diff --git a/sras_average.py b/sras_average.py
index d997a15..42aeaca 100644
--- a/sras_average.py
+++ b/sras_average.py
@@ -47,7 +47,7 @@ def read_header_sections(sras: SrasFile) -> bytes:
     lost in a re-encode."""
     with open(sras.path, "rb") as f:
         f.seek(HDR_SIZE)
-        return f.read(sras._data_offset - HDR_SIZE)
+        return f.read(sras.data_offset - HDR_SIZE)
 
 
 def average_rows(block: np.ndarray, n: int, discard_remainder: bool) -> np.ndarray:
diff --git a/sras_compute.py b/sras_compute.py
index fc7b30e..0305c54 100644
--- a/sras_compute.py
+++ b/sras_compute.py
@@ -768,7 +768,7 @@ def _skimage_phase_cross_correlation():
 
 # ---- Geometry: local mm, ref mm, and the one affine builder ---------------
 
-def _pixel_pitch_mm(sras: SrasFile, angle_idx: int) -> tuple[float, float]:
+def pixel_pitch_mm(sras: SrasFile, angle_idx: int) -> tuple[float, float]:
     """(dx, dy) mm/pixel for one angle: dx is the file-wide constant
     pixel_x_mm; dy is this angle's own row spacing (assumed uniform, the same
     assumption _redraw_image makes when it builds the display extent). dy keeps
@@ -803,7 +803,7 @@ def _local_half_extent_mm(sras: SrasFile, angle_idx: int) -> tuple[float, float]
     """(half width, half height) in mm from this angle's array center to the
     center of its outermost pixel."""
     n_rows, n_frames = sras.image_shape(angle_idx)
-    dx, dy = _pixel_pitch_mm(sras, angle_idx)
+    dx, dy = pixel_pitch_mm(sras, angle_idx)
     return (n_frames - 1) / 2.0 * abs(dx), (n_rows - 1) / 2.0 * abs(dy)
 
 
@@ -813,7 +813,7 @@ def _rotation_matrix(theta_deg: float) -> np.ndarray:
     return np.array([[c, -s], [s, c]])   # CCW rotation acting on (x, y)
 
 
-def _nominal_delta_deg(sras: SrasFile, angle_idx: int, ref_idx: int) -> float:
+def nominal_delta_deg(sras: SrasFile, angle_idx: int, ref_idx: int) -> float:
     """The rotation-stage's own reported angle change between two angles.
 
     Used only to *seed* the rotation search, never as the answer: the stage's
@@ -888,7 +888,7 @@ def apply_alignment(result: AlignmentResult, angle_idx: int, img: np.ndarray,
         mode="constant", cval=0.0)
 
 
-def _block_mean_2d(img: np.ndarray, fy: int, fx: int) -> np.ndarray:
+def block_mean_2d(img: np.ndarray, fy: int, fx: int) -> np.ndarray:
     """Block-mean by independent row/column factors. Independent factors matter
     because the raw grid is strongly anisotropic (5 µm along x, 50 µm along y):
     a single square factor would either alias along x or throw away rows."""
@@ -944,10 +944,10 @@ def _prepare_reg_image(sras: SrasFile, angle_idx: int, img: np.ndarray,
     resampled onto the registration grid. Pre-averaging matters: the raw grid
     is 10x finer along x than along y, so sampling it directly at the (much
     coarser) isotropic registration pitch would alias badly along x."""
-    dx, dy = _pixel_pitch_mm(sras, angle_idx)
+    dx, dy = pixel_pitch_mm(sras, angle_idx)
     fx = max(1, int(pitch_mm / abs(dx)))
     fy = max(1, int(pitch_mm / abs(dy)))
-    small = _block_mean_2d(np.asarray(img, dtype=np.float32), fy, fx)
+    small = block_mean_2d(np.asarray(img, dtype=np.float32), fy, fx)
     n_rows, n_frames = img.shape
     return _RegImage(
         small, dx * fx, dy * fy,
@@ -1087,7 +1087,7 @@ def _reg_pitch_and_size(sras: SrasFile, max_dim: int,
     span = diag * margin
     native = float(np.sqrt(
         abs(sras.pixel_x_mm)
-        * max(abs(_pixel_pitch_mm(sras, a)[1]) for a in range(sras.n_angles))))
+        * max(abs(pixel_pitch_mm(sras, a)[1]) for a in range(sras.n_angles))))
     pitch = max(span / max_dim, native)
     n = int(scipy_fft.next_fast_len(max(16, int(np.ceil(span / pitch)))))
     return pitch, n
@@ -1111,6 +1111,16 @@ def _registration_workers(sras: SrasFile, fine_dim: int) -> int:
                           _TOTAL_BYTES_BUDGET // max(1, per_worker))))
 
