Beam drift: what to do

Sometimes the parallelism between the two crystals of the monochromator can be partially lost. This causes the beam to move vertically and the intensity of the beam to drop significantly.
To check whether this happened during your experiment, do the following:

1) From the Beamline Browser (sunflowers window), start the beamwatch and, from there, click on the Diode

 
A new tab will open: from there, select the CLOSE command and enter. This will put a diode in the beam path, therefore the reading of the BMS will go to zero. This diode will be used to monitor the beam intensity shortly after the monochromator.

2) From the BL browser, open Diode (Current Trends):

3) In the TrendPlot window that will open, double click on Channel 1 to make this line disappear (you want to follow Channel 2 only!).
Then, right click on the plot and set a forced reading period to 300 ms:

4) From the BL browser, open the Si Pitch window:

   

5) Move a StepUp and follow the reading of the Channel2 in the TrendPlot. If the signal increases, keep going - slowly, step by step - until it starts to go down. Then move a step back (StepDown).
If the signal decreases, move a StepDown and keep going - slowly, step by step - until it starts to go down. Then move a step back (StepUp) and you're done.

6) Remember to re-open the Diode from the beamwatch window, otherwise you will have no photons reaching the UHVC!