Two-Tool Collimation: Laser & Cheshire
The everyday collimation. The laser sets the focuser axis and ballparks the primary; the reflective Cheshire sets the primary precisely.
Quick steps = just do this. Full guide = the same steps plus the reasoning and theory.
Core procedure. This is the everyday collimation — it takes the telescope from out of alignment to well collimated. The autocollimator (a separate guide) is an optional precision finish on top of this.
What this does
Two tools, used in order, working from rough to fine: the laser sets the focuser axis and gets the primary into the ballpark, then the Cheshire sets the primary precisely. Done well, this is enough for most telescopes.
The two errors you are correcting
- Primary error — primary-mirror tip/tilt. Corrected with the primary mirror's collimation screws.
- Focuser error — focuser / secondary-mirror tip/tilt. Corrected by tilting the secondary mirror — not the focuser hardware itself.
| Tool | Sets | By adjusting |
|---|---|---|
| Laser | Focuser axis, then a ballpark primary | Secondary tilt, then primary |
| Cheshire | Primary, precisely | Primary |
Before you start
- The primary is center-marked with a triangle.
- The primary cell holds its adjustment (knobs turn easily, settings stay put, lock).
- The secondary is roughly centered and rotated under the focuser (Preparation layer). If not, do that first.
Have a light source handy — the Cheshire has no battery and is lit by outside light (daylight, or a dim flashlight at night). See Step 3 for how.
Step 1 — Laser: set the focuser axis
- Insert the laser and seat it square and flush in the focuser.
- Turn it on. The beam crosses to the secondary and lands somewhere on the primary.
- Adjust the secondary mirror's tilt until the beam lands exactly on the primary's center spot.
Step 2 — Laser: ballpark the primary
The beam reflects off the primary and travels back toward the laser. You adjust the primary until that return beam comes back to where it started. How you see the return depends on your telescope:
- Open / truss tube (you can see the secondary): watch the laser spot on the secondary mirror, and the returning beam alongside it. Adjust the primary until the return comes back on itself.
- Closed tube (you can't see in): use a white card with a small hole punched in it. Hold the card so the beam leaving the laser passes through the hole; the return beam then falls somewhere on the card. Taping the card to a small stick (a take-out chopstick works well) makes it easy to hold in the light path. Adjust the primary until the return beam comes back to the hole.
This step is coarse on purpose. The Cheshire does the precise primary alignment next.
Step 3 — Cheshire: set the primary precisely
The Cheshire has no battery. Its face is a highly reflective surface — a 3M micro-retroreflective vinyl — that glows brightly under ordinary light. Light it like this:
- In daylight: aim the telescope toward a bright area — an open window, a white wall, or the daytime sky — and move it until the reflective face glows brightly and evenly.
- At night: shine a dim flashlight (a red astro-light is ideal) down the telescope tube to light the face. Cupping your hand over the light gives a softer, more even glow.
- Remove the laser and insert the Cheshire, then light its face as above. You will see a white ring with the red triangle center-spot reflection inside it.
- Adjust the primary mirror until the red triangle is centered in the white ring, with all three apexes just touching the ring's inner edge at the same time.
The triangle is sized so its three points reach the ring's inner edge exactly when centered — so the moment one apex touches and another gaps, you can see the misalignment. It is a very sensitive cue.
Finish and verify
The telescope is collimated when, taken together:
- the laser dot sits on the center spot (focuser axis), and
- the Cheshire shows the center-spot reflection centered in its ring (primary).
Quick troubleshooting
- The laser dot can't reach the center spot: the secondary needs centering/rotating first — go back to the Preparation layer.
- The return dot misses the target face entirely: the primary is far off; rough it in, and re-check that the laser is seated square.
- You can't see the Cheshire's bright ring: not enough light reaching the reflective face — brighten or reposition your light, or aim the scope at a brighter area.
- Collimation won't hold: the problem is mechanical (springs/knobs), not optical — address the primary cell (Preparation layer).
Full guide. This is the everyday collimation procedure, with the reasoning behind each step. If you just want the steps, use the Core procedure instead.
What this does ·The two errors ·Before you start ·Step 1 — Laser: focuser axis ·Step 2 — Laser: ballpark primary ·Step 3 — Cheshire: precise primary ·Finish & verify ·Troubleshooting ·How good is good enough
What this does
Two tools, used in order, working from rough to fine. The laser sets the focuser axis and gets the primary into the ballpark; the Cheshire then sets the primary precisely. Done well, this is enough for most telescopes. The autocollimator (a separate guide) is an optional precision finish on top of it.
Why two tools, in this order
Each tool is good at one thing. A laser shows the focuser axis directly and unambiguously — you can see exactly where the beam strikes — which makes it ideal for the secondary. But a laser is only a coarse primary tool: small errors in the beam and in the laser's own alignment limit how finely it can set the primary. The Cheshire, lit by daylight, gives a high-contrast, very sensitive read of the primary. So we use each for what it does best, and in the order that doesn't undo earlier work: focuser axis first (secondary), then primary.
The two errors you are correcting
- Primary error — primary-mirror tip/tilt. Corrected with the primary mirror's collimation screws.
