An optimized collimator for X‐ray diffraction from very small samples using synchrotron radiation

The high intensity available from synchrotron radiation sources like DORIS allows one to reduce substantially the sample size required for biologicalstructure determination by X-ray techniques. One particular example is the possibil i~ (Bartunik, Fourme & Phillips, 1982) of collecting on a double-focusing X-ray camera, like the X l l at DORIS, highresolution data for three-dimensional protein-structure analysis from crystals as small as 10-20 t~m. Such applications require a collimator system which provides an optimum signal-to-noise ratio, and which may be aligned with high accuracy, possibly under remote control using the synchrotron-radiation beam. Such an optimized collimator has been developed and is described in the following. Two of these collimators are in routine use in diffraction studies of twoand three-dimensionally ordered biological structures on a modified Arndt-Wonacott rotation camera on the X l l and on a fourcircle diffractometer on the X31 at DORIS (Bartunik & Bartels, 1984). Fig. 1 shows the collimator system mounted on the rotation camera. The collimator incorporates two x-ytantalum slits (purchased from Huber, Rimsting) which serve as defining (DS) and guard slits (GS), respectively. DS defines the