Angle of astigmatism convention

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 or nocaCTF (novaCTF has also the option to read CTFFIND4 output file). The angle of astigmatism in IMOD is equal to the angle in CTFFIND4 plus 90 degrees:

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