[d2n-analysis-talk] BPM Calibration

[Diana Parno]

After discussion with Vince, I think I understand the source of the  
discrepancy between my coordinate transformation (reported on  
Thursday) and Transversity's, though I still need to redo my own  
transformation.

I have appended Vince's email because I think the problem may be of  
general interest.

It turns out that the EPICS variables for beam position use the MCC  
coordinate system, where positive X is to beam right. However, the  
beam positions in the Hall A detector system place positive X to beam  
left, which means that you need to flip the sign of the epics BPM X  
variables before comparing them to the positions measured in the DAQ  
scalers.

Vince says that the Harp scans use the Hall A coordinate system, so  
(with a beam generally centered at 0,0) there  wouldn't be any  
confusion -- unless the Harp scans can't be used for some reason.  
That's where I ran into trouble.

Diana

> Hi Diana,
>
> First the EPICS data for the BPM's is exactly what is shown on the  
> Hall A Tools page from MCC. Hence the X position that we refer has  
> as sign flip compared to the HRS and BB DAQ's.  In the coordinate  
> system that we use +X in terms of the beam position points to beam  
> left (left HRS at small angles).  Just for completeness +Y points  
> vertically up.
>
> Apparently MCC's definition of +X points to beam right.  So there  
> has been some confusion in the past about this.  But since the beam  
> is usually centered around zero in terms of X, then it does not  
> usually cause any significant problems.
>
> I am not entirely sure where the sign flip comes from and I was only  
> aware of it for the past couple of years.  If I remember correctly,  
> the Harp scans provided the beam position in the same coordinate  
> system as the HRS. Since we always used these for the absolute  
> calibration, there was never any confusion.
>
> Cheers,
> Vince