 
+def registration_workers(sras: SrasFile) -> int:
+    """Public: concurrent-registration cap at the default fine grid."""
+    return _registration_workers(sras, _DEFAULT_FINE_DIM)
+
+
+def default_max_workers() -> int:
+    """Public: the module-wide worker cap (SRAS_MAX_WORKERS or cpu count)."""
+    return _MAX_WORKERS
+
+
 def _rotation_candidates(nominal_deg: float, search_deg: float,
                          step_deg: float) -> list[float]:
     """Coarse rotation candidates: a window around *both* signs of the stage's
@@ -1254,7 +1264,7 @@ def register_angle_to_reference(
     if not candidates:
         return RigidFit(0.0, (0.0, 0.0), -1.0, "none")
 
-    nominal = _nominal_delta_deg(sras, angle_idx, ref_angle_idx)
+    nominal = nominal_delta_deg(sras, angle_idx, ref_angle_idx)
     thetas = _rotation_candidates(nominal, search_deg, coarse_step_deg)
 
     # ---- Stage 1: coarse sweep, every source ------------------------------
@@ -1363,7 +1373,7 @@ def build_canvas_affine(sras: SrasFile, angle_idx: int, ref_angle_idx: int,
     (rows, cols) block-mean factor already applied to the image the caller will
     resample — 1:1 for the raw image, coarser for the manual-alignment
     preview's downsampled masks."""
-    dx_a, dy_a = _pixel_pitch_mm(sras, angle_idx)
+    dx_a, dy_a = pixel_pitch_mm(sras, angle_idx)
     n_rows, n_frames = sras.image_shape(angle_idx)
     fy, fx = (max(1, int(v)) for v in src_downsample)
     if (fy, fx) != (1, 1):
@@ -1398,7 +1408,7 @@ def _result_from_params(sras: SrasFile, ref_angle_idx: int,
     the shared canvas, then build each angle's canvas->raw affine. Pure matrix
     and bbox math, so it is cheap enough to call synchronously on the GUI
     thread on every manual edit."""
-    pitch = _pixel_pitch_mm(sras, ref_angle_idx)
+    pitch = pixel_pitch_mm(sras, ref_angle_idx)
     canvas_origin_mm, canvas_shape = canvas_for_params(
         sras, ref_angle_idx, pitch, params, snap=True)
 
diff --git a/sras_edit_scans.py b/sras_edit_scans.py
index 8774daf..040e0d5 100644
--- a/sras_edit_scans.py
+++ b/sras_edit_scans.py
@@ -25,15 +25,17 @@ import struct
 import sys
 from pathlib import Path
 
-from sras_format import (
-    GEO_FMT_V6, GEO_SIZE_V6, HDR_FMT, HDR_FMT_V6, HDR_SIZE, HDR_SIZE_V6,
-    SrasFile,
-)
+from sras_format import GEO_FMT_V6, HDR_FMT, HDR_FMT_V6, HDR_SIZE, SrasFile
 
 _LEGACY_VERSIONS = (2, 3, 4, 5)
 _V6_VERSIONS = (6, 7)
 
