Angle of astigmatism convention

The convention for the angle of astigmatism, as is it is defined in IMOD and novaCTF, differes from the convention used in CTFFIND4. This difference should be considered when the defocus was estimated in CTFFIND4 and is then used in IMOD/novaCTF. The angle of astigmatism in IMOD/novaCTF is equal to the angle in CTFFIND4 plus 90 degrees (see figure below). It should be noted, that novaCTF has the option to read the format of CTFFIND4 output files.

The angle of astigmatism in IMOD is equal to the angle in CTFFIND4 plus 90 degrees. IMOD measures the angle of the smaller defocus (f1) while CTFFIND4 measures the angle of the larger defocus (f2).