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Dear all,

we're recently facing a very strange behavior on one of our spinning 
disk systems. One of our users noticed a repeated fluctuation in 
recorded timelapse experiments (this was a FRAP experiment, but we don't 
think that's related to FRAP).

The timelapse was recorded every 6 seconds, and after background 
subtraction a repeating drop of signal is visible in the intensity plot 
(please find the corresponding graph under [1], see below). The 
fluctuation is also visible by eye when scrolling through the timelapse.

We noticed a frequency of 66 seconds and therefore think we can exclude 
any external vibrations as source of this problem (plus the system is 
mounted on a Melles Griot optical table). On some experiments the 
oscillation almost follows a sine-like function, on others (like the 
mentioned one) it looks more rectangular with a very rapid drop and 
raise. Also, the frequency does *not* change if we double the 
acquisition frequency, it's still on 66 seconds when acquiring every 3 
seconds.

One very weird thing is the stability of the background. The plots from 
above show the intensity in selected ROI's over cells. The same plot for 
an "empty" region in the sample shows a more or less constant signal (at 
least we could figure a pattern in it, even after normalization and 
using a big scaling factor to visualize the deltas).

Has anyone of you come across a similar behavior?

Any ideas about this would be highly appreciated!

Many Thanks,
Niko

[1] 
https://www.biozentrum.unibas.ch/fileadmin/redaktion/Forschung/Research_Groups/IMCF/3i_intensity_fluctuation.png

-- 
Niko Ehrenfeuchter
Imaging Core Facility
Kragenbau, Room G1055
Biozentrum, University of Basel
Klingelbergstrasse 50/70
CH-4056 Basel (Switzerland)

Office:  +41 (61) 26 72673
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