 
+def _die(msg: str):
+    print(f"Error: {msg}", file=sys.stderr)
+    sys.exit(1)
+
+
 def parse_args():
     p = argparse.ArgumentParser(
         description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter)
@@ -106,7 +108,7 @@ def _write_legacy(sras: SrasFile, keep: list[int], out_path: Path):
     angle_table_size = sras.n_angles * 4
     with open(sras.path, "rb") as f:
         f.seek(HDR_SIZE + angle_table_size)
-        shared_mid = f.read(sras._data_offset - (HDR_SIZE + angle_table_size))
+        shared_mid = f.read(sras.data_offset - (HDR_SIZE + angle_table_size))
 
     angle_bytes = n_rows * n_ch * n_frames * spf * bps
 
@@ -115,128 +117,55 @@ def _write_legacy(sras: SrasFile, keep: list[int], out_path: Path):
         fout.write(sras.angles_deg[keep].astype(">f4").tobytes())
         fout.write(shared_mid)
         for a in keep:
-            _copy_range(fin, fout, sras._data_offset + a * angle_bytes, angle_bytes)
+            _copy_range(fin, fout, sras.data_offset + a * angle_bytes, angle_bytes)
 
 
 # ---------------------------------------------------------------------------
 # v6/v7: per-angle geometry, ragged waveform blocks
 # ---------------------------------------------------------------------------
 
-def _read_v6_sections(path: Path) -> dict:
-    """Raw, low-level read of everything before the waveform data.
-
-    SrasFile._parse_v6 reads and discards each angle's x_delta (it's not
-    part of the display-facing geometry it exposes), so a round-trip through
-    SrasFile would silently drop that field. Re-parsing here keeps every
-    byte of the Per-Angle Geometry Table intact.
-    """
-    with open(path, "rb") as f:
-        hdr_raw = f.read(HDR_SIZE_V6)
-        (magic, ver, n_angles_declared, x_start_nom, y_start_nom, x_delta_nom,
-         y_delta_nom, row_spacing, vel, freq, spf, sr, bps,
-         n_ch) = struct.unpack(HDR_FMT_V6, hdr_raw)
-
-        angles_deg = list(struct.unpack(f">{n_angles_declared}f",
-                                        f.read(n_angles_declared * 4)))
-
-        geo = [struct.unpack(GEO_FMT_V6, f.read(GEO_SIZE_V6))
-               for _ in range(n_angles_declared)]   # (x_start, x_delta, n_frames, n_rows)
-
-        row_table = [f.read(geo[a][3] * 4) for a in range(n_angles_declared)]
-
-        preamble_start = f.tell()
-        for _ in range(n_ch):
-            (length,) = struct.unpack(">H", f.read(2))
-            f.read(length)
-        preambles_raw = _reread_span(f, preamble_start)
-
-        bg_start = f.tell()
-        (n_bg,) = struct.unpack(">I", f.read(4))
-        f.read(n_bg)
-        background_raw = _reread_span(f, bg_start)
-
-        data_offset = f.tell()
-
-    return {
-        "version": ver, "n_angles_declared": n_angles_declared,
-        "x_start_nom": x_start_nom, "y_start_nom": y_start_nom,
-        "x_delta_nom": x_delta_nom, "y_delta_nom": y_delta_nom,
-        "row_spacing": row_spacing, "vel": vel, "freq": freq,
-        "spf": spf, "sr": sr, "bps": bps, "n_ch": n_ch,
-        "angles_deg": angles_deg, "geo": geo, "row_table": row_table,
-        "preambles_raw": preambles_raw, "background_raw": background_raw,
-        "data_offset": data_offset,
-    }
-
-
-def _reread_span(f, start: int) -> bytes:
-    end = f.tell()
-    f.seek(start)
-    span = f.read(end - start)
-    f.seek(end)
-    return span
-
-
-def _v6_angle_offsets(sections: dict, file_size: int) -> list[tuple[int, int]]:
-    """(offset, nbytes) of each declared angle's waveform block, stopping at
-    the first angle whose data isn't fully on disk (aborted scan)."""
-    n_ch, spf, bps = sections["n_ch"], sections["spf"], sections["bps"]
-    offsets = []
-    offset = sections["data_offset"]
-    for xs, xd, nf, nr in sections["geo"]:
-        nbytes = nr * n_ch * nf * spf * bps
-        if offset + nbytes > file_size:
-            break
-        offsets.append((offset, nbytes))
-        offset += nbytes
-    return offsets
-
-
-def _write_v6(in_path: Path, sections: dict, keep: list[int], out_path: Path):
-    file_size = in_path.stat().st_size
-    offsets = _v6_angle_offsets(sections, file_size)
-    geo = sections["geo"]
-
+def _write_v6(sras: SrasFile, keep: list[int], out_path: Path):
     header = struct.pack(
-        HDR_FMT_V6, b"SRAS", sections["version"], len(keep),
-        sections["x_start_nom"], sections["y_start_nom"],
-        sections["x_delta_nom"], sections["y_delta_nom"],
-        sections["row_spacing"], sections["vel"], sections["freq"],
-        sections["spf"], sections["sr"], sections["bps"], sections["n_ch"],
+        HDR_FMT_V6, b"SRAS", sras.version, len(keep),
+        sras.x_start_nominal_mm, sras.y_start_nominal_mm,
+        sras.x_delta_nominal_mm, sras.y_delta_nominal_mm,
+        sras.row_spacing_mm, sras.velocity_mm_s, sras.laser_freq_hz,
+        sras.samples_per_frame, sras.sample_rate_hz,
+        sras.bytes_per_sample, sras.n_channels,
     )
 
