On-axis viewing: Sample visualization along the synchrotron X-ray beam

The SBC on-axis visualization system allows viewing of biological crystal samples along the X-ray beam direction, without image degradation and without parallax distortion. The on-axis system was constructed using a long working distance Maksutov–Cassegrain (MC) reflective microscope, and a right angle (45°) externally reflecting mirror. The minimum size of the right angle mirror and the maximum size of the hole through which X-rays pass depend on the sample to mirror distance as well as the required field of view and optics working distance. The on-axis system allows biological crystal visualization during diffraction data collection with full Kappa geometry without image degradation (paraxial cone rays are excluded from the image plane). On the basis of these observations it is recommended that users seriously consider on-axis reflective rather than refractive optics as the primary visualization technique.