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[Te-Chuan Huang <tchuang.phys AT gmail.com>]

Hi Guannan (and maybe Jason),

I have basically finished implementing the current mechanical design into 
AgML geometry.
Now, I am working on modifying the fast simulator according to the new 
geometry.
However, I met few questions and need some helps from you:

1.) There is one procedure in the simulator that I can not fully understand.
The simulator reconstructed the hits followed the procedure below:
First, shifted the GEANT hit by:
        xc = mRaster * TMath::Cos(idisk * 60 * TMath::DegToRad());
        yc = mRaster * TMath::Sin(idisk * 60 * TMath::DegToRad());
and then,
        xx = x - xc;
        yy = y - yc;
Second, use xx and yy to find the corresponding segment on the disk.
Third, find the middle point of the corresponding segment (r0 for the radius 
and p0 for the azimuthal angel).
Finally, shifted back the hit position by:
        x0 = r0 * cos(p0) + xc;
        y0 = r0 * sin(p0) + yc
where x0 and y0 are the finally reconstructed hit position in x-axis and 
y-axis.
Somewhere in the codes commented this shift as “Raster each disk by 1mm, 60 
degree offset for every disk”.
But I can not understand why do we need this “raster” procedure.

2.) Second question is about the assignment of the hit position error.
I found in the codes, because of the design of silicon strips, first find the 
error on the radius and azimuthal direction (dr and dphi).
In the end, radius and the azimuthal angel are transform to x and y position.
But somehow the dr and dphi are directly assigned as the error on the 
x-direction and y-direction, respectively.
Any assumption or approximation is applied on this error assignment?

Cheers,
Te-Chuan