-    with open(in_path, "rb") as fin, open(out_path, "wb") as fout:
+    blocks = {a: (offset, nbytes) for a, offset, nbytes in sras.iter_angle_blocks()}
+    with open(sras.path, "rb") as fin, open(out_path, "wb") as fout:
         fout.write(header)
-        fout.write(struct.pack(f">{len(keep)}f",
-                               *[sections["angles_deg"][i] for i in keep]))
+        fout.write(sras.angles_deg[keep].astype(">f4").tobytes())
         for i in keep:
-            fout.write(struct.pack(GEO_FMT_V6, *geo[i]))
+            fout.write(struct.pack(
+                GEO_FMT_V6, float(sras.x_start_mm[i]),
+                float(sras.x_delta_mm_per_angle[i]),
+                int(sras.n_frames[i]), int(sras.n_rows[i])))
         for i in keep:
-            fout.write(sections["row_table"][i])
-        fout.write(sections["preambles_raw"])
-        fout.write(sections["background_raw"])
+            fout.write(sras.y_pos_per_angle[i].astype(">f4").tobytes())
+        fout.write(sras.preambles_raw)
+        fout.write(sras.background_raw)
         for i in keep:
-            off, nbytes = offsets[i]
-            _copy_range(fin, fout, off, nbytes)
+            offset, nbytes = blocks[i]
+            _copy_range(fin, fout, offset, nbytes)
 
 
 def main():
     args = parse_args()
     in_path = Path(args.input)
     if not in_path.exists():
-        print(f"Error: input file not found: {in_path}", file=sys.stderr)
-        sys.exit(1)
+        _die(f"input file not found: {in_path}")
 
     print(f"Reading {in_path} ...", flush=True)
     try:
         sras = SrasFile(str(in_path))
     except ValueError as e:
-        print(f"Error: {e}", file=sys.stderr)
-        sys.exit(1)
+        _die(str(e))
 
     if sras.version not in (*_LEGACY_VERSIONS, *_V6_VERSIONS):
-        print(f"Error: unsupported .sras version: {sras.version}", file=sys.stderr)
-        sys.exit(1)
+        _die(f"unsupported .sras version: {sras.version}")
 
     aborted_note = " (scan aborted; trailing angle(s) already excluded)" if sras.scan_aborted else ""
     print(f"  Version : v{sras.version}", flush=True)
@@ -247,16 +176,13 @@ def main():
         return
 
     if not args.output:
-        print("Error: output path required unless --list is given.", file=sys.stderr)
-        sys.exit(1)
+        _die("output path required unless --list is given.")
     if not (args.drop or args.keep):
-        print("Error: specify --drop or --keep (see --list for indices).", file=sys.stderr)
-        sys.exit(1)
+        _die("specify --drop or --keep (see --list for indices).")
 
     out_path = Path(args.output)
     if out_path.resolve() == in_path.resolve():
-        print("Error: output path must differ from input path.", file=sys.stderr)
-        sys.exit(1)
+        _die("output path must differ from input path.")
 
