ess.sans.types.BeamCenter

ess.sans.types.BeamCenter = ess.sans.types.BeamCenter

Position of the beam center relative to the sample.

The beam passes through the sample, but its direction, set by the collimation, in general deviates from the nominal beam axis defined by source and sample. Together with the sample position, the beam center defines that direction: it is a point on the actual beam, given as an offset from the sample.

Its component along the nominal beam is therefore the distance from the sample at which the transverse offset was determined, typically the distance of the detector bank used for the beam-center determination. Recording that distance is essential, since the same angular deviation of the beam corresponds to a proportionally smaller transverse offset on banks closer to the sample.

Only the direction of this vector enters the correction, its magnitude cancels. A value determined on one detector bank therefore applies to all banks, and, as long as the beam direction is set by the collimation alone, also to a detector that has been moved to a different distance from the sample. Changing the slits changes the beam direction, so re-determine the beam center whenever the slits change.

Set to a zero vector to apply no beam-center correction. May be set directly or computed using a beam-center finder, see ess.sans.beam_center_finder.