A l i g n m e n t |

Last updated by Thomas Naumann

` - an external alignment of the BST
in the H1 coordinate system`` - an internal alignment of the disks
in the BST coordinate system .`

`The external position of the BST is characterized by:`

` - 3 translation parameters DR, DPHI, DZ and`` - 2 rotation parameters THX and THY around the x and y
axes.`

`It is determined from the mispointing dz of a BST track to the
vertex``and checked with the difference dth of the polar angles measured
by BST and BDC.`

`Before alignment dz
and dth strongly depend on the azimuthal angle phi.``Assuming the BST gravicenter as the rotation center ZROT this
is parametrized as`

` DZ = (DR*COS(PHI-DPHI) + (THX*SIN(PHI)+THY*COS(PHI))*(Z-ZROT))*
COT(TH) + DZ`

`The 5 external alignment constants are fitted from the``3D distribution dz(phi,z). We find`

` DR = 1.2 mm DZ = 0.5 mm
THX = 1.2 mrad THY = 0.0 mrad`

`After external alignment no z or theta biases and no phi dependence``of dz
and dth
are visible. Integrating over phi we get``the z and theta resolutions of the
BST:`

` z BST - z CT = 0.01
+/- 0.31 mm`` th BST - th BST = 0.07 +/- 0.43
mrad`

`The z
and theta
resolutions are also available for BST1 and BST2 separately.``dth is dominated by the BDC resolution.
dz contains 2 mm z error of the CT.`

`The external and internal alignment of
the r and u detectors is given``in the banks BSTA
and BSTU
.`