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This also doesn't include the "slippage resolution." That is, if they're using the full undulator, then by the end the x-rays can have slipped out of the electron slice by ~3 fs for soft x-rays. Obviously not a small number if trying to make 5 fs pulses. They've been advised to not use the full undulator when shorter pulses are more important than number of photons.

Algorithm Details

(courtesy of Mihir Mongia)

We believe XTCAVRetrieval.SetCurrentEvent(evt) calls the following three "steps":

Routine ProcessShotStep1:

  • overall goal: subtract dark, denies, signal ROI, bunch split
  • subtract dark image
  • run denoting algorithm median filter
  • median filter (for smoothing):
    • looks at pixel and its neighbors (number can be specified in some manner not yet understood)
    • take median of that set and set the center pixel value to the median
  • look at noise region
  • subtract mean of noise from whole image
  • anything >10 (can be over-ridden) standard deviation of noise, keep it.  < 10 stddevs set it to zero.
  • normalize image so sum=1
  • assumes dark image is larger than the shot image ROI (xmin,xmax,ymin,ymax probably coming from EPICS)
  • looks at max value in normalized image
  • takes all pixels>0.2*max then you "stay" in the ROI.  the software "draws a rectangle" around these pixel to keep ROI rectangular
  • expands rectangle dimensions by 2.5 (from the center) (user-settable) to bring in all interesting pixels into ROI, with a high likelihood
  • calls splitimage (says 'not done' in code?) to handle the bunches
    • this calls IslandSplitting which calls scipy.measurement.label on a boolean image, where the threshold for computing the boolean is zero (this is not really Otsu's method, we believe)
  • splitimage returns a 3D array where first dimension is the bunch (i.e. a set of rectangular ROIs)

ProcessShotStep2

  • overall goal: convert x to time, and y to energy (calibration, probably use EPICS variables)

ProcessShotStep3

  • overall goal: calculate power profile (power, time arrays)
  • calculate center-of-mass vector (1 number per time bin) and ERMS (energy RMS).  this may be related to the current-projection.
  • take the lasing-on image project onto time axis to get the current, and normalize
  • loop through the lasing-off images and do dot-products to find the most similar
  • subtract the lasing-on center-of-mass vector from lasing-off vector to get the power, and similarly for the sigma method (although for some reason the sign of the subtraction is opposite).
  • things not understood: bunch delay
  • calculate the power profile (delta and sigma methods)

Then Call XRayPower

  • averages the delta/sigma results (not weighted)