Beam Line Alignment Methods at the National Synchrotron Light Source

 

We will report on automatic alignment protocols that have been in place for some years at three different beamlines at the NSLS, two of them primarily for macromolecular crystallography. Beam line alignment techniques fall generally into two categories: those that address initial alignment for a particular kind of experiment, and those that address maintenance or recovery of the initial alignment after the experiment has begun. Although the automatic methods we employ fall in the last category, they represent the final step in an ab initio beamline alignment.

At beamline X25 we provide users with automatic optimization methods to steer a monochromatized and focussed wiggler beam through a pinhole. The algorithm makes use of theta tilt of a focussing mirror for vertical motion and chi tilt of the second monochromator crystal for horizontal motion. Fine theta tuning of the second monochromator crystal is included in the scheme to maximize the flux throughput.

At beamline X12-B for some time we have employed a search and lock-in algorithm for the tuning of the second monochromator crystal. We have a reliable beam-dropout monitor that will pause and then resume an experiment during filling of the ring. Recently we have devised a non-intrusive video visualizer to fiduciate beam position relative to the spindle, to allow first manual and eventually automatic steering of the beam, permitting optimization of the beam and spindle alignment with a specimen in place.

Beamline X12-C has employed automatic alignment of the downstream portion of the beamline for 14 years, and we will describe this experience. The same protocols devised for use at X12-C have been transferred to X25 when it is being used for PX. These include an algorithm to optimize the alignment of the optical table, on which the diffractometer or other apparatus sits, and a curve-fitting algorithm intended to set the monochromator to a wavelength that optimizes particular features of a heavy atom's absorption spectrum. This feature provides especially strong signals for MAD phasing. A principal feature of the robustness of the system is reliance on a fixed-collimator diffractometer. This allows one to decouple the difficulty of placing the beam at the center of a three-axis diffractometer and perpendicular to its major axis, from the problem of hanging the diffractometer on a focussed beam from the synchrotron. We also will describe the use of GUIs to provide user control of the process.