Optics: parameterize PSF relative tail

README.md

@@ -234,6 +234,11 @@ cached/direct-fallback image counts.  Pass `--psf-direct` to use the slower
 whose required HDR-tail support exceeds the cache radius selects that reference
 path automatically.
 
+`--psf-relative-tail R` controls the maximum omitted PSF tail fraction per
+image; it defaults to `1e-8`.  Larger values intentionally shorten the Moffat
+support and rebuild the immutable cache at the corresponding radius, which is
+useful when a faster, lower-fidelity render is acceptable.
+
 For bounded preview renders, `--max-cache-psf-flux F` (default `1`) allows
 images with `1 < flux <= F` to use the existing cache instead of the direct
 evaluator. This does not rebuild or enlarge the cache: the image retains its