Fix: correct ICRS camera orientation

src/catalog.c

@@ -91,9 +91,11 @@ int catalog_load_csv(StarCatalog *catalog, const char *path)
         const double lat = latitude * PI / 180.0;
         const double cos_lat = cos(lat);
         Star *star = &catalog->stars[catalog->count++];
+        /* Standard right-handed ICRS Cartesian axes: X is (RA, Dec) =
+         * (0, 0), Y is (90, 0), and Z is the north celestial pole. */
         star->direction[0] = cos_lat * cos(lon);
-        star->direction[1] = sin(lat);
-        star->direction[2] = cos_lat * sin(lon);
+        star->direction[1] = cos_lat * sin(lon);
+        star->direction[2] = sin(lat);
         star->temperature_K = temperature;
         star->amplitude = amplitude;
     }
@@ -170,10 +172,10 @@ int catalog_load_all_sky(StarCatalog *catalog, const char *directory)
 static void lon_lat_from_direction(const double direction[3], double *longitude,
                                    double *latitude)
 {
-    *longitude = atan2(direction[2], direction[0]) * 180.0 / PI;
+    *longitude = atan2(direction[1], direction[0]) * 180.0 / PI;
     if (*longitude < 0.0)
         *longitude += 360.0;
-    *latitude = asin(fmax(-1.0, fmin(1.0, direction[1]))) * 180.0 / PI;
+    *latitude = asin(fmax(-1.0, fmin(1.0, direction[2]))) * 180.0 / PI;
 }
 
 static double dot(const double a[3], const double b[3])
@@ -195,8 +197,8 @@ static void direction_from_lon_lat(double longitude, double latitude,
     const double lat = latitude * PI / 180.0;
     const double cos_lat = cos(lat);
     direction[0] = cos_lat * cos(lon);
-    direction[1] = sin(lat);
-    direction[2] = cos_lat * sin(lon);
+    direction[1] = cos_lat * sin(lon);
+    direction[2] = sin(lat);
 }
 
 /* The minimum of a plane dot-product over a longitude/latitude rectangle is

src/main.c

@@ -183,8 +183,10 @@ static int parse_args(int argc, char **argv, Settings *s,
                   .verbose = 1,
 #endif
                   .horizontal_fov_deg = 30.0,
-                  .look_ra_deg = 270.0,
-                  .look_dec_deg = 0.0,
+                  /* Preserve the legacy -Z view after switching to standard
+                   * right-handed ICRS Cartesian axes. */
+                  .look_ra_deg = 90.0,
+                  .look_dec_deg = -90.0,
                   .exposure = 1e-3,
                   .max_magnification = INFINITY,
                   .max_cache_psf_flux = 1.0,

src/observer.c

@@ -19,9 +19,11 @@ ObserverState observer_fixed_at_origin_look_at(double ra_deg, double dec_deg) {
   const double dec = dec_deg * pi / 180.0;
   const double cos_ra = cos(ra), sin_ra = sin(ra);
   const double cos_dec = cos(dec), sin_dec = sin(dec);
-  const double forward[3] = {cos_dec * cos_ra, sin_dec, cos_dec * sin_ra};
-  const double up[3] = {-sin_dec * cos_ra, cos_dec, -sin_dec * sin_ra};
-  const double right[3] = {-sin_ra, 0.0, cos_ra};
+  const double forward[3] = {cos_dec * cos_ra, cos_dec * sin_ra, sin_dec};
+  const double up[3] = {-sin_dec * cos_ra, -sin_dec * sin_ra, cos_dec};
+  /* forward cross celestial north is celestial west: with north up, image
+   * right is decreasing RA. */
+  const double right[3] = {sin_ra, -cos_ra, 0.0};
   return (ObserverState){.coordinate_time = 0.0,
                          .coordinate_position = {0.0, 0.0, 0.0},
                          .tetrad = {{1.0, 0.0, 0.0, 0.0},
@@ -41,14 +43,14 @@ static int schwarzschild_look_direction(double ra_deg, double dec_deg,
   const double cos_ra = cos(ra), sin_ra = sin(ra);
   const double cos_dec = cos(dec), sin_dec = sin(dec);
   direction[0] = cos_dec * cos_ra;
-  direction[1] = sin_dec;
-  direction[2] = cos_dec * sin_ra;
+  direction[1] = cos_dec * sin_ra;
+  direction[2] = sin_dec;
   up[0] = -sin_dec * cos_ra;
-  up[1] = cos_dec;
-  up[2] = -sin_dec * sin_ra;
-  right[0] = -sin_ra;
-  right[1] = 0.0;
-  right[2] = cos_ra;
+  up[1] = -sin_dec * sin_ra;
+  up[2] = cos_dec;
+  right[0] = sin_ra;
+  right[1] = -cos_ra;
+  right[2] = 0.0;
   return 0;
 }
 

src/observer.h

@@ -9,8 +9,9 @@ typedef struct {
 } ObserverState;
 
 ObserverState observer_fixed_at_origin(void);
-/* Point the fixed inertial observer at an ICRS-style RA/Dec direction.
- * The local spatial axes remain (forward, celestial north, increasing RA). */
+/* Point the fixed inertial observer at a standard right-handed ICRS RA/Dec
+ * direction.  The local spatial axes are (forward, celestial north,
+ * celestial west), so a north-up image has decreasing RA to the right. */
 ObserverState observer_fixed_at_origin_look_at(double ra_deg, double dec_deg);
 /* Static camera at Cartesian Kerr--Schild position -radius * look_direction,
  * directed toward the Schwarzschild black hole at the origin.  look_direction