This pull request implements a simple model for the spatially varying CTF in tilted images. The model is described in
Philippsen, Engel, and Engel, ‘The Contrast-Imaging Function for Tilted Specimens’, Ultramicroscopy 107, no. 2 (1 February 2007): 202–12, https://doi.org/10.1016/j.ultramic.2006.07.010.