July 17, 2007
C. Duke
Coordinate Systems used in GrISU, Kascade, Eventdisplay and Vegas

Ground System

Kascade/cherenkf uses a ground coordinate system with z(down), y(South), and x(East).  This system differs from the standard ground system used in grisudet, eventdisplay and vegas with z(up), y(North), and x(East). Thus, for example, grisudet requires a coordinate system transformation when reading photon records from cherenkf.  Note that Corsika uses x(North), y(West), and z(up).

Telescope System

With the telescope in stow position, the telescope system used in grisudet, eventdisplay, and vegas has: z(North), x(East), y(down). Thus, a 90-degree rotation about the telescope's x axis will align the telescope coordinate system with the ground-based coordinate system.

In grisudet, to rotate the telescope to its pointing position, start with the telescope aligned with the ground-based system. Rotate about telescope -z axis through the azimuthal angle. Then, rotate about the telescope's new x axis through the elevation angle. This sequence of rotations is identical to the actual telescope rotation starting from its stow position: rotation about the x(East) axis through the elevation angle followed by a rotation about the downward pointing axis through the aximuthal angle.

Camera System

The natural (in my opinion, that is) camera coordinate system has its origin at the camera center and its x and y axes parallel to the  telescope-based system. Thus, up to the latest release of GrISU, the GrISU telescope configuration file which gives the x and y locations of the pixels on our cameras uses this system.  

However, our standard camera coordinate system as specified in eventdisplay and vegas has its y-axis antiparallel to the telescope y axis; that is, when standing in front of the camera on the platform with the telescope in stow position, the camera y-axis is up while the telescope y-axis is down.  The latest GrISU configuration version, 4.1.3, uses this standard pixel coordinate system and is thus a change from prior versions.

Since the camera coordinate axes are not parallel to the telescope coordinate system axes, our reconstruction algorithms require a coordinate transformation so that the camera system axes are parallel to the telescope system axes. This transformation is just a reflection of the y-axis; however, it's easy to forget to do this.

So, we now have the same pixel, ground, and telescope coordinate systems in GrISU, eventdisplay, and vegas which should facilitate code comparisons.