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faq.md

@@ -74,15 +74,31 @@ considerations:
 
 ### What does the BCT measure?
 
-### Why is the BCT cabling delay so large?
-
-### You have calibrated using an LHC beam. How do those measurements apply to the CNGS beam?
+The BCT is made of a toroidal magnetic core with a winding around it
+that delivers a voltage when the beam goes through the center of the
+toroid. It's a bit like a conventional transformer, with the beam
+playing the role of the primary winding and its effect being observed in
+the secondary winding.
 
-## Measurement set up.
+### Why is the BCT cabling delay so large?
 
-### How do you measure the neutrino speed?
+There are no electronics near the beam pipe for radiation reasons. The
+original mission of that BCT was to make sure the beam had the required
+characteristics for CERN Neutrinos to Gran Sasso (CNGS) operation. Using
+it as a means to provide a time-stamped acquisition for neutrino time of
+flight experiments was an afterthought. In any case, the cable length
+and the amplifier combined with the BCT still provide a bandwidth of
+several hundreds of MHz so there is no appreciable distortion in the
+waveform.
 
-### How could you remove any systematics errors?
+### You have calibrated using an LHC beam. How do those measurements apply to the CNGS beam?
 
-### Is there anyone else measuring the neutrino velocity?
+Lab measurements indicate that there is a variation in beam-to-output
+delay with respect to transverse beam position. We measured an effect of
+almost 10 ns for a 10 cm transverse displacement. But the different in
+transverse position between an LHC beam and a CNGS beam is less than 2
+mm. So this effect is not a problem. Also, for the purpose of measuring
+BCT delay, the actual time structure of the beam is irrelevant. This is
+why we chose the LHC beam, with its properly separated bunches that make
+analysis of the delay much simpler.