Microdiffractometer

ID29 is equipped with a microdiffractometer (MD2), driven by PMAC electronics. The MD2 has a set of five beam defining apertures which allows to change very quickly the beam size at the sample position. Aperture sizes are 75 (full beam), 50, 30, 20 and 10 microns in diameter. An additional beam cleaning aperture at the end of the collimator reduces scattering.

A MiniKappa goniometer is permanently available to reorient the crystal for different purposes (measure of cuspids, smart spot separation for large cells, measure of Friedel's pairs on the same image).

Beam sizes and photon flux

ID29 beam is permanently focused at 40x30 um² FWHM with an elliptical beam profile.

The 75 um aperture delivers a full beam, the 50um a 50x30 um² with sharper tails, the 30 um a round 30x30 um², 20 um a 20x20 um² and 10 um a 10x10 um².

Since the beam is collimated and the flux density is constant, a smaller beam delivers a lower photon flux. The measured photon flux over the whole energy range for each beam size is shown in figure.

MiniKappa goniometer

The MiniKappa goniometer head is usually mounted on all beamlines (except the microfocus beamline ID23-2).

It is used to re-orient crystals for an optimal data collection:

• The crystal can be aligned such that Bijvoet mates are measured on the same image for the optimal measurement of anomalous differences in MAD/SAD phasing experiments.
• The spot separation can be optimised for a particularly large unit cell axis.
• The total necessary oscillation range can be minimised.

(Figure from Brockhauser et al., Acta Cryst. D69, 1241-1251 (2013). )

The most convenient way to use the MiniKappa is through the visual reorientation and kappa reorientation workflows.

The diffractometer automatically changes the intensity of the light according to the zoom level. However, this might be a problem when working with light-sensitive samples. You can manually set the intensity of the light, which will not change automatically anymore.

The controller for the front and back lights sits underneath the experimental table (see photo below). The back and front lights should be properly labelled. Change the two switches on the front panel from "Remote" to "Local" and adjust the light intensity as desired with the knob.

Once you are finished, do not forget to switch them back to "Remote"!