From: Tom Laue <tom.laue@unh.edu>
  To  : Neil Errington <neil.errington@nottingham.ac.uk>
  Date: Mon, 6 Aug 2001 09:38:13 -0400

RE: wiggles in dc/dt plots

Hi Folks,
The wiggles referred to can come from a couple of sources.
1- Jitter in timing the laser pulse: there was a firmware problem in setting
up the timing of the laser pulse. The software loop to do this required ~1.5
milliseconds, which is longer than the period of rotation at high rotor
speeds, and caused problems in data collected at rotor speeds above ~55,000
rpm. There was a firmware fix released about 18 months ago that fixed this
problem. Be sure you have the most recent version of the EPROMs in the XLI.
2- There is an outstanding issue with the heaters and baseplate flexing that
David Yphantis and Jeff Lary have documented. This problem is likely due to
the mounting of this lens to the baseplate... a problem stemming from my
design. The problem was reported to BCI four years ago, and I have been told
that BCI has a possible fix. To my knowledge the fix hasn't seen the light
of day. A related problem is caused by thermal gradients across this lens,
resulting in a 'twisting' of the fringe image. The fix from BCI should help
this problem, too. The magnitude of these fluctuations is about 0.01-0.1
fringe, far worse than the intrinsic capabilities of the design (~0.0006
fringe).
3- The Fourier analysis of the fringe patter can result in oscillations in
the reduced data. The cause is a mismatch in the frequency of analysis and
the spatial frequency of the fringes, and it leads to a periodic oscillation
in concentration calculation. Typically, the magnitude of this oscillation
is 0.003 fringe or so, though it can be worse if the image is not good (i.e.
there is dust on the camera). This problem can be minimized by calibration
of the pixels per fringe (available under the detail settings for the
interference optics). An even better fix is to replace the camera with one
containing more pixels in the vertical direction, though this adds a
one-time cost of about $12,000. David Yphantis has shown that with a larger
array, and elimination of the temperature fluctuations will result in at
least a ~100-fold  improved precision. Are you interested in having the
improvements available?
Best wishes,
Tom

University of New Hampshire
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