     try:
         if args.drop:
@@ -265,12 +191,10 @@ def main():
         else:
             keep = sorted(_parse_index_list(args.keep, sras.n_angles))
     except ValueError as e:
-        print(f"Error: {e}", file=sys.stderr)
-        sys.exit(1)
+        _die(str(e))
 
     if not keep:
-        print("Error: at least one angle must remain.", file=sys.stderr)
-        sys.exit(1)
+        _die("at least one angle must remain.")
 
     dropped = [a for a in range(sras.n_angles) if a not in keep]
     print(f"\nDropping angle(s): {dropped}")
@@ -280,7 +204,7 @@ def main():
     if sras.version in _LEGACY_VERSIONS:
         _write_legacy(sras, keep, out_path)
     else:
-        _write_v6(in_path, _read_v6_sections(in_path), keep, out_path)
+        _write_v6(sras, keep, out_path)
 
     in_mb = in_path.stat().st_size / 1024**2
     out_mb = out_path.stat().st_size / 1024**2
diff --git a/sras_format.py b/sras_format.py
index 0b2715d..46d8003 100644
--- a/sras_format.py
+++ b/sras_format.py
@@ -375,21 +375,34 @@ class SrasFile:
             angles = np.frombuffer(f.read(n_angles * 4), dtype=">f4").astype(np.float32)
 
             x_start = np.empty(n_angles, dtype=np.float64)
+            x_delta = np.empty(n_angles, dtype=np.float64)
             n_frames = np.empty(n_angles, dtype=np.int64)
             n_rows = np.empty(n_angles, dtype=np.int64)
             for a in range(n_angles):
                 xs, xd, nf, nr = _read_struct(f, GEO_FMT_V6)
-                x_start[a], n_frames[a], n_rows[a] = xs, nf, nr
+                x_start[a], x_delta[a], n_frames[a], n_rows[a] = xs, xd, nf, nr
 
             y_pos_per_angle = [
                 np.frombuffer(f.read(int(n_rows[a]) * 4), dtype=">f4").astype(np.float32)
                 for a in range(n_angles)
             ]
 
+            # Verbatim on-disk spans of the preamble and background sections,
+            # kept so file-rewriting tools (sras_edit_scans) can carry them
+            # over byte-for-byte without re-parsing.
+            span_start = f.tell()
             self._set_calibration(_read_preambles(f, n_ch), n_ch)
-            self.background = _read_background(f)
+            span_end = f.tell()
+            f.seek(span_start)
+            self.preambles_raw = f.read(span_end - span_start)
 
-            data_offset = f.tell()
+            span_start = span_end
+            self.background = _read_background(f)
+            span_end = f.tell()
+            f.seek(span_start)
+            self.background_raw = f.read(span_end - span_start)
+
+            data_offset = span_end
 
         self._data_offset = data_offset
 
@@ -428,6 +441,7 @@ class SrasFile:
         self.scan_aborted = n_complete < n_angles_declared
         self.angles_deg = angles[:n_complete]
         self.x_start_mm = x_start[:n_complete]
+        self.x_delta_mm_per_angle = x_delta[:n_complete]
         self.n_frames = n_frames[:n_complete]
         self.n_rows = n_rows[:n_complete]
         self._y_pos_per_angle = y_pos_per_angle[:n_complete]
@@ -436,6 +450,37 @@ class SrasFile:
         if self.version == 7 and offset < file_size:
             self._parse_cach_section(offset)
 
