Although the procedure is non-invasive for Hall A, let the shift leader know
when you start and finish the scan.
The scan is done in contact with MCC (7047) by checking the online evolution
of the counting rate using "spy_acq".
The optimal Y-position is at the upper part of the bell appearing
on the middle of the spy_acq picture below ("Counting Rates versus DAQ Y
at vertex"). As a first pass, one can use bigger step size to locate
the maximum and then go back to small steps to fine tune the position.
When this procedure is over, come back to the right Y-position and
ask to the machine operator to lock the Y-position of the beam. Then an
automatic magnetic feed-back will run to keep the electron beam Y-position
within 50 micro m of this optimal position. Click on Set X and
Set Y buttons in the frame "Beam drift alarm on Epics pos at vertex".
Click on Alarm ON to set it green. It will bip when the 50 micons limit
is reached. This is an important task to avoid sensitivity to beam position
false asymmetry.
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