[3dem] TEM: object distance

Philip Köck Philip.Koeck at ki.se
Mon Feb 21 06:07:14 PST 2011


I have a rather technical question concerning TEM imaging.

In the weak phase object approximation (for high resolution imaging
of thin biological specimens) one assumes that the back focal plane
contains the Fourier transform of the exit wave.
According to Fourier optics (e.g. in Goodman) this is only strictly
true if the object is placed in the front focal plane (moving the
image plane to infinity). Otherwise one has to deal with additional
quadratic phase factors.

My question: How close to the front focal plane is the object plane
in a properly aligned Tem (when imaging at about 1000 nm underfocus)?
Is it close enough to approximate the diffraction pattern by a simple FT
without extra phase factors?
If I assume a magnification of about 50, then the lens law gives me a ratio
of about 1.02 between the object distance and the focal length. Does that
seem reasonable?


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