+    # ------------------------------------------------------------------
+    # Byte-layout accessors (public: used by file-rewriting tools)
+    # ------------------------------------------------------------------
+
+    @property
+    def data_offset(self) -> int:
+        """File offset where the waveform data begins (headers end)."""
+        return self._data_offset
+
+    @property
+    def y_pos_per_angle(self) -> list[np.ndarray]:
+        """Per-angle Y row positions (mm). The list and its arrays are the
+        live parsed state — tools that reproject may replace entries."""
+        return self._y_pos_per_angle
+
+    @y_pos_per_angle.setter
+    def y_pos_per_angle(self, value: list[np.ndarray]):
+        self._y_pos_per_angle = value
+
+    def iter_angle_blocks(self):
+        """Yields (angle_idx, byte_offset, byte_count) for each complete
+        angle's waveform block. Works for every version: legacy files have
+        uniform per-angle geometry, so the same walk applies."""
+        offset = self._data_offset
+        for a in range(self.n_angles):
+            nbytes = (int(self.n_rows[a]) * self.n_channels
+                      * int(self.n_frames[a]) * self.samples_per_frame
+                      * self.bytes_per_sample)
+            yield a, offset, nbytes
+            offset += nbytes
+
     # ------------------------------------------------------------------
     # v7 cache tail (CACH section: precomputed DC / FFT images)
     # ------------------------------------------------------------------
@@ -445,12 +490,10 @@ class SrasFile:
         start), derived purely from the header + Per-Angle Geometry Table —
         independent of whether a cache tail is actually present. Used by
         both the parser and the in-place writer."""
-        waveform_bytes = sum(
-            int(self.n_rows[a]) * self.n_channels * int(self.n_frames[a])
-            * self.samples_per_frame * self.bytes_per_sample
-            for a in range(self.n_angles)
-        )
-        return self._data_offset + int(waveform_bytes)
+        end = self._data_offset
+        for _, offset, nbytes in self.iter_angle_blocks():
+            end = offset + nbytes
+        return end
 
     def _read_cache_block(self, f, hdr_fmt: str, magic: bytes,
                           stores: list[list]) -> int | None:
diff --git a/sras_viewer.py b/sras_viewer.py
index c1c232f..23f9344 100644
--- a/sras_viewer.py
+++ b/sras_viewer.py
@@ -1127,7 +1127,7 @@ class ManualAlignmentDialog(QDialog):
         threshold = self.spin_mask_threshold_mv.value()
         fy, fx = self._downsample
         self._masks_small = {
-            a: compute._block_mean_2d((img >= threshold).astype(np.float32), fy, fx)
+            a: compute.block_mean_2d((img >= threshold).astype(np.float32), fy, fx)
             for a, img in self._dc4_mv.items()
         }
 
@@ -1143,7 +1143,7 @@ class ManualAlignmentDialog(QDialog):
         mask-threshold change, Auto De-rotate, a rotation nudge/edit of the
         active angle. NOT triggered by a translation-only nudge — see
         _refresh_active_preview_layer."""
-        dx_ref, dy_ref = compute._pixel_pitch_mm(self._sras, self._ref_angle_idx)
+        dx_ref, dy_ref = compute.pixel_pitch_mm(self._sras, self._ref_angle_idx)
         fy, fx = self._downsample
         pitch = (dx_ref * fx, dy_ref * fy)
         origin, shape = compute.canvas_for_params(
@@ -1288,7 +1288,7 @@ class ManualAlignmentDialog(QDialog):
         for a in range(self._sras.n_angles):
             if a == self._ref_angle_idx:
                 continue
-            self._angle_params[a].rotation_deg = sign * compute._nominal_delta_deg(
+            self._angle_params[a].rotation_deg = sign * compute.nominal_delta_deg(
                 self._sras, a, self._ref_angle_idx)
             n_changed += 1
         self._sync_active_spinboxes()
@@ -1364,7 +1364,7 @@ class ManualAlignmentDialog(QDialog):
                  f"{worst[1][1]})."]
         drifted = []
         for a, _note in rows:
-            nominal = compute._nominal_delta_deg(self._sras, a, self._ref_angle_idx)
+            nominal = compute.nominal_delta_deg(self._sras, a, self._ref_angle_idx)
             got = self._angle_params[a].rotation_deg
             dev = min(abs(got - nominal), abs(got + nominal))
             if dev > 1.0:
diff --git a/sras_workers.py b/sras_workers.py
index 5919b79..4a75dd8 100644
--- a/sras_workers.py
+++ b/sras_workers.py
@@ -253,7 +253,8 @@ class BatchCacheWorker(QObject):
         for path in paths:
             try:
                 err = cache_file(path, self._mode, self._apply_bg_sub,
-                                 compute.get_fft_backend(), compute._MAX_WORKERS)
+                                 compute.get_fft_backend(),
+                                 compute.default_max_workers())
             except Exception as exc:
                 err = str(exc)
             done += 1
@@ -382,7 +383,7 @@ class CrossCorrelateWorker(_PooledWorker):
         self._search_deg = search_deg
 
