Offline Alignment

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Introduction Offline alignment is the process to make the geometry (used in simulation and in reconstruction) as close as possible to the real one: detectors and detectors'parts cannot be placed exactly where they are expected, some positioning can be verified only after mounting, some parts are subject to deformation because of the assembly process or as time goes on. To take into account this variations to the ideal geometry and thus correctly associate signals to positions in space, simulation and reconstruction need:

  • 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.

This pages include

  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

  MUON PHOS TRD TPC HMPID VZERO T0
surveyed volumes quadrants and slats 5 modules 18 supermodules 72 RO chambers 7 chambers 2 scintillator arrays 2 arrays of 12 PMDs
other alignable nodes 156 (quadrants+slats+chambers) no 90 stacks, 540 chambers central membrane no no no
survey precision 30 μm 1 mm 1mm w.r.t. MARS - 0.3mm w.r.t. TPC 100 μm  1mm 1 mm 1 mm
survey update frequency once per day once only at mounting at mounting at mounting once a year once a year - depends on V0
source of survey data photogrammetry optical survey optical survey optical survey optical survey optical survey optical survey
monitoring system GMS using BCAMs no no no no no no
internal alignment procedure B off / B on depends on TPC ? cosmics / B off / B on - depends on TPC cosmics, laser, beam gas B=0/B≠0 based on tracks no no
alignment runs frequency daily   monthly monthly   / /
fiducial marks' positions known yes yes yes yes yes yes no
adjustable positions no no no no no no no

 

List of survey/alignment parameters for the ALICE detectors. Red cells indicate missing information.
  ZDC ITS TOF FMD PMD
surveyed volumes no staves and ladders 2 support cones w.r.t. TPC 18 supermodules 6 (coupled) half-rings 4 steel plates
other alignable nodes no 2198 modules (240+260+1698)   90 chambers, 1638 strips no no
survey precision / see this table see this table 0.5mm (5 mm including rails and spaceframe deformations) ~100 μm  
survey update frequency once per fill off-shelf + at mounting at mounting ~monthly at mounting at each mounting
source of survey data manual adjustment optical 3D survey   B-cams optical survey optical survey
monitoring system no ITSAMS (status?) no no no no
internal alignment procedure no pp or periferal - depends on TPC depends on TPC depends on spaceframe no no
alignment runs frequency /       no no
fiducial marks' positions known / yes yes not fully no to be agreed with survey
adjustable positions yes (ZN, ZP vertical) no no no no yes

 

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

The following table presents the status of detector's code required by the alignment framework; presently in the to-do list:

  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.

 

For the status of the code of the subdetectors concerning alignment-awareness of simulation and reconstruction, you can also see this table . The colors are given on the basis of a simple test which checks that simulation (in the construction of digits) and reconstruction (in the calculation of the global coordinates of recpoints) produce different signals when using a geometry with a misalligned sensitive volume w.r.t. the signal produced with ideal geometry. The idea on which the test is based is that the recpoint should appear at the position of the generating hit(s) when simulation and reconstruction are using both the same geometry (either ideal or misaligned). When only simulation uses the misaligned geometry, the recpoint should appear shifted by the inverse of the transformation applied to the sensitive module hitted; when only the reconstruction is using the misaligned geometry he recpoint should appear shifted by the same transformation applied to the sensitive module hitted.

Table last updated: 13 June 2008

  ACORDE EMCAL FMD HMPID ITS MUON PHOS
alignment objects yes yes yes yes yes yes yes
symbolic volume names yes yes yes yes yes yes yes
alignment-aware simulation no OK OK OK OK OK OK
alignment-aware reconstruction not OK not OK to be tested OK OK OK OK
sim-rec local RS to TGeo local RS not req. no not req. yes yes not required yes

 

List of the alignment offline tasks for the ALICE detectors. Red cells indicate items still to be done or not correctly done.
  PMD T0 TOF TPC TRD VZERO ZDC
alignment objects yes yes yes yes yes yes yes
symbolic volume names yes yes yes yes yes yes yes
alignment-aware simulation OK OK OK OK to be tested OK OK
alignment-aware reconstruction not OK OK OK OK to be tested not OK OK *
sim-rec local RS to TGeo local RS not required not required OK OK OK not required not required