Optics: add PSF luminance cutoff

README.md

@@ -239,6 +239,11 @@ 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.
 
+`--psf-min-y Y` defaults to `0` (disabled).  A positive value is a linear-HDR
+luminance cutoff: a PSF stops where its continuous Moffat Y profile falls below
+`Y`, and an image whose central value is already below `Y` is omitted.  The
+final PSF report counts such omitted events and emits a warning when any occur.
+
 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