length of the tube, the secondary is too far up or down. Adjust the centre bolt that holds the secondary on its spider so that the oval moves toward the focuser, and turn the bolt in small steps. Hold the secondary still with your fingers on the back of the holder while you turn the bolt, so the mirror does not spin.

Check for symmetry. The secondary should look like a circle when seen from the focuser, not an ellipse leaning to one side. If it looks stretched, the rotation of the holder is wrong. Turn it by hand until the shape is round and the reflection looks even.

What to check

• The secondary's outline should be centred in the focuser view along the tube.
• You should see the primary's whole circular reflection inside the secondary.
• The secondary must be tight enough that it does not move when you press gently on the tube, but not so tight that the holder twists.

Step 2: Aim the secondary at the primary

Now look for the reflection of the primary mirror inside the secondary. You should see the primary's edge, its clips, and the small ring mark at its centre, all inside the oval of the secondary.

The three small screws on the secondary holder tilt it. Turn them in small steps, loosening one before tightening another, until the reflected primary appears centred inside the secondary. You may also see the reflection of the secondary holder, the spider vanes, and your own sight tube. At this stage, ignore those and focus on getting the primary's full outline inside the secondary's outline.

Keep these points in mind:

• Make adjustments of an eighth to a quarter of a turn. Larger moves overshoot quickly.
• After each change, look again before moving a second screw.
• If a screw is already tight and the picture is still wrong, back off another screw rather than forcing the first.

When the outline of the primary looks even all the way around inside the secondary, this step is complete.

Step 3: Align the primary mirror

Look down the sight tube again. You should see the secondary, the primary mirror reflected in it, and a reflection of the sight tube itself with the centre mark in the middle. The centre mark is the target. Turn the primary's adjusting screws until the reflection of the sight tube and the centre mark fall on top of each other, so the dark spot of the tube sits right in the ring.

Work with two screws at a time:

1. Pick one screw and turn it a small amount, such as a quarter turn.
2. Watch which way the reflection moves.
3. If it moves farther from the ring, reverse the turn, and try a different screw.
4. When the reflection of the sight tube sits inside the ring, tighten the locking screws gently.

Do not over-tighten. Over-tightening pinches the mirror and can distort the star image into a triangle, which looks like a collimation fault but is not.

When you are done, the view should show concentric rings: the secondary, the primary's edge, the primary's centre mark, and the reflection of the sight tube, all lined up around a common centre. This is the picture to aim for before you go outside.

Step 4: Refine with a star test

The desk work gets you close. A star test confirms the result under the real sky and corrects any small remaining error. Do it on a calm, clear night after the telescope has cooled outdoors for 20 to 30 minutes, so air currents inside the tube do not blur the pattern.

Set up the test

1. Point the telescope at a moderately bright star well above the horizon. Avoid low stars, which are distorted by the atmosphere.
2. Use a medium-power eyepiece, giving roughly 150 to 250 times in this aperture range, and centre the star in the field.
3. Defocus slightly until you see a ring pattern, known as the diffraction pattern.
4. Keep the star in the centre of the field while you work. If the telescope is on a mount that tracks, switch tracking on. On a manual mount, nudge the tube so the star stays near the middle.

Read the pattern

A well-collimated telescope shows a round disc with evenly spaced concentric rings, and a dark shadow of the secondary in the middle of the pattern. Both sides of focus should look nearly identical.

If the rings are uneven, with the dark centre pushed to one side, the optics are still tilted. Work out which way the shadow moves, and make small changes at the primary screws only. Adjust the screws that move the shadow toward the centre of the rings. After every change, recentre the star and look again.

A common mistake is to adjust the secondary at this stage. Leave it alone unless the daytime step showed a clear fault. The primary screws do the fine work.

Check both sides of focus

Look at the pattern slightly inside focus and slightly outside focus. They should look like mirror images of each other. If one side is clearly lopsided, repeat the primary adjustment. Small differences can come from thermal currents, so wait a few minutes and try again before you make further changes.

Common problems and what they mean

• The star shows a comet-like tail. This is coma that has been made worse by collimation error. Repeat the primary step and check that the secondary has not shifted.
• The pattern looks triangular. Often this means the primary cell is pinching the mirror. Loosen the locking screws and clips slightly, then refocus.
• The pattern looks like a squashed oval. This can be astigmatism from a pinched mirror or from a tilted focuser. Check that the clips on the primary are not too tight.
• The shadow is a different size on each side of focus. The mirrors are still tilted relative to each other. Work through the primary adjustment again.
• The view improves for a moment and then gets worse. Thermal currents inside the tube are likely. Allow more time to cool, and if you used a locking screw, check that it did not shift the mirror as it tightened.

When to check collimation

A quick check each time you set up is enough, and it takes only a minute with a sight tube. Do a full alignment when the telescope has been transported, when the view looks softer than usual, or when the star test shows an uneven ring pattern. A telescope that stays on the same stand in a dry shed may hold its alignment for months.

Keep a note of your screw positions. A small pen mark on each knob tells you where you started, so you can return to a good position if an adjustment goes the wrong way.