ZPD info from Jamie:
--------------------
Each ZPD covers 3/8 in phi with the central 8th being the one
corresponding to the numbering so we have.

ZPD     TSF x           TSF y
------------------------------
0       14,15           7
        0,1             0  ** central setion used for finding seed tracks
        2,3             1

1       0,1             0
        2,3             1 **
        4,5             2

2       2,3             1
        4,5             2 **
        6,7             3

3       4,5             2
        6,7             3 **
        8,9             4

4       6,7             3
        8,9             4 **
        10,11           5

5       8,9             4
        10,11           5 **
        12,13           6

6       10,11           5
        12,13           6 **
        14,15           7

7       12,13           6
        14,15           7 **
        0,1             0

so each ZPD has input from 9 TSFs (6 x and 3 y).