- Focuser error — focuser / secondary-mirror tip/tilt. Corrected by tilting the secondary mirror — not the focuser hardware itself.
| Tool | Sets | By adjusting |
|---|---|---|
| Laser | Focuser axis, then a ballpark primary | Secondary tilt, then primary |
| Cheshire | Primary, precisely | Primary |
Before you start
- The primary is center-marked with a triangle.
- The primary cell holds its adjustment (knobs turn easily, settings stay put, lock).
- The secondary is roughly centered and rotated under the focuser (Preparation layer). If not, do that first.
Have a light source handy — the Cheshire has no battery and is lit by outside light (daylight, or a dim flashlight at night). See Step 3 for how.
Step 1 — Laser: set the focuser axis
- Insert the laser and seat it square and flush in the focuser.
- Turn it on. The beam crosses to the secondary and lands somewhere on the primary.
- Adjust the secondary mirror's tilt until the beam lands exactly on the primary's center spot.
Seat the laser square — a cocked laser lies
If the laser sits tilted in the drawtube, its beam points slightly off, and everything you set from it inherits that error. Before trusting it, rotate the laser in the focuser and watch the dot: if the dot swings in a circle, the laser (or the focuser) isn't square — re-seat it, snug the compression ring evenly, and re-check. A laser that holds its dot in one place as you rotate it is telling the truth.
What "the focuser axis" means here, and offset
Landing the beam on the center spot points the focuser's optical axis at the middle of the primary — that is the focuser axis, set by the secondary's tilt. Centering and rotating the secondary (Preparation layer) is a different job: it governs even illumination and seeing the whole mirror, not the axis. You may notice the secondary looks slightly offset away from the focuser; that offset is normal and happens on its own when you aim by the center spot — you do not calculate or chase it.
Step 2 — Laser: ballpark the primary
The beam reflects off the primary and travels back toward the laser. You adjust the primary until that return beam comes back to where it started. How you see the return depends on your telescope:
- Open / truss tube (you can see the secondary): watch the laser spot on the secondary mirror and the returning beam alongside it. Adjust the primary until the return comes back on itself.
- Closed tube (you can't see in): use a white card with a small hole punched in it. Hold the card so the beam leaving the laser passes through the hole; the return beam then falls somewhere on the card. Taping the card to a small stick (a take-out chopstick works well) makes it easy to hold in the light path. Adjust the primary until the return beam comes back to the hole.
Why the laser is only a ballpark for the primary
The return-beam method sends the beam on a long round trip, so any small imperfection — the beam not landing dead-center on the spot, the laser not perfectly collimated, a trace of slop in the focuser — gets folded into where the return lands. It is plenty to get the primary close, but it can't resolve the last small fraction of error. That is the Cheshire's job, which is why we don't stop here.
Step 3 — Cheshire: set the primary precisely
The Cheshire has no battery. Its face is a highly reflective surface — a 3M micro-retroreflective vinyl — that glows brightly under ordinary light. Light it like this:
- In daylight: aim the telescope toward a bright area — an open window, a white wall, or the daytime sky — and move it until the reflective face glows brightly and evenly.
- At night: shine a dim flashlight (a red astro-light is ideal) down the telescope tube to light the face. Cupping your hand over the light gives a softer, more even glow.
- Remove the laser and insert the Cheshire, then light its face as above. You will see a white ring with the red triangle center-spot reflection inside it.
- Adjust the primary mirror until the red triangle is centered in the white ring, with all three apexes just touching the ring's inner edge at the same time.
The triangle is sized so its three points reach the ring's inner edge exactly when centered. The moment one apex touches while another gaps, the misalignment is obvious — it is a very sensitive cue.
How the retroreflective face works
The Cheshire's bright face is coated with a micro-retroreflective vinyl — the same kind of material used on road signs and safety gear, which sends light back toward wherever it came from. That is why it lights up so readily from ordinary daylight or a flashlight and shows as a clean, bright white ring reflected in the optics, with no battery and no fussing over angles. You read the primary by centering the red center-spot reflection in that ring; small primary moves don't disturb the focuser axis you set with the laser, so you can fine-tune freely here.
Finish and verify
The telescope is collimated when, taken together:
- the laser dot sits on the center spot (focuser axis), and
- the Cheshire shows the red triangle centered in its white ring (primary).
Troubleshooting
- The laser dot can't reach the center spot: the secondary needs centering/rotating first — go back to the Preparation layer.
- The dot swings in a circle when you rotate the laser: the laser or focuser isn't square — re-seat the laser and snug the ring evenly before trusting it.
- The return beam misses the card entirely: the primary is far off; rough it in, and re-check the laser seating.
- You can't see the Cheshire's bright ring: not enough light reaching the reflective face — brighten or reposition your light, or aim the scope at a brighter area.
- Collimation won't hold: the problem is mechanical (springs/knobs), not optical — address the primary cell (Preparation layer).
How good is good enough
Collimation tolerance tightens sharply with focal ratio — roughly with its cube. A long, slow scope (f/8 and up) has a generous margin, and a careful laser-and-Cheshire alignment is more than enough. A fast scope (f/5 and faster) has a small margin where the last fraction of error matters; there, finish with the autocollimator to confirm and refine what the laser and Cheshire have set.
Tools for this step
