[from Isaac Upsal's instructions to star-ops, dd.May 17, 2023]

If stable collisions are available and the BBC (ZDC) is ready to use as a trigger then we can take the VPD gain scan. The procedure is as follows:
Make sure that you are using a **non-VPD-based** Trigger for all data taking.

First :
On the sc3 machine there needs to be a "small GUI" for the VPD with 4 buttons - upVPD HV A, upVPD HV B, upVPD HV C, and Default. If it is not open or you are unsure open a terminal and execute "upvpd_settings". I just opened this ~an hour ago.
"large GUI" shows all the channel values, and where one powers them on/off.
"small GUI" selects between different gain sets A, B, C, & default.

start with the VPD HV **OFF**

1) On the small GUI choose the HV Gain setting *A*
2) On the large GUI power on the VPD HV
3) Make sure that the upper-left corner (channel 13-0) of the large GUI reports a voltage of ( 1101, 1251, 1401 ) for gain setting A, B, and C respectively.
4) Wait about a minute or two for the VPD PMT channel voltages to settle
5) Start run: TRG+DAQ+TOF (it is OK to include other detectors in if you want, but those are required), 200k events. Use string "VPD HV A" (or B, C as needed) in the shiftlog entry
6) Use the "large GUI" to power off the VPD HV
7) Wait for all channels to read a voltage of 0

Repeat these steps for each of the gain settings (A, B, C) - making the appropriate entry in the shiftlog to identify which gain setting is used (upVPD HV A, upVPD HV B, upVPD HV C ).

Once this is complete, use the "small GUI" to select "default" then power the VPD HV back on. Now you can close the "small GUI".