The backstop is made of 300 microns thick Tungsten carbide with a G ring shape. It is mounted on two motorised translation stage (bstopz and bstopy, vertical and horizontal respectively). A prealignment is performed by fitting accurately the metal pin on the backstop support right at the centre of the V shape by a strong magnet. The collimator aperture is 300 microns diameter. The alignment of the backstop is performed remotely by moving bstopy and bstopz and also manually if needed with the yaw and tilt screws on the backstop support (don't forget the locking screw buried below the backstop in the aluminium support!).

Note - If the beamstop is broken outside working hours then go to bed, as there are no spares on the beamline. These are kept in a safe place as they cost 1500 Euros!

How to align the beamstop

• Put the YAG screen in place and focus to see the surface clearly, remember to move the cryostream away first.
• Open the safety shutter and fast shutter (mxcube Hutch tab).
• Use the "Expert" mode of mxcube put the beamstop "IN"; click on the logo next to the "IN" , a new window will appear with arrows in 4 directions (up and down, left and right): clicking on the arrows will allow you to move by a specific step (can be modified by clicking on the button "step size"; 0.02 is a good starting point).
• Check that the beam is coming through without the collimator cutting it by moving 20 microns (0.02 step size) right / left until you see the edge of the collimator, move to the centre position of both and save the new position (bottom right button of the dialogue window; if done properly a new dialogue window should pop up to confirm it.
• The collimator should be ~300 microns, if you get less than this or worse (for example see a rugby ball shape) you may need to manually adjust the screws keeping the backstop aligned on the support. Be aware that these screws are very sensitive and should be moved with caution (one eighth turns are good starting points).
• Take an image and usually all is well, but you may see the following image (taken on EH4 - the big shadow is their X-ray camera):

Image of the YAG screen with the EH4 detector diode in. The scatter rings come from the beamstop, and are only at low resolution (~2.54A, ~2.18A, ~1.93A and others).

Adjust the bstopz motor in exes (+/- 10 microns) and take another image. Remember to set 'bstopz' motor to 0 in spec before taking another image as the data collection procedure will move the beamstop to zero first!