[esa-t474] Energy resolution for the ESA spectrometer

[Michele Viti]

Dear all,
after my first reply I continued thinking on what Alexey did and I have 
some other comments/questions.
> * take the same data as Michele used, run 2743 (energy scan with 
> magnets on)
> * take I's and Q's from BPMs 1,2,3,5 and regress them against I's and 
> Q's from BPM 4 for a "quiet" period, where the energy does not jump 
> (magnets are on!)
> * use the constants to predict the I's and Q's in BPM4 during the 
> energy scan, get dI and dQ, which means jitter-subtracted measurements 
> of BPM4
> * establish the IQ rotation from the energy scan - all the points lie 
> on a straight line in the IQ space, any offset means some change not 
> related to energy
I dont understand this: I guess you are talking about the top right 
figure in file bpmPlot.pdf. For DeltaE = 0 (no change in energy), I 
should expect both value of I/Q in 0 with no offset (which seems to be 
the case in figure), since this corresponds to the situation where you 
made you regression between I/Q from BPM 4 and I/Q of the other BPMs.
> * go to dAmp and dPhase in order to do the phase rotation and 
> projection on that straight line in the IQ plane
A question for my understandig: After you plot dI and dQ (top right 
figure in file bpmPlot.pdf) you rotate all the points so they lie along 
the I axis and you call then "I" "Amplitude" which is plotted in 
top/bottom left figure in bpmPlot.pdf
> * get the energy scale for the resulting value from the energy scan 
> with known steps (see plots in bpmPlot.pdf)
> * the resulting plot of the energy scan is very clean, so I again took 
> some data from the quiet piece to estimate the resolution (see 
> enePlot.pdf), it turned out to be about 0.73 MeV, but this measurement 
> still included the energy jitter of the beam itself
> * so, in the next step I regressed the energy against the I's and Q's 
> of BPMs 12 and 24 to exclude the energy jitter and expose the noise of 
> the spectrometer system
Why dont you try to regress and subtract during the energy scan, where 
any relation between "Amplitude" of BPM4 and I/Q values from BPM12 and 
BPM24 appears more clearly?
> * there is a clear visual improvement when one looks at the plots, 
> although the resolution is still about 0.63 MeV
> * now, 0.63 MeV/28.5 GeV = 2.2e-5, which is still not bad!

cheers:-)))))))))))))))))))

Michele