Segmented Space Telescope Mirror Phasing

Deployment

Misalignment amplitude 400 nm

Phasing

Actuator gain 0.60

Observing wavelength 2000 nm

Live Readouts

RMS wavefront error

0 nm

Strehl ratio

100%

Segments phased

18 / 18

Phasing step

Idle

How it works

A large space telescope mirror (this one is modelled on JWST's 6.5 m primary) is too big to launch as one piece, so it folds into 18 hexagonal beryllium segments that unfold in orbit. Each segment has 7 actuators — 6 for piston/tip/tilt positioning of the whole segment and 1 that bends its radius of curvature — and right after deployment every segment points in a slightly different direction, so a single star produces 18 separate blurry images instead of one sharp point.

Ground control aligns them in stages: segment identification, image stacking (bringing the 18 dots close together), coarse phasing (removing large piston offsets using specially dispersed light), and fine phasing (nulling the last nanometres of tip/tilt/piston error). This simulator models that convergence directly on each segment's piston p and tip/tilt angles.

RMS wavefront error   σ = sqrt( mean(pᵢ²) )  over all 18 segments
Strehl ratio (Maréchal approx.)
                      S ≈ exp( -(2πσ/λ)² )
Correction per frame  pᵢ ← pᵢ · (1 − gain·dt)

Misalignment amplitude — the initial random piston/tilt error injected by "Scatter Segments", in nanometres.
Actuator gain — how aggressively the phasing control loop drives each segment's error toward zero once started.
Observing wavelength — used only in the Strehl-ratio formula above; shorter wavelengths are far less forgiving of the same nanometre-scale error.
Focal-plane panel (top-right of the view) — each segment's un-phased tilt shifts its own copy of the star image; as phasing runs, all 18 dots collapse onto one point, exactly as the real commissioning images do.

Focal-plane image

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