     def _plan(self) -> int:
-        return compute._registration_workers(self._sras, compute._DEFAULT_FINE_DIM)
+        return compute.registration_workers(self._sras)
 
     def _items(self):
         return self._angles
diff --git a/tests/test_alignment.py b/tests/test_alignment.py
index 18be785..04bb85f 100644
--- a/tests/test_alignment.py
+++ b/tests/test_alignment.py
@@ -40,7 +40,7 @@ def mm_transform(sras: SrasFile, result, angle_idx: int) -> np.ndarray:
     per-angle pixel pitches; undoing both must leave something orthonormal, or
     the transform is smuggling in a scale or a shear.
     """
-    dx_a, dy_a = compute._pixel_pitch_mm(sras, angle_idx)
+    dx_a, dy_a = compute.pixel_pitch_mm(sras, angle_idx)
     A_out = np.array([[0.0, result.canvas_dx_mm], [result.canvas_dy_mm, 0.0]])
     D = np.array([[0.0, 1.0 / dy_a], [1.0 / dx_a, 0.0]])
     return np.linalg.inv(D) @ result.per_angle[angle_idx].matrix @ np.linalg.inv(A_out)
@@ -99,7 +99,7 @@ def test_stage_coordinates_are_not_consulted(rig):
     moved = SrasFile(str(rig.path))
     for a in range(1, moved.n_angles):
         moved.x_start_mm[a] += 13.5 * a
-        moved._y_pos_per_angle[a] = moved._y_pos_per_angle[a] - 9.25 * a
+        moved.y_pos_per_angle[a] = moved.y_pos_per_angle[a] - 9.25 * a
     moved_dc4 = dc4_images(moved)
     moved_fits = {a: compute.register_angle_to_reference(
                       moved, a, 0, moved_dc4, dc_threshold_mv=_THRESHOLD_MV)
@@ -118,7 +118,7 @@ def test_canvas_is_reference_grid_extended(rig):
     assert np.allclose(t0.offset, np.round(t0.offset)), \
         f"angle 0 does not land on whole canvas pixels: {t0.offset}"
     assert ((result.canvas_dx_mm, result.canvas_dy_mm)
-            == compute._pixel_pitch_mm(sras, 0)), \
+            == compute.pixel_pitch_mm(sras, 0)), \
         "canvas pitch is angle 0's own pitch"
 
