[3DEM] microscope indicated defocus values
Paul Chipman
paulrc at bilbo.bio.purdue.edu
Wed May 23 09:13:59 PDT 2007
Dear Colleagues,
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?
All comments appreciated.
Thanks,
Paul
--
Paul Chipman
Director, Electron Microscopy Facility
Project Leader, Structural Virology EM Studies
Dept. of Biology, Purdue University
Lilly Hall, Rm. B216
Phone: 765-494-1487
Fax:765-496-1189
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