Hi Bill, the picture does not show Fa" (as a vector), but the vector addition Fp+Fa+iFa" (it might be a naming convention of the picture to write Fa" instead of iFa", but that's a matter of taste really). Furthermore Fa" has the same phase as Fa plus the contribution of i, which corresponds to the rotation of 90deg. The phase of Fa results from the offset of the anomalous scatterer from the origin within the unit cell.
Does this answer your question? Tim On Wed, Oct 13, 2010 at 10:48:00AM -0700, William Scott wrote: > Hi Citizens: > > Try not to laugh. > > I have an embarrassingly simple MAD phasing question: > > Why is it that F" in this picture isn't required to be vertical (purely > imaginary)? > > http://www.doe-mbi.ucla.edu/~sawaya/tutorials/Phasing/phase.gif > > (Similarly in the Harker diagram of the intersection of phase circles, one > sees this.) > > I had a student ask me and I realized that there is this fundamental gap in > my understanding. > > Many thanks in advance. > > -- Bill > > > > > William G. Scott > Professor > Department of Chemistry and Biochemistry > and The Center for the Molecular Biology of RNA > 228 Sinsheimer Laboratories > University of California at Santa Cruz > Santa Cruz, California 95064 > USA > > phone: +1-831-459-5367 (office) > +1-831-459-5292 (lab) > fax: +1-831-4593139 (fax) -- -- Tim Gruene Institut fuer anorganische Chemie Tammannstr. 4 D-37077 Goettingen phone: +49 (0)551 39 22149 GPG Key ID = A46BEE1A
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