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<div><font color="#000000">Dear Colleagues,</font><br>
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<div><font color="#000000">We've known for years that the indicated
amount of defocus on our CM200 and CM300 was not accurate but was
reproducible. Since we generally only used a couple of different
magnifications, it was pretty straightforward to plot out the actually
values (linear) versus those indicated on the microscope.
Absolute precision was not really a requirement since the user
determined minimum focus point could also be slightly off the target
and a range of different defocus values was needed for the
reconstructions. Close was good enough and an accurate value
could be determined during the data processing phase. Recently
we've noticed a change in the slope of the plotted values and have
been using some previously unused magnifications. Some of the
indicated values can be quite far off the actual (underestimated by up
to a factor of 4). Again we can compensate but now I'm curious
if anyone knows of a way to alter the indicated defocus (similar to
changing the indicated magnification with a LensEdit program) or if it
is dependent upon any of the microscope calibrations. Also, how
do other labs handle this problem? </font></div>
<div><font color="#000000"><br>
All comments appreciated.<br>
<br>
Thanks,</font></div>
<div><font color="#000000">Paul </font><br>
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<div><font face="Courier New" size="-3" color="#000000">Paul
Chipman<br>
Director, Electron Microscopy Facility</font></div>
<div><font face="Courier New" size="-3" color="#000000">Project
Leader, Structural Virology EM Studies<br>
Dept. of Biology, Purdue University<br>
Lilly Hall, Rm. B216<br>
Phone: 765-494-1487<br>
Fax:765-496-1189</font></div>
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