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Offline Alignment

Links

• Survey Information (including status table for detector procedures)

Content

• Introduction
• Survey parameters table
• Offline alignment status
• the ALICE Offline Alignment framework
• Initial knowledge of ITS sensitive volumes'positions

• survey and monitoring systems to gather information on the position of survey fiducial marks, later converted into positions of the volumes on which those targets were fixed to by "survey-to-alignment procedures";
• alignment procedures which improve the knowledge on the position of different volumes based on the signals generated on those volumes by tracks, lasers, cosmics ...
• a way to apply displacements (shifts and tilts, possibly deformations) to the initial (ideal) geometry, in order to make simulation and reconstruction aware of the real geometry.

1. two tables:
   1. the survey parameters table presents the status of and the knowledge about those survey issues directly related to offline alignment, divided by detector;
   2. the offline alignment table presents the status of offline alignment issues which need to be completed by every detector
2. a presentation of the ALICE Offline Alignment Framework;
3. a tentative table listing the parameters affecting the knowledge and precision of ITS sensitive volumes' positions .

Survey Parameters Table

The following table collects and presents the status of the relevant survey and alignment parameters relevant for a precise understanding of survey systems and alignment procedures detector by detector.

Table last updated: 18th June 2006

Survey/alignment parameters explanation:

• Surveyed volumes: the volumes whose position will be determined by survey means, i.e. the volumes on which reference targets will be put
• Other alignable nodes: other volumes which are alignable, i.e. their position can change w.r.t. their mother volume but is not surveyed
• Survey precision: estimate of the survey sensitivity, can be different in different directions, for different surveyed volumes.
• Survey update frequency:
• Source of survey data: technology on which the survey is based, like e.g. photogrammetry, optical survey, … specify the name of the survey system if any.
• Monitoring system: only a few subdetectors will put in place a monitoring system continously monitoring the position of given reference marks
• internal alignment procedure: specify in this row wheter the subdetector has (plans to have) an alignment procedure or will rely solely on survey data. In case of an alignment procedure, is it dependent on other subdetectors, will need B=0 data, B≠0 data, cosmics data, laser tracks …
• Alignment runs frequency:
• Fiducial marks' positions: indicate whether they are known or not.
• Adjustable positions: only few detectors foresee to manually adjust the position of some volume, usually by means of a screw, possibly periodically; the volume then will not be surveyed, but the position of the volume has eventually to be stored somewhere.

Offline Alignment Status

1. Misalignment objects: simulated alignment objects ( AliAlignObj -ects) to be saved as AliCDBEntry and to be used to check the framework and to be applied during the Physics Data Challenge. In particular:
   1. zero misalignment objects: objects representing no displacement (zero translation, identity rotation). This are intended to check the framework, in the sense their application should return the same ideal geometry to which they are applied;
   2. residual misalignment objects: objects to simulate the final achievable precision in the knowledge of the geometry; this is the precision of the survey or of the position-tuning device or the precision achieved by the alignment procedure (if any);
   3. full misalignment objects: objects to simulate the possible displacements before any alignment procedure has been applied, e.g. the size of the displacements is of the order of what could be produced by the survey.
2. the symbolic volume names: each detector declares which are its alignable volumes (physical volumes in TGeo, identifiedand by their volume path) and, at the same time, assignes a symbolic name to them. The reasons for the symbolic volume names and how they are implemented is explained in a dedicated paragraph of the Offline Alignment Framework pages.
3. the transformation relating the local RS used in simulation and reconstruction to the local RS known by the geometry (TGeo) for the same volume: this is an important and yet missing link between the offline geometry and the specific reference systems introduced in simulation and reconstruction. How to store these transformations is explained in the multiple local RSs paragraph of the Offline Alignment Framework pages.

Table last updated: 13 June 2008