The geometrical information is in type CHART. In PTC, the chart contains a MAGNET_FRAME. The magnet frame locates the front, the centre and the end of the fibre.
The reader geometry of a planar fibre is shown below: please click on picture to magnify.
Looking at this picture we see that PTC has amount of redundant information about standard planar magnets.
The fibre desired location is specified by the position FIBRE%CHART%F%A(3) and by the vector triad basis FIBRE%CHART%F%ENT(3,3). The other two sets of affine bases are redundant and can be deduced from the magnet geometry.
One notices that the actual magnet is displaced (misalignments) from its desired position and that a totally independent set of affine bases are located on the magnet. In the absence of misalignments they merge with the fibre bases.
We showed here a typical planar magnet. Things are more complex with the vertical bends which follow the MAD8 convention. We delay this discussion.
Now some operators acting on the affine bases:
We now describe the purely geometrical operations of PTC on affine bases of the type (A(3),ENT(3,3)) . These operations are crucial in order to compute patches and misalignments.