Original reporter: ISIS / T. Williams

Describe the bug
A few years ago (~2021?), the disk chopper controller on MERLIN was replaced / upgraded. Unfortunately this lead to an offset (of 4500 us) in the time-delay set by Mantid compared to what PyChop believes the time delay (chopper phase) is. Although it is a feature not often used, PyChop allows users to set the delay time of the disk chopper on MERLIN to govern which "reps" (incident energies) are allow to become incident on the sample. Because of the offset, if users set the delay time which PyChop suggests, they would be setting the wrong chopper phase and may not end up measuring anything and so lose time during experiments.

To Reproduce

1. Open PyChop and select Merlin as the instrument. Put 50 in the Ei text box. Click Calculate and Plot.
2. Click on the Time-Distance tab near the top.
3. Go to the Options menu and select Instrument Scientist Mode - you should see an addition box Disk chopper phase delay time added to the GUI.
4. Change the value 1500 in this box to 3000 and see that the first two "reps" (at 133 and 50meV) are now blocked.

In fact in real life, a phase delay time of 3000 set in the IBEX control actually corresponds to 7500 us in PyChop terms so many more reps will be blocked and it's likely that the user will not be measuring anything.

Expected behavior
The phase input in PyChop matches the phase set by the IBEX controller program.

Screenshots
N/A

Platform/Version (please complete the following information):
All, since Mantid 3.x

Additional context