feat(flomni): acquire empty frames at start of tomo_scan

Add collect_empty_frames(), which acquires 10 flat-field frames at
angle 0 with fsamx shifted out of the beam by fovx/2 (opposite the
normal alignment x-offset). Called once at the start of a new
tomo_scan() (not on resume). Frames are excluded from the ptycho
reconstruction queue and logged to tomography_scannumbers.txt with
subtomo_number=0.

csaxs_bec/bec_ipython_client/plugins/flomni/flomni.py

@@ -2259,6 +2259,68 @@ class Flomni(
                 end_scan_number=end_scan_number,
             )
 
+    def collect_empty_frames(self):
+        """Acquire 10 empty-frame (flat-field) images at angle 0, with fsamx
+        shifted out of the beam by half the field of view (fovx/2), in the
+        direction opposite the normal alignment x-offset at angle 0.
+
+        Called once at the start of a new tomo_scan() (not on resume).
+        These frames are deliberately not passed through tomo_reconstruct()
+        -- they are flat fields, not ptycho projections, and must not be
+        added to the reconstruction queue. Logged to
+        tomography_scannumbers.txt with subtomo_number=0 so they remain
+        traceable but are clearly distinguished from the angular grid
+        (subtomo_number 1-8).
+        """
+        scans = builtins.__dict__.get("scans")
+
+        angle = 0
+
+        # --- rotation ---
+        fsamroy_current_setpoint = dev.fsamroy.user_setpoint.get()
+        if angle != fsamroy_current_setpoint:
+            umv(dev.fsamroy, angle)
+        else:
+            print("No rotation required")
+
+        # --- alignment offset at angle 0, then push x out of the beam by
+        # fovx/2, in the direction opposite the normal offset ---
+        offsets = self.get_alignment_offset(angle)
+        normal_offset_x = offsets[0]
+        direction = -1 if normal_offset_x >= 0 else 1
+        sum_offset_x = normal_offset_x + direction * (self.fovx / 2) + direction * 10
+        sum_offset_y = (
+            offsets[1]
+            - self.compute_additional_correction_y(angle)
+            - self.compute_additional_correction_y_2(angle)
+            + self.manual_shift_y
+        )
+        # sum_offset_z = offsets[2]
+        # Todo
+        sum_offset_z = 0
+        
+        
+        dev.rtx.controller.laser_tracker_on()
+        umv(dev.rtx, sum_offset_x, dev.rty, sum_offset_y, dev.rtz, sum_offset_z)
+        tracker_signal = dev.rtx.controller.laser_tracker_check_signalstrength()
+        # checks that the fsamx coarse stage is at a position that leaves
+        # sufficient piezo range on the fine (rtx) stage
+        dev.rtx.controller.move_samx_to_scan_region(sum_offset_x)
+
+        if tracker_signal == "low":
+            logger.warning("Signal strength of the laser tracker is low. Realignment recommended!")
+        elif tracker_signal == "toolow":
+            raise FlomniError(
+                "Signal strength of the laser tracker is too low for scanning. Realignment required!"
+            )
+
+        print("Acquiring 10 empty frames at angle 0, fsamx shifted out of the beam.")
+        start_scan_number = bec.queue.next_scan_number
+        scans.acquire(exp_time=self.tomo_countingtime, frames_per_trigger=10)
+        end_scan_number = bec.queue.next_scan_number
+        for scan_nr in range(start_scan_number, end_scan_number):
+            self._write_tomo_scan_number(scan_nr, angle, subtomo_number=0)
+
     def tomo_scan(self, subtomo_start=1, start_angle=None, projection_number=None):
         """start a tomo scan"""
 
@@ -2311,6 +2373,7 @@ class Flomni(
             self.progress["estimated_finish_time"] = None
             self.progress["accumulated_idle_time"] = 0.0
             self.progress["heartbeat"] = None
+            self.collect_empty_frames()
 
         with scans.dataset_id_on_hold:
             if self.tomo_type == 1: