Reading the tomography graphs » History » Revision 4
Revision 3 (Anchi Cheng, 07/15/2010 09:11 PM) → Revision 4/14 (Anchi Cheng, 07/15/2010 09:15 PM)
h1. Reading the tomography graphs
The graphs found in http://your_host/myamiweb/tomography/ are very useful for trouble shooting.
Figure 1 shows a typical result.
*Figure 1*
!http://emg.nysbc.org/software/leginon/images/images/tomoxyz1.png!
* x and y axes contains the tilt axis and are referenced to columns and rows on the images acquired.z-axis is parallel to the electron beam.
* The "Prediction" data in the x and y-axis plot is the extrapolated result of the smooth curve fitting from preceding (usually 4) tilts to the current tilt angle.
* The "Position" is where features on the specimen end up to be relative to the origin of the image shift at the scope as determined by correlation of images. Both "Prediction" and "Position" are expressed in microns.
* The "Feature" is the shift of the feature on the image. It corresponds to the difference between "Prediction" and "Position" and is expressed in pixels of the image acquired.
* The z-axis "Prediction" is the correction of defocus, i.e., specimen z-height change, the current model suggests and applied for each tilt angle to keep the specimen focused. Originally, it is possible to measure defocus at each tilt and obtain "Measured" values in this plot. However, the function is currently disabled because the procedure does give accurate results.
By activate "show model parameters" check box in the viewer, more plots are shown (Figure 2).
*Figure 2*
!http://emg.nysbc.org/software/leginon/images/images/tomomodel1.png!
To first approximation, if the object tomography application try to track is perfect in xy but off in z ( off eucentric, shown as z0 in the model parameter), the movement of the z-projected object during tilting follows a sine curve. On the other hand, if the object is perfect in z but offset on the xy plane in direction perpendicular to the tilt axis, the movement of the projected object follows a cosine curve, as show here:
!tilt.png!
Therefore, the curve below on x-axis which is almost perpendicular to the tilt axis suggests the specimen is 10 um ( = 7 um / sin(45 degrees) ) off in z and the tilt axis passes quite close to the center of the CCD.
!zoff.png!