Draft: add ERICH_sensor_dist to eRICH compact file

• no change in geometry or implementation
• this variable is for more convenient access from a script in reconstruction_benchmarks
• add FIXME labels indicating the doubling of sensor sizes (needed for graphics performance)

compact/erich.xml

@@ -13,10 +13,11 @@
 <constant name="ERICH_window_thickness"   value="0.1*cm"/>  <!-- thickness of entrance and exit walls -->
 <!-- additional parameters -->
 <constant name="ERICH_aerogel_thickness"  value="3.0*cm"/>  <!-- aerogel thickness -->
-<constant name="ERICH_sensor_active_size" value="2*24.0*mm"/> <!-- sensor side length (effective area) -->
-<constant name="ERICH_sensor_full_size"   value="2*25.8*mm"/> <!-- sensor side length (full size, with enclosure) -->
+<constant name="ERICH_sensor_active_size" value="2*24.0*mm"/> <!-- sensor side length (effective area) (FIXME: x2 for graphics performance) -->
+<constant name="ERICH_sensor_full_size"   value="2*25.8*mm"/> <!-- sensor side length (full size, with enclosure) (FIXME: x2 for graphics) -->
 <constant name="ERICH_sensor_thickness"   value="1.5*mm"/> <!-- sensor thickness -->
-<constant name="ERICH_num_px"             value="2*8"/> <!-- number of pixels along one side of the sensor -->
+<constant name="ERICH_sensor_dist"        value="40*cm"/> <!-- distance between aerogel exit plane and sensor entrance plane -->
+<constant name="ERICH_num_px"             value="2*8"/> <!-- number of pixels along one side of the sensor (FIXME: x2 for graphics) -->
 <!-- debugging switches -->
 <comment>
 - `ERICH_debug_optics`:  1 = all components become vacuum; test opticalphotons from IP
@@ -144,13 +145,12 @@
 ##### Sensor plane
 - sensors will be placed on a plane
   - plane dimensions:
-    - `frontplane`: z-position of the sensor plane active surface (e.g., photocathode), w.r.t aerogel backplane;
-      absolute value of this number is the distance between aerogel/filter boundary and sensor surface
+    - `sensordist`: distance between sensor plane active surface (e.g., photocathode) and aerogel backplane
     - `rmin`: minimum radial position of a sensor's centroid
     - `rmax`: maximum radial position of a sensor's centroid
 </documentation>
 <plane
-  frontplane="-40*cm"
+  sensordist="ERICH_sensor_dist"
   rmin="ERICH_rmin1 + 2*cm"
   rmax="ERICH_rmax  - 4*cm"
   />