CONVENTIONS FOR PHOBOS PAD COORDINATES
Baker, Verdier
28-Aug-1997

Global Coordinate System:
-------------------------
The origin of the PHOBOS global coordinate system is the nominal vertex
position. The Z axis is the beam axis, with positive Z in the direction of the
PHOBOS spectrometer which is clockwise when the RHIC ring is viewed from above.
The Y axis is vertical with the positive Y direction being up. The X-axis
completes a right-handed coordinate system. This means that the "left arm" is
the positive arm SPxn when the spectrometer is viewed from the vertex. The
"right arm" is the negative arm SNxn.

Detector Elements:
------------------
Pad:    A pad is a single pixel
Sensor: A sensor is a contiguous array of Silicon pads from one wafer.
Plane:  A plane is a collection of sensors which are nearly coplanar. Typically
        they are slightly offset from each other and overlapping.
Layer:  A layer is a group of planes which will only be traversed once by a
        normal particle trajectory. These planes are on one side of a given
        frame (front or back). For instance the horseshoe layers in the
        spectrometer comprise 3 or 4 planes each, while the spectrometer layers
        closest to the beampipe have only one plane each.
Subdetector: Left spectrometer arm, Right spectrometer arm, multiplicity
             barrel etc.

Naming conventions:
-------------------
1st two letters specifies the subdetector:
    SP - positive (X) spectrometer arm
    SN - negative (X) spectrometer arm
    MO - multiplicity barrel (octagonal prism)
    MP - positive (Z) multiplicity ring
    MN - negative (Z) multiplicity ring
    VP - positive (Y) vertex plane
    VN - negative (Y) vertex plane
    ?? - TOF, trigger counters?
3rd letter specifies the layer:
    A is closest to the beamline.
    The spectrometer arms contain 16 layers A-P
4th character (a number) specifies the plane within a given layer.
    Plane numbers start at 1. 
    In the spectrometer, plane 1 is the plane in a given layer farthest from 
    theta=0 and the plane #s increase with decreasing |theta|.
    In the multiplicity octagon The planes are numbered as if the 
    frames were fully loaded even though there are places where we don't
    currently plan to have Silicon. E.g. there is no SPN1, but the name is
    reserved in case Silicon is later placed on the back of the 7th frame.

Sensor numbering conventions:
-----------------------------
Within a plane, the sensor closest to the beampipe in theta is sensor 0 and the
sensor numbers increase away from the beampipe.

Pixel numbering conventions:
----------------------------
Each pad in a given sensor is identified by 2 integer indices i,j which start
from 0. In the spectrometer: i is the coordinate in the xz-plane with i=0
referring to the column of pixels closest to the beamline (in theta) and
increasing as |theta| increases; j is the y-coordinate with j=0 being the
bottom-most row (-y) and  increasing with increasing y.

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Questions:

- Should the spectrometer plane numbering convention be changed so that 1 is  
  closest to the beam pipe? Should they start at 1 or 0?