Fix: correct ICRS camera orientation

README.md

@@ -210,9 +210,12 @@ newly inserted vertices and geometry-only longest-edge probes for its new
 leaves are collected together, so both ray sets use the same parallel
 `RayPool` pass.
 
-`--look-ra-deg` and `--look-dec-deg` rotate that fixed tetrad so its forward
-axis is the corresponding catalog direction; their defaults reproduce the
-original `-Z` view. `--exposure` converts a catalog's physical flux
+Catalog directions and `--look-ra-deg`/`--look-dec-deg` use standard
+right-handed ICRS Cartesian axes: `+X` is RA 0 degrees/Dec 0 degrees, `+Y` is
+RA 90 degrees/Dec 0 degrees, and `+Z` is the north celestial pole.  The local
+camera axes are forward, celestial north, and celestial west, so an image with
+north up has decreasing RA to the right.  The defaults preserve the original
+`-Z` view. `--exposure` converts a catalog's physical flux
 normalization to the prototype HDR scale. The current synthetic catalog is
 calibrated for default exposure `1e-3`; a 2MASS blackbody normalization in
 steradians requires a much larger display exposure such as the example above.