design 121 LAST GROUP (Lens S25-S27): exit error vs the EXACT ray trace, ACROSS THE DOE FAN
  every row is the chain's OWN element call for that order (captured, not synthesised)

--- HALF=96 px (patch half-width 3.19 mm), amp threshold 0.02
   order  dec/w   w(mm)  NAexit  grad a rms   RAYMAP    RESID    TOTAL  Strehl  AMPerr  Ppatch   Pkeep  nn p99
--------------------------------------------------------------------------------------------------------------
   (0,0)  0.000  3.1336  0.2778    0.000693  0.00016  0.01448  0.00023  1.0000  0.0029  1.0000  0.9996  0.0002   [19s]
  (-1,0)  0.241  3.1337  0.2829    0.000812  0.00025  0.01803  0.00442  0.9992  0.0028  1.0000  0.9996  0.0045   [42s]
  (-2,0)  0.481  3.1322  0.2878    0.001113  0.00023  0.02627  0.00413  0.9993  0.0028  1.0000  0.9996  0.0044   [42s]
  (-3,0)  0.723  3.1295  0.2915    0.001533  0.00020  0.03785  0.00347  0.9995  0.0137  1.0000  0.9992  0.0041   [42s]
  (-4,0)  0.965  3.1261  0.2944    0.002054  0.00027  0.05128  0.00323  0.9996  0.0278  1.0000  0.9978  0.0048   [42s]
 (-4,-2)  1.079  3.1246  0.2955    0.002335  0.00039  0.05789  0.00321  0.9996  0.0317  1.0000  0.9971  0.0060   [41s]

grad a rms = amplitude-weighted rms of the INPUT residual's own transverse direction cosines (rad).  This is the quantity
             preserve_input_phase='remap' drops: it launches along grad(W) alone and evaluates the residual at THAT ray's foot.
TOTAL      = unwrap-free equivalent rms wavefront error of the SHIPPED field vs rays launched along grad(W + a), waves.
Ppatch/Pkeep = fraction of the element's whole-grid exit power inside the patch / inside the measured mask.
