__________________________________________________________________________ DRIFT CHAMBER ALIGNMENT version 0.1 January 26, 1998 By: R. Feuerbach, J. McNabb, M. Mestayer, R. Schumacher, A. Skabelin CLAS-Note 98-002 __________________________________________________________________________ Results of the CLAS drift chamber alignment based only on survey data are presented in this memo, together with a definition of the coordinate systems used, as well as a brief discussion of work that remains to be done. For some time now RECSIS has had the correct internal geometry (wire layouts) for each of the drift chamber packages. However, RECSIS has also assumed ideal alignment between the different drift chamber packages. That is, the drift chamber offsets in translation and rotation with respect to the CLAS torus were set to zero, and the torus was assumed to be perfectly aligned with the overall CLAS coordinate system. The results given here are the first step in removing this idealization by giving offsets for the detectors and the torus as determined by survey data; these results do not involve any software track alignments. The J_Lab survey group (K. Tremblay et al) provided data for the location of each detector and of the torus with respect to the "Hall Coordinate System" (HCS) which is defined below. These data were analyzed by M. Mestayer, A. Skabelin (CLAS_note 98-xxx), and R. Schumacher (CLAS_note 98-001), and L. Weinstein, R. Niyazov, A. Skabelin (CLAS_note 98-yyy). The results given below take the HCS data for each sector and simply rotate the values into a sector-based system defined by exact (N-1)x60 degree rotations (N = 1...6) about the Z axis of the HCS. This is called the Hall Sector Coordinate System (HSCS). __________________________________________________________________________ DEFINITION OF COORDINATE SYSTEMS: Hall Coordinate System (HCS): X - horizontal towards beam left Y - vertical opposite gravity Z - horizontal along beam line in the direction of the beam Origin: At the nominal torus center which is also the nominal target positon. It is defined by the Survey Group according to fixed marks in the Hall and beam enclosure. Angles: In this right-handed system, positive angles follow the right-hand rule for rotation about any given axis. Hall Sector Coordinate System (HSCS): X - radially outward in the ideal mid-plane of a drift chamber sector Y - such that system is right-handed with X and Z Z - horizontal along beam line in the beam direction Origin: The same as the HCS. Angles: In this right-handed system, positive angles follow the right-hand rule for rotation about any given axis. Remark: One could say there are actually 6 coordinate systems here, one for each of the six sectors. For Sector 1 the X axis of the HCS is identical with the X axis of the HSCS. Torus Sector Coordinate System (TSCS): X - radially outward along the mid-plane between two torus coils. Y - such that system is right-handed with X and Z Z - along the axis of the torus Origin: At the center of the torus/target. Angles: In this right-handed system, positive angles are follow the right-hand rule for rotation about any given axis. Remark: This would be identical with the HSCS if the Torus were ideally positioned. Torus Coordinate System (TCS): X - such that system is right-handed with Y and Z Y - through "12 o'clock" cryostat Z - along the axis of the torus Origin: At the center of the torus/target. Angles: In this right-handed system, positive angles are follow the right-hand rule for rotation about any given axis. Remark: To an excellent approximation, this is the same as the HCS with a +4 mrad rotation of the HCS coordinates about Z, and small displacements (given below) in X, Y, and Z. __________________________________________________________________________ ISSUES IN USING THE ALIGNMENT DATA AS GIVEN: We have recently decided to do all tracking in the Hall Sector Coordinate System (HSCS). Thus, RECSIS takes the numbers given below in the HSCS and translates and rotates each of the sectors away from their ideal positions accordingly. The numbers given are in every case the displacement or rotation needed to move the object in question from its ideal location to its measured location in the HSCS. While displacements and offsets are given for the torus, they are presently not used in the tracking. Thus, we are ignoring the roughly 1 mm shifts in the torus centroid, as well as the 4 mrad rotation of the torus. We presently believe that neglect of these effects is negligible in terms of the results of tracking. The TSCS and TCS are not used. Results returned by the tracking code is therefore also in the HSCS or the HCS, as appropriate, for direct matching to the outside detectors. More alignment work needs to be done. The result given below is only a first step using only survey data. Next we need to do a software alignment using the field-off, target-empty data. We expect sub- millimeter shifts to result from that procedure. Subsequently we may need to improve the magnetic field model, although the evidence for this is not yet clear. The results are given below in the HSCS, and these are the same numbers which should appear in the database map DC_GEOM.map. In every case, the numbers tell which way a detector element is displaced from its ideal position. _____________________________________________________________________________ TORUS NUMBERS ARE IN THE HALL COORDINATE SYSTEM Xoffset Yoffset Zoffset Xrotation Yrotation Zrotation (cm) (cm) (cm) (rad) (rad) (rad) Torus 0.1310 0.0030 0.0330 0.0000 0.0000 0.0040 DC ALL NUMBERS ARE IN THE HALL SECTOR COORDINATE SYSTEM Xoffset Yoffset Zoffset Xrotation Yrotation Zrotation (cm) (cm) (cm) (rad) (rad) (rad) Region Sector 1 1 0.0660 -0.0940 -0.1400 -0.0008 0.0001 -0.0042 2 -0.0480 -0.1040 -0.1400 -0.0005 -0.0007 -0.0042 3 -0.1140 -0.0100 -0.1400 0.0003 -0.0008 -0.0042 4 -0.0660 0.0940 -0.1400 0.0008 -0.0001 -0.0042 5 0.0480 0.1040 -0.1400 0.0005 0.0007 -0.0042 6 0.1140 0.0100 -0.1400 -0.0003 0.0008 -0.0042 2 1 0.9630 0.0790 -0.1530 0.0001 -0.0001 0.0029 2 0.8730 0.0840 -0.1710 0.0003 -0.0001 0.0030 3 0.8680 -0.2160 -0.2010 -0.0003 -0.0005 0.0049 4 0.9300 -0.0010 -0.2170 -0.0002 -0.0007 0.0043 5 0.9990 0.0380 -0.2870 -0.0007 -0.0005 0.0033 6 0.9940 0.0410 -0.1590 0.0005 0.0003 0.0036 3 1 1.3550 -1.0950 -0.2750 0.0017 0.0007 0.0035 2 0.9150 0.1660 -0.1450 -0.0003 0.0007 0.0023 3 1.0320 0.5740 -0.1130 0.0000 0.0001 0.0038 4 1.1840 1.0640 0.5230 -0.0004 0.0004 0.0038 5 1.4150 -0.0970 0.1180 -0.0003 0.0009 0.0053 6 1.5750 -0.3810 0.2790 0.0001 0.0006 0.0041