EMAN
Refine 3d Model
Determine CTF parameters
Once the particles are boxed we may use ctfit
to determine the CTF parameters. Knowing the CTF parameters is necessary
in order to classify the particles and to
align the particles
within these classes. The CTF parameters include microscope parameters
(voltage, spherical aberration coefficient, the defocus, B-factor and amplitude contrast)
as well as the parameters that describe the noise present in the images
(4 parameters using an empirical model). This information is written into the headers for
the particle files and are read automatically by the classification and alignment programs.
The phase-flipping portion of the CTF correction process is also
performed here. While this could be postponed until the amplitudes
are corrected later in the reconstruction process, correcting the phases
at this point means the particles can be used in reconstructions without
CTF amplitude correction, and produce more accurate (but not perfect) results.
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Last Modified: Feb 7 14:43:58 CST 1999