Difference between revisions of "Alignment"

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==Definitions==
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Alignment describes the corrections to the geometric descriptions of actual detector pieces compared to their nominal as-engineered description.  An example
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is ''rigid body alignment'', which describes the detector piece's position and orientation within some larger frame, ignoring any internal distortions to the
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piece.  Alignment is often built up out of a nested set of corrections, moving outwards to larger frames.
  
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The specifics of the tracker alignment are described on the page [[TrackerAlignment]].
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==Technical Definition==
 
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This section explains how the tracker alignment is defined. 
 
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* There are three places that alignment transforms are inserted
 
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** between the whole tracker and the nominal geometry
 
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** between each plane and the aligned tracker
 
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** between each panel and the aligned plane
 
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* transforms
 
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** apply rotation before displacement
 
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** a rotation is defined as right-handed about the local x, then y, then z axes (rad)
 
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** add an offset in position (mm)
 
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* tracker geometry object are in coordinate centered at the nominal tracker center
 
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Here is how the geometry is conceptually built up, including the alignment
 
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<ol>
 
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<li>create straws within the panel
 
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* straw direction is +y
 
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* straws are placed around the +x axis with the longer (innermost) straw near x=0
 
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* the origin is the average position of the two innermost straws
 
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* straw z is plus or minus half the layer spacing
 
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</li>
 
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<li>Apply the panel alignment of rotations and displacements.  The coordinates are these current local coordinates. For example, to move a panel to a larger radius in the final detector, apply a +x alignment.</li>
 
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<li>Place the panel in the plane.  This involves rotating the panel to its nominal phi position around the z axis, and adding its nominal displacement of about <code>x += 380*cos(phi)</code> and <code>y += 380*sin(phi)</code> </li>
 
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<li>Apply the plane alignment of rotations and displacements.  The coordinates are the center of the plane. For example, to move a plane vertically up in the final detector, apply a +y alignment.</li>
 
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<li>Place the plane within the tracker.  This only involves moving it along the z axis.</li>
 
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<li>Place the tracker by applying the tracker alignment.  The coordinates are all the same as the final detector.  To move it closer to the calorimeter, apply a +z alignment. To spin it about its axes apply a z roation.</li>
 
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</ol>
 
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[[Category:Computing]]
 
[[Category:Computing]]
 
[[Category:Code]]
 
[[Category:Code]]
 
[[Category:Database]]
 
[[Category:Database]]

Latest revision as of 22:38, 13 January 2021

Definitions

Alignment describes the corrections to the geometric descriptions of actual detector pieces compared to their nominal as-engineered description. An example is rigid body alignment, which describes the detector piece's position and orientation within some larger frame, ignoring any internal distortions to the piece. Alignment is often built up out of a nested set of corrections, moving outwards to larger frames.

The specifics of the tracker alignment are described on the page TrackerAlignment.

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