Spectrometer offsets 03/16/06

Here are the existing spectrometer offsets:
;These are offsets applied before the call to the single arm montecarlos.
begin parm spect_offset
  spec.e.offset.x = 0.        ;  x offset (cm)
  spec.e.offset.y = 0.        ;  y offset (cm)
  spec.e.offset.z = 0.        ;  z offset (cm)
  spec.e.offset.xptar = -2.62    ;  xptar offset (mr)    !x(y)ptar is slope, so
  spec.e.offset.yptar = 0.    ;  yptar offset (mr)    !it's really unitless.
  spec.p.offset.x = 0.        ;  x offset (cm)
  spec.p.offset.y = 0.        ;  y offset (cm)
  spec.p.offset.z = 0.        ;  z offset (cm)
  spec.p.offset.xptar = 0.    ;  xptar offset (mr)
  spec.p.offset.yptar = 0.    ;  yptar offset (mr)
end parm spect_offset

The y offsets need to be fit by looking at the mean of ytar in the experimental data.  The y offsets will then be the negative of the mean of the data.

The existing xptar offset is very wrong.  A POSITIVE xptar corresponds to the spectrometer being below the target.

It is not clear whether the HMS should be given an xptar offset.  Tanja includes an HMS xptar offset of 1.1 mrad because in the HMS optics data, the surveyed-reconstructed xptar were shifted relative to each other by 1.1 mrad.

The heep data shows that to make the reconstructed particles coplanar, the SOS xptar has an offset of
0.00363+0.000001078*exp((spcentral+1.0)**2.1)
with respect to the HMS xptar=0 plane.  If the HMS xptar offset is not included, then this coplanarity will not be preserved when the spectrometers are rotated.    One can picture that when a spectrometer is rotated very close to the beam pipe, the lab phi angle will need a larger correction for a given xptar offset compared to when the spectrometer is at 90 deg.

Therefore, an HMS xptar offset of 1.1 mrad will be included in simc.