Optics: scale PSF cache radius by parameters

psf_lookup_optimization_plan.md

@@ -66,10 +66,12 @@ flux so that both of these hold:
 - the PSF value at the support boundary is below a configured per-pixel HDR
   error budget.
 
-The cache contains weights through the largest radius required by the selected
-configuration.  A dim image uses a cropped subset; an unusually bright image
-uses a larger subset.  If an event requires a radius beyond the cache maximum,
-use the direct Moffat evaluator for that event and record it in diagnostics.
+The cache contains weights through the radius implied by the selected PSF
+parameters at the reference scaled flux of one.  A dim image uses a cropped
+subset; an unusually bright image uses a larger subset.  If an event requires
+a radius beyond that cache maximum, use the direct Moffat evaluator for that
+event and record it in diagnostics.  The radius is not a fixed pixel cap:
+low-beta Moffat profiles need broad wings even when their FWHM is small.
 This prevents a visible hard cutoff in bright-star wings without silently
 discarding their flux.