ToF lookup table is wrong for negative chopper phase

[jl-wynen]

(Found by Celine)

The bcc chopper of DREAM is symmetric by 180deg. But in the dream chopper notebook, the lookup tables are different when subtracting 180deg from the chopper phase. They should be the same.

That is, the default values of

bcc = DiskChopper(
    frequency=sc.scalar(112.0, unit="Hz"),
    beam_position=sc.scalar(0.0, unit="deg"),
    phase=sc.scalar(240 - 180, unit="deg"),
    axle_position=sc.vector(value=[0, 0, -66.77], unit="m"),
    slit_begin=sc.array(dims=["cutout"], values=[-36.875, 143.125], unit="deg"),
    slit_end=sc.array(dims=["cutout"], values=[36.875, 216.875], unit="deg"),
    slit_height=sc.scalar(10.0, unit="cm"),
    radius=sc.scalar(30.0, unit="cm"),
)

give the expected result. But

bcc = DiskChopper(
    frequency=sc.scalar(112.0, unit="Hz"),
    beam_position=sc.scalar(0.0, unit="deg"),
    phase=sc.scalar(240 - 2 * 180, unit="deg"),
    axle_position=sc.vector(value=[0, 0, -66.77], unit="m"),
    slit_begin=sc.array(dims=["cutout"], values=[-36.875, 143.125], unit="deg"),
    slit_end=sc.array(dims=["cutout"], values=[36.875, 216.875], unit="deg"),
    slit_height=sc.scalar(10.0, unit="cm"),
    radius=sc.scalar(30.0, unit="cm"),
)

gives wrong tables.

Interestingly, this only happens when subtracting 180. Adding 180 or multiples of 180 gives the same lookup table as the default values. So I suspect that this is a problem with negative phases.

[nvaytet]

Isn't this just an issue in tof?

[jl-wynen]

Probably. I didn't debug beyond what I wrote.

[nvaytet]

I checked tof, and it's not that.

I think it's because of the convention in nexus with the sign of the frequency that determines direction of rotation. The phase also has a sign.
If the frequency is positive and the phase negative the phase will be applied in the opposite direction to the rotation.

In tof, the phase is applied always in the same direction as the chopper rotation.