Preparation: Center-Spot, Cell & Secondary
Before you collimate: mark the primary's center, make the mirror cell hold its adjustment, and position and rotate the secondary under the focuser.
Quick steps = just do this. Full guide = the same steps plus the reasoning and theory.
Core procedure. Three one-time (or once-in-a-while) jobs that must be done before collimating: mark the primary's center, make the primary cell adjust and hold, and position the secondary. With these done, the laser, Cheshire, and autocollimator will work — and hold.
Why preparation comes first
Collimation is optical; preparation is mechanical. You are getting the hardware right so the optical tools have something honest to read. You do these steps rarely — the center spot once, the cell upgrade once, the secondary when it's disturbed — but skipping them is the usual reason collimation "won't take."
- Primary error — primary-mirror tip/tilt (the primary's collimation screws).
- Focuser error — focuser/secondary tip/tilt (the secondary's tilt).
1 — Center-spot the primary mirror
- Get the mirror somewhere you can work safely — ideally remove the mirror and cell to a clean, padded surface. Handle the mirror by its edges; never touch the coated face.
- Find the exact center with a center-finding template sized to your mirror. A template and triangle targets come with Farpoint collimation tools and the center-marking kit, so you likely already have what you need.
- Apply the triangle target at the center. Use the triangle, not a round dot — the later steps read the triangle (the Cheshire's apex-touch cue and the autocollimator's orientation both depend on it).
- Re-seat the mirror and cell in the telescope.
2 — Make the primary cell reliable
The cell must do three things: adjust easily, stay put after adjustment, and lock when done. If it fails any of these, fix it before you collimate — no optical procedure will hold on a cell that drifts.
- Identify your cell's collimation screws (they tilt the mirror) and locking screws (they secure it).
- If collimation drifts as you move the scope, the usual cause is weak factory springs that sag under the mirror's weight. Stronger springs fix it; tool-free knobs make adjustment easier. Farpoint offers collimation knob-and-spring upgrade kits for common telescopes — a drop-in fix for both problems at once.
- When you collimate later: set the collimation screws, then snug the locking screws and re-check — locking can shift things slightly.
- Test it now: make an adjustment, lock it, gently tip the tube up and down, and look again. It should not have moved.
3 — Position and rotate the secondary mirror
The secondary has three mechanical adjustments to set first — centering it in the tube, placing it along the tube's length, and rotating it. Once those are right, you confirm the result by looking down the focuser. (A fourth adjustment, tilt, is set with the laser in the next layer.)
3a — Center it in the tube
Look down the front of the telescope. The secondary should sit in the middle of the tube — the same distance to the tube wall on every side. Check it with a ruler or tape measure. Most telescopes let you center it using adjustments on the spider (the vanes that hold it).
3b — Place it along the length of the tube
The secondary also has to sit at the right point along the tube's length, so it lines up directly under the focuser. Most secondary holders have screws that slide the mirror along the tube — adjust until it is centered under the focuser opening.
3c — Rotate it to face the focuser
Finally, twist the holder so the elliptical mirror faces squarely up the focuser. Most holders allow this rotation. Mis-rotated, the mirror looks egg-shaped or clipped when you look down the focuser.
Confirm — look down the focuser
With those three set, look down the focuser with a sight tube or collimation cap. Your Farpoint Cheshire works perfectly well as a collimation cap for this — no separate tool needed. You should see a stack of concentric circles — the sight-tube edge, the round secondary outline, and the primary's reflection — all sharing one center. If the secondary still looks egg-shaped, fine-tune the rotation (3c); if the circles are off to one side, re-center (3a) or re-place it (3b).
You may notice the secondary ends up slightly offset away from the focuser and toward the primary. That offset is normal and happens on its own — you don't measure or chase it.
Preparation done
You're ready to collimate when:
- the primary has a triangle center spot,
- the cell adjusts, holds, and locks, and
- the secondary is round and centered under the focuser.
Next: the laser and Cheshire (the two-tool collimation layer).
Quick troubleshooting
- The secondary outline won't go round: check both its rotation and its placement along the tube, and that the spider isn't skewed.
- The center spot looks off after re-seating the mirror: re-do it — being concentric with the mirror edge matters more than anything.
- Collimation won't hold later: the cause is almost always here — weak springs or loose locks. Address the cell.
Full guide. The three preparation jobs, with the reasoning behind each. If you just want the steps, use the Core procedure instead.
Why preparation comes first ·The two errors ·1 — Center-spot the primary ·2 — Make the cell reliable ·3 — Position & rotate the secondary ·Preparation done ·Troubleshooting
Why preparation comes first
Collimation is optical; preparation is mechanical. You are getting the hardware right so the optical tools have something honest to read. You do these steps rarely — the center spot once, the cell upgrade once, the secondary when it's disturbed — but skipping them is the single most common reason collimation "won't take." A mirror with no center mark gives the tools no reference; a cell that won't hold lets your careful work drift away the moment you point at the sky; a badly placed secondary clips the light and confuses every later reading.
- Primary error — primary-mirror tip/tilt (the primary's collimation screws).
