diff --git a/README.md b/README.md
index 8b8f614..3f1ef07 100644
--- a/README.md
+++ b/README.md
@@ -256,7 +256,10 @@ the ray counts before each slab is loaded.
 To preserve a render for later exposure and tone-mapping work, build the desired
 backend with `ENABLE_HDR=1`, for example `make SPACETIME=schwarzschild
 ENABLE_HDR=1`. This produces `build/Release/schwarzschild_sky`,
-which accepts `--hdr-output PATH`. It writes the pre-tone-mapping RGB
+which accepts `--hdr-output`.  This switch writes the HDR file next to the
+ordinary output, replacing its extension with `_HDR.fits`; for example,
+`--output output/imgs/ring.png --hdr-output` writes
+`output/imgs/ring_HDR.fits`.  It writes the pre-tone-mapping RGB
 framebuffer as a three-plane, 32-bit float FITS image. Values remain linear HDR
 at the renderer's arbitrary scale; no tone mapping or per-frame normalization
 is applied. The ordinary
diff --git a/src/main.c b/src/main.c
index 380a03b..16f5000 100644
--- a/src/main.c
+++ b/src/main.c
@@ -32,7 +32,8 @@ typedef struct {
   const char *all_sky_catalog_path;
   const char *output_path;
 #ifdef ENABLE_HDR_OUTPUT
-  const char *hdr_output_path;
+  int write_hdr_output;
+  char hdr_output_path[PATH_MAX];
 #endif
   const char *observer_track_path;
   const char *frames_dir;
@@ -143,6 +144,26 @@ static int validate_tonemapped_output_path(const char *path) {
   return -1;
 }
 
+#ifdef ENABLE_HDR_OUTPUT
+static int make_hdr_output_path(char path[PATH_MAX], const char *output_path) {
+  static const char hdr_suffix[] = "_HDR.fits";
+  const char *filename = strrchr(output_path, '/');
+  const char *extension;
+  size_t stem_length;
+
+  filename = filename == NULL ? output_path : filename + 1;
+  extension = strrchr(filename, '.');
+  stem_length = extension != NULL && extension != filename
+                    ? (size_t)(extension - output_path)
+                    : strlen(output_path);
+  if (stem_length + sizeof hdr_suffix > PATH_MAX)
+    return -1;
+  memcpy(path, output_path, stem_length);
+  memcpy(path + stem_length, hdr_suffix, sizeof hdr_suffix);
+  return 0;
+}
+#endif
+
 static int parse_args(int argc, char **argv, Settings *s,
                       const char **write_path) {
 #ifdef ENABLE_PNG
@@ -190,8 +211,8 @@ static int parse_args(int argc, char **argv, Settings *s,
     else if (!strcmp(argv[i], "--output") && i + 1 < argc)
       s->output_path = argv[++i];
 #ifdef ENABLE_HDR_OUTPUT
-    else if (!strcmp(argv[i], "--hdr-output") && i + 1 < argc)
-      s->hdr_output_path = argv[++i];
+    else if (!strcmp(argv[i], "--hdr-output"))
+      s->write_hdr_output = 1;
 #endif
     else if (!strcmp(argv[i], "--width") && i + 1 < argc &&
              !parse_int(argv[++i], &s->width)) {
@@ -450,7 +471,7 @@ static int render_observer_frame(const Settings *s, StarCatalog *catalog,
   if (s->draw_mesh)
     frame_draw_mesh(&mesh, hdr, s->width, s->height, 0.5, 0.5);
 #ifdef ENABLE_HDR_OUTPUT
-  if (s->hdr_output_path != NULL &&
+  if (s->write_hdr_output &&
       write_hdr_fits(s->hdr_output_path, hdr, s->width, s->height,
                      s->horizontal_fov_deg)) {
     fprintf(stderr, "Failed to write HDR FITS image: %s\n", s->hdr_output_path);
@@ -667,7 +688,7 @@ int main(int argc, char **argv) {
             "[--max-magnification M] [--max-cache-psf-flux F] "
             "[--psf-direct] [--verbose] "
 #ifdef ENABLE_HDR_OUTPUT
-            "[--hdr-output PATH] "
+            "[--hdr-output] "
 #endif
             "[--coarse-cell-pixels N] [--refine-max-level N "
             "--refine-angle-abs-deg D --refine-angle-rel R "
@@ -693,7 +714,7 @@ int main(int argc, char **argv) {
                ? 0
                : (perror(settings.write_minkowski_accel_track_path), 1);
 #ifdef ENABLE_HDR_OUTPUT
-  if (settings.frames_dir != NULL && settings.hdr_output_path != NULL) {
+  if (settings.frames_dir != NULL && settings.write_hdr_output) {
     fputs("--hdr-output is available only for a single-frame render.\n", stderr);
     return 2;
   }
@@ -701,6 +722,13 @@ int main(int argc, char **argv) {
   if (settings.frames_dir == NULL &&
       validate_tonemapped_output_path(settings.output_path))
     return 2;
+#ifdef ENABLE_HDR_OUTPUT
+  if (settings.write_hdr_output &&
+      make_hdr_output_path(settings.hdr_output_path, settings.output_path)) {
+    fputs("HDR output path is too long.\n", stderr);
+    return 2;
+  }
+#endif
   StarCatalog catalog = {0};
   if (settings.all_sky_catalog_path != NULL) {
     if (catalog_load_all_sky(&catalog, settings.all_sky_catalog_path)) {