Changeset 1626

Timestamp:

11/15/02 21:52:19 (22 years ago)

Author:

bigongia

Message:

*** empty log message ***

Location:

trunk/MagicSoft/Simulation/Detector/ReflectorII/doc

Files:

• Tdas0211.ps.gz (modified) ( previous)
• Tdas0211.tex (modified) (7 diffs)
• eps/timing.eps (deleted)

trunk/MagicSoft/Simulation/Detector/ReflectorII/doc/Tdas0211.tex

@@ -59 +59 @@
 information contained in this document is also present in MAGIC-TDAS
 02-05 dealing with the previous version of the program, but it has also
-been included here for clarity. An important bug regarding the
-ray-tracing routine has been corrected in this new version.
+been included here for clarity. Two important bugs regarding the
+ray-tracing routine have been corrected in this new version.
 \end{abstract}
 
@@ -89 +89 @@
 routine was checked in detail and some inconsistencies were
 found. As a result of this, an important bug was detected which
-produced a systematic blurring of all the images. This bug, fixed in
-this version, is discussed in detail in section \ref{notes}.
+produced a systematic blurring of all the images. Later another bug
+was found in the calculation of the photon arrival time on the
+camera.These bugs, fixed in this version, are discussed in detail in
+section \ref{notes}. 
 
 %------------------------------------------------------------
@@ -126 +128 @@
 \section{Notes on version 0.6 \label{notes}}
 
-\subsection {The bug fixed in the ray tracing}
+\subsection {First bug fixed in the ray tracing}
 
 The previous versions of the reflector were extensively checked and a
@@ -262 +264 @@
 2$\pi$). When the direction cosines of a particle's trajectory are
 given, they refer also to its momentum vector (a downgoing
-vector). The same is true for the Cherenkov photons.
+vector)\footnote{In Corsika the third component of the momentum is
+measured along the negative z-axis and this is probably the origin of
+the confusion which resulted in the bug}. The same is true for the
+Cherenkov photons.
 %
 \begin{figure}[h]
@@ -307 +312 @@
 course a paralel beam of light was focused at the right distance (1697
 cm) since in that case u = v = 0, and both the upgoing and downgoing
-versors have the same direction.
+versors have the same direction. 
 \par
 The example in figure \ref{parabola} shows the case in which the
@@ -408 +413 @@
 dramatic effect than the defocusing which the bug was producing.
 %
+\subsection {Second bug: photon timing}
+After a first release of Reflector 0.6 had been made public, another
+important bug was found. We tried to check whether the simulated
+reflecting dish was really ``isochronous''. A paralel beam of photons,
+all sharing the same arrival time on the ground, were processed by the
+simulation program and their arrival times on the camera plane were
+histogrammed. The result can be seen in fig. \ref{timing} (dashed
+histograms). This time the bug was quite evident: since Corsika gives
+us the arrival time of photons on ground, the path from the point
+where the photon hits the dish to the ground has to be subtracted (or
+added, because since the center of the dish in the MC is at $z = 0$,
+the mirror reflecting the photon may have $z < 0$ when the dish is
+inclined). The sign in this subtraction (in {\it ph2cph.c}) was
+wrong. This bug was present in both versions 0.5 and 0.4, and might
+be related to the other one (a change of orientation of the z axis
+at some point may have produced it). Since in the simulation made for
+the design report the timing played little or no role (the camera
+simulation did not consider the arrival times of photons) it is not
+possible to know whether the bug was already in the code by then.
+\par
+This bug means that all the studies made up to now regarding photon
+arrival times on the camera are completely useless (for instance, the
+optimization of the time parameters in the L1 trigger has to be redone
+from scratch!).
+%
+\begin{figure}[h]
+  \begin{center}
+    \epsfig{file=eps/timing.eps,width=\textwidth}
+    \caption{Test of reflector isochrony. The arrival time
+distributions of photons in the camera are shown for (buggy) Reflector
+0.5 and for Reflector 0.6. The sketch in the center shows the test for
+the case in which the light beam is paralel to the telescope axis
+(left plot). On the right, the same test has been made with light
+arriving 1 degree off axis.
+ \label{timing}}
+  \end{center} 
+\vspace*{-1cm}
+\end{figure}
+
 \subsection {Performance of the new version}
 In figure \ref{coma} we show the images of a point-like source at 10
@@ -1026 +1070 @@
 MagicProgs/Simulation/Detector/Reflector_0.6/doc/eps/spot_inf_f1697.eps.gz
 MagicProgs/Simulation/Detector/Reflector_0.6/doc/eps/telecoor.eps.gz
+MagicProgs/Simulation/Detector/Reflector_0.6/doc/eps/timing.eps.gz
 
 MagicProgs/Simulation/Detector/Reflector_0.6/tester/Makefile