- Focuser error — focuser/secondary tip/tilt (the secondary's tilt).
1 — Center-spot the primary mirror
Why a mark at the center is invisible to the image
Light reaches the focuser only from the parts of the primary the secondary can "see." Directly in front of the mirror's center, the secondary itself blocks the path — that small zone is always in shadow and contributes nothing to the image. A spot there is purely a reference for your tools, not part of the optical surface that matters.
- Get the mirror somewhere you can work safely — ideally remove the mirror and cell to a clean, padded surface. Handle the mirror by its edges; never touch the coated face, and don't let any adhesive touch the coating.
- Find the exact center with a center-finding template sized to your mirror. A template and triangle targets come with Farpoint collimation tools and the center-marking kit, so you likely already have what you need.
- Apply the triangle target at the center, then re-seat the mirror and cell.
Why a triangle, not a round dot
Every later step reads this mark. The Cheshire centers the triangle in its ring — and because the triangle's three points reach the ring's edge at once, you get a far more sensitive "centered" signal than a round dot could give. The autocollimator goes further: it needs the triangle's orientation to tell its reflections apart and to form the tell-tale hexagram in the CDP method. Spot with the triangle from the start and every tool downstream works better.
2 — Make the primary cell reliable
The cell must do three things: adjust easily, stay put after adjustment, and lock when done. If it fails any of these, fix it before you collimate — no optical procedure will hold on a cell that drifts.
- Identify your cell's collimation screws (they tilt the mirror) and locking screws (they secure it).
- If collimation drifts as you move the scope, the usual cause is weak factory springs that sag under the mirror's weight. Stronger springs fix it; tool-free knobs make adjustment easier. Farpoint offers collimation knob-and-spring upgrade kits for common telescopes — a drop-in fix for both problems at once.
- When you collimate later: set the collimation screws, then snug the locking screws and re-check — locking can shift things slightly.
- Test it now: make an adjustment, lock it, gently tip the tube up and down, and look again. It should not have moved.
Why springs matter, and the stiffness trade-off
The spring at each point keeps the mirror pressed firmly against the collimation screw so there's no free play. When a spring is too soft, the mirror's own weight compresses it, and as you swing the tube from horizon to zenith the load shifts and the mirror nods — your collimation literally changes with where you point. Stronger springs resist that. The trade-off is feel: very stiff springs hold beautifully but make the knobs harder to turn, and over-tightening can stress the mirror. The goal is firm and repeatable, not bone-tight. (Some cells skip springs and use a true push-pull pair of screws instead; the same principle applies — no free play, and lock it down.)
3 — Position and rotate the secondary mirror
The secondary has three mechanical adjustments to set first — centering it in the tube, placing it along the tube's length, and rotating it. Once those are right, you confirm the result by looking down the focuser. (A fourth adjustment, tilt, is set with the laser in the next layer.)
3a — Center it in the tube
Look down the front of the telescope. The secondary should sit in the middle of the tube — the same distance to the tube wall on every side. Check it with a ruler or tape measure. Most telescopes let you center it using adjustments on the spider (the vanes that hold it).
3b — Place it along the length of the tube
The secondary also has to sit at the right point along the tube's length, so it lines up directly under the focuser. Most secondary holders have screws that slide the mirror along the tube — adjust until it is centered under the focuser opening.
3c — Rotate it to face the focuser
Finally, twist the holder so the elliptical mirror faces squarely up the focuser. Most holders allow this rotation. Mis-rotated, the mirror looks egg-shaped or clipped when you look down the focuser.
Confirm — look down the focuser
With those three set, look down the focuser with a sight tube or collimation cap. Your Farpoint Cheshire works perfectly well as a collimation cap for this — no separate tool needed. You should see a stack of concentric circles — the sight-tube edge, the round secondary outline, and the primary's reflection — all sharing one center.
If the secondary still looks egg-shaped, fine-tune the rotation (3c); if the circles are off to one side, re-center (3a) or re-place it (3b).
The offset, and why you don't chase it
When correctly placed, the secondary ends up shifted very slightly away from the focuser and toward the primary — the "offset." It exists because the cone of light from the primary is wider on the focuser side. The good news: if you align by making the view concentric and aiming at the primary's center (which the laser does in the next layer), the right offset falls out on its own. For visual telescopes it is small, and there is no need to measure or dial it in.
Preparation done
You're ready to collimate when:
- the primary has a triangle center spot,
- the cell adjusts, holds, and locks, and
- the secondary is round and concentric under the focuser.
Next: the laser and Cheshire (the two-tool collimation layer).
Troubleshooting
- The secondary outline won't go round: check both its rotation (3c) and its placement along the tube (3b), and that the spider isn't skewed.
- The circles won't go concentric: re-center in the tube (3a) and re-check placement (3b) — they interact, so work back and forth in small moves.
- The center spot looks off after re-seating the mirror: re-do it — being concentric with the mirror edge matters more than anything.
- Collimation won't hold later: the cause is almost always here — weak springs or loose locks. Address the cell (Section 2).
Tools for this step
