As of this afternoon (2pm 1-26-98) the CLAS drift chamber
alignment using survey data is in place in the generally-used map
(DC_GEOM.map). To use this information successfully you need to remake
your Recsis, with either a cvs checkout or an update of your dc area.
The alignment changes have been tested over the weekend (by Stepan) and
have been shown to significantly improve our W resolution, as well as
removing some false correlations seen in the data. If you do not remake
your Recsis, your resolutions may get worse.
The drift_chamber people will get the full text of CLAS-Note 98-002
appended to this message which describes details of what was done. The
off_line people can get a copy of the Note if they care to read about
the details.
Regards, Reinhard.
__________________________________________________________________________
DRIFT CHAMBER ALIGNMENT
version 0
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 1.7410 -1.0950 -0.2750 0.0017 0.0007 0.0035
2 1.2990 0.1660 -0.1450 -0.0003 0.0007 0.0023
3 1.4180 0.5740 -0.1130 0.0000 0.0001 0.0038
4 1.5700 1.0640 0.5230 -0.0004 0.0004 0.0038
5 1.8020 -0.0970 0.1180 -0.0003 0.0009 0.0053
6 1.9610 -0.3810 0.2790 0.0001 0.0006 0.0041
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