Coordinate systems

Internal coordinate system inside LAr volume

In the hpge_strings, top (and partly also calibration) modules, a special internal coordinate system is used for some calculations. Unlike in many other conventions, this is a left handed coordinate system! It is a cylindrical coordinate system with an angle \(\phi\), a radius \(r\) and a \(z\) coordinate.

  • \(z = 0\) denotes the top of the top plate in the ’birds nest‘ The positive \(z\) direction extends upwards (in the same direction as the GDML Z-axis)

  • \(\phi\) is the clockwise angle. \(\phi = 0\) denotes the direction of string 11, i.e. ’warm north‘.

  • \(r\) is measured to the central axis, as one would expect.

To convert this coordinate system, one can use:

\[ \begin{align}\begin{aligned}Z = z + z0\\X = r \cdot \cos\phi\\Y = - r \cdot \sin\phi\end{aligned}\end{align} \]

X/Y/Z is then a typical right handed cartesian coordinate system that is centered at the barycenter of the argon volume. This is an intrinsic property of the GDML file format and cannot be changed.

Important: The fibers module internally uses a normal, “right handed” coordinate system. But this module does not consume external data. In technical documents concerning the fiber module positioning, the above left handed coordinate system is used.

Global coordinate system

TBD