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. 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. This 2D view looks straight down the optical axis: each hexagon is shaded by its own piston (depth) error and carries a small offset glint showing its tip/tilt error.
Ground control aligns them in stages: image stacking (bringing the 18 focal-plane dots close together), coarse phasing (removing large piston offsets), and fine phasing (nulling the last nanometres of error). Drag the main view to pan and scroll to zoom in on individual segments.
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) [same tiny update applies to tiltX, tiltY]
Note on the source model: the original 3D simulator's own "How it works" box states the correction rule above, but its code actually applied an undocumented extra ×2.2 multiplier to the decay rate, converging roughly 30× faster than the stated formula predicts (checked numerically: after a 6s sweep at gain 0.6, the documented formula leaves ≈10.7 nm of a 400 nm error, the shipped code left ≈0.13 nm). This build implements the formula exactly as documented above, with phase-timing recalibrated to match the slower, physically-labelled convergence rate.
- 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) — each segment's un-phased tilt shifts its own copy of the star image; as phasing runs, all 18 dots collapse onto one point.
- Segment errors panel (bottom-right) — a live bar per segment showing |piston| error, so you can see individual actuators converge at different rates.
- History panel (top-left) — RMS wavefront error and Strehl ratio traced over the last ~12 seconds.