What is collimation?
Collimating a Newtonian means lining up its two mirrors with the focuser axis. The primary, at the bottom of the tube, brings the light to a focus; the small tilted flat, the secondary, sends it sideways to the eyepiece. If either one leans, a star stops being a fine point and turns into a small comma-shaped blur.
A car trip, a knock or a change of temperature is enough to shift the mirrors. Checking before observing takes a minute once you know how to read what the focuser shows. This simulator lets you build that eye without touching a real telescope.
Why are fast Newtonians so demanding?
A paraboloid is perfect on its axis and only there. Away from it, a star smears into coma, all the faster as the focal ratio f/D (written N) gets smaller. The allowed distance between the primary’s axis and the eyepiece centre therefore scales as N³: halve N and the margin shrinks eightfold.
This simulator takes a moderate loss of sharpness as the limit (Strehl ratio of 0.8 in green light), which gives a tolerance of about 0.0107 × N³ millimetres at focus. The table below applies it to the preset telescopes.
| Telescope | f/D | Tolerance at focus |
|---|
| 114/900 | f/7,9f/7.9 | 5,25 mm5.25 mm |
| 130/650 | f/5,0f/5.0 | 1,33 mm1.33 mm |
| 150/750 | f/5,0f/5.0 | 1,33 mm1.33 mm |
| 200/1000 | f/5,0f/5.0 | 1,33 mm1.33 mm |
| 250/1200 | f/4,8f/4.8 | 1,18 mm1.18 mm |
| 300/1500 | f/5,0f/5.0 | 1,33 mm1.33 mm |
In what order?
- Centre the secondary under the focuser with the centre bolt: its outline should look round and in the middle of the drawtube.
- Aim the secondary with its three tilt screws until the primary’s reflection is whole and centred in the secondary, with its three clips.
- Loosen the primary’s locking screws, then turn its knobs until the centre spot surrounds the dark reflection of the peephole, or sits in the middle of the Cheshire ring. Snug the locking screws without forcing and check again.
- Confirm on a star, slightly out of focus: the secondary’s shadow should sit in the middle of the disc.
Why this order? You judge the primary through the secondary. Touching the secondary afterwards moves the reference and forces you to redo the primary. So set the reference first, then what depends on it.
What each view shows
- Collimation cap
- A cover with a small hole in its centre. All it does is put your eye on the focuser axis, yet that is enough for a good visual collimation. From the edge inwards you see: the drawtube, the secondary’s outline, the primary and its clips reflected in it, the shadow of the secondary and spider inside that reflection, and finally the centre spot around the black dot.
- Sight tube
- A long narrow tube with a crosshair at the far end. The small field and the crosshair make it easier to centre the secondary and aim it at the spot.
- Cheshire
- A tube with a 45° face around the peephole, lit through a side window. Seen in the primary, that face becomes a bright ring: the target in which you centre the spot by turning the knobs.
- Laser
- The beam follows the focuser axis. Its dot on the primary tells whether the secondary aims right; its return on the target tells whether the primary is aligned. A laser that is itself off-axis misleads: rotate it in the focuser, the dot must not draw a circle.
- Star
- Just inside or outside focus, a star becomes a disc with a hole made by the secondary’s shadow. This final test does not lie. Here the computation is purely geometric: no diffraction rings, no seeing.
Reading the Cheshire
Light the side window with a torch and look through the hole. Three marks matter: the white spot stuck in the middle of the primary, the bright ring (the reflection of the Cheshire’s face) and the black dot in the middle (the reflection of the hole, so of your eye).
If the spot is not in the middle of the ring, the primary leans. Turn a knob and watch the reflection: the spot, stuck to the mirror, barely moves, but the ring and the black dot move clearly. A mirror that pivots by some angle sends the light off by twice that angle, hence the sensitivity.
Do not try to centre the secondary’s shadow in the primary’s reflection. With an offset secondary, as in this model, it looks slightly off-centre even when everything is right. The secondary’s outline may also look a little off-centre, more so from a distance (Cheshire, sight tube).
Collimated, but the secondary looks crooked?
Sometimes the collimation is good, every mark nicely centred in the focuser, and yet the secondary, looked at directly through the open end of the tube, seems clearly askew: strongly tilted, and turned at an angle to the focuser.
This means the secondary started out rotated on its centre bolt: its face was not looking at the focuser. The three tilt screws made up for that rotation the only way they could, by tilting the mirror a lot. The optical axis is right, but the secondary no longer offers its whole surface to the light cone: some of the light from the edge of the primary can miss it, and the image loses illumination, mostly towards the edge of the field.
The cure: loosen the centre bolt slightly, rotate the secondary on itself until it squarely faces the focuser (round and centred in the sight tube), tighten again, then start the collimation over from the beginning: centring, aiming, then the primary.
Conventions and limits of the simulation
- Primary knobs, seen from behind the telescope: clockwise, the knob moves in and pushes the mirror towards the sky where it sits. M6 thread (1 mm per turn), three knobs at 120° on a circle of radius 0.42 × D. The mirror rests on their three tips: their plane gives its tilt.
- Secondary tilt screws, seen from the front of the tube: clockwise, the screw moves in and pushes that side of the holder towards the primary. M4 thread (0.7 mm per turn), three screws at 120° on a circle of radius 0.32 × minor axis. The centre bolt (M5, 0.8 mm per turn) moves the secondary along the tube: clockwise, towards the open end.
- The focuser view has the open end of the tube at the top. Each pixel is a ray from the peephole, folded by the secondary and then by the parabolic primary, in their positions in the 3D view. The secondary is offset with the classic Newtonian design formula.
- Success thresholds: secondary centred to better than 3% of its minor axis (at least 1 mm), focuser axis within 2% of D of the primary’s centre (at least 2 mm), primary axis within the N³ tolerance above.
- Not simulated: the squareness of the focuser, the sideways centring of the secondary (set with the spider vanes) and its rotation about the centre bolt, rarely needed on a factory-made instrument.