     n_rows, n_cols = result.canvas_shape
@@ -169,7 +169,7 @@ def test_downsampled_preview_lands_with_full_res(rig):
         result.canvas_origin_mm, result.canvas_shape)
     fy, fx = 4, 16
     small = compute.reproject_mask(
-        sras, a, 0, compute._block_mean_2d(full_mask, fy, fx),
+        sras, a, 0, compute.block_mean_2d(full_mask, fy, fx),
         p.rotation_deg, p.shift_mm, (pitch[0] * fx, pitch[1] * fy),
         result.canvas_origin_mm,
         (result.canvas_shape[0] // fy, result.canvas_shape[1] // fx),
diff --git a/tests/test_gui.py b/tests/test_gui.py
index 7b1ebec..cb0bc28 100644
--- a/tests/test_gui.py
+++ b/tests/test_gui.py
@@ -261,7 +261,7 @@ def test_manual_alignment_geometry(ctx):
     assert np.allclose(compute._center_idx(s, 0),
                        [(n_rows - 1) / 2, (n_frames - 1) / 2]), \
         "array center is the geometric center of the pixel grid"
-    dx0, dy0 = compute._pixel_pitch_mm(s, 0)
+    dx0, dy0 = compute.pixel_pitch_mm(s, 0)
     assert np.allclose(compute._local_half_extent_mm(s, 0),
                        [(n_frames - 1) / 2 * abs(dx0), (n_rows - 1) / 2 * abs(dy0)]), \
         "local half-extent is derived from shape and pitch alone"
@@ -270,7 +270,7 @@ def test_manual_alignment_geometry(ctx):
     moved = SrasFile(str(ctx.path))
     for a in range(1, moved.n_angles):
         moved.x_start_mm[a] += 7.5
-        moved._y_pos_per_angle[a] = moved._y_pos_per_angle[a] + 3.25
+        moved.y_pos_per_angle[a] = moved.y_pos_per_angle[a] + 3.25
     origin_b, shape_b = compute.canvas_for_params(moved, 0, (dx0, dy0), identity)
     assert shape_a == shape_b and np.allclose(origin_a, origin_b), \
         ("moving every non-reference angle's scan window must leave the canvas "
@@ -356,7 +356,7 @@ def test_auto_derotate(ctx):
     dlg, s, active = ctx.dlg, ctx.s, ctx.active
     shift_before_derotate = dlg._angle_params[active].shift_mm
     dlg._on_auto_derotate()
-    nominal = compute._nominal_delta_deg(s, active, dlg._ref_angle_idx)
+    nominal = compute.nominal_delta_deg(s, active, dlg._ref_angle_idx)
     assert abs(dlg._angle_params[active].rotation_deg - nominal) < 1e-6, \
         "auto de-rotate seeded rotation from the stage's reported angle"
     assert dlg._angle_params[active].shift_mm == shift_before_derotate, \
diff --git a/tools/check_equivalence.py b/tools/check_equivalence.py
index 37934dc..9488040 100644
--- a/tools/check_equivalence.py
+++ b/tools/check_equivalence.py
@@ -47,8 +47,8 @@ def row_slice(sras: SrasFile, angle_idx: int, n_rows: int) -> SrasFile:
     view.n_rows[angle_idx] = n
     view.data = list(sras.data)
     view.data[angle_idx] = sras.data[angle_idx][:n]
-    view._y_pos_per_angle = list(sras._y_pos_per_angle)
-    view._y_pos_per_angle[angle_idx] = sras._y_pos_per_angle[angle_idx][:n]
+    view.y_pos_per_angle = list(sras.y_pos_per_angle)
+    view.y_pos_per_angle[angle_idx] = sras.y_pos_per_angle[angle_idx][:n]
     return view
 
 
diff --git a/tools/make_test_sras.py b/tools/make_test_sras.py
index 92b0b54..0a3651b 100644
--- a/tools/make_test_sras.py
+++ b/tools/make_test_sras.py
@@ -11,12 +11,19 @@ Usage:
 
 import argparse
 import struct
+import sys
 from pathlib import Path
 
 import numpy as np
 
-HDR_FMT_V6 = ">4sBHfffffffIdBB"
-GEO_FMT_V6 = ">ffIH"
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+# Single source of truth for the byte layout: the reader's own constants.
+# The byte *assembly* below stays independent, so a writer bug can't be
+# masked by a matching reader bug.
+from sras_format import HDR_FMT as HDR_FMT_LEGACY          # noqa: E402
+from sras_format import GEO_FMT_V6, HDR_FMT_V6             # noqa: E402
+from sras_compute import _rotation_matrix as _rot          # noqa: E402
 
 # Per-angle (n_rows, n_frames) — deliberately different per angle so ragged
 # geometry handling is actually exercised.
@@ -171,12 +178,6 @@ def _sample_shape_mv(u: np.ndarray, v: np.ndarray) -> np.ndarray:
     return img
 
 
-def _rot(theta_deg: float) -> np.ndarray:
-    t = np.radians(theta_deg)
-    c, s = np.cos(t), np.sin(t)
-    return np.array([[c, -s], [s, c]])
-
-
 def write_rotating(path: Path, n_angles: int = 5, samples_per_frame: int = 4,
                    seed: int = 0) -> dict:
     """Write a v6 file whose CH4 DC image is one sample seen at n_angles known
@@ -260,7 +261,6 @@ def write_rotating(path: Path, n_angles: int = 5, samples_per_frame: int = 4,
             "y_starts": y_starts, "dx_mm": _ROT_DX_MM, "dy_mm": _ROT_DY_MM}
 
 
-HDR_FMT_LEGACY = ">4sBHHffffIIdBB"
 
 
 def write_legacy(path: Path, version: int = 4, n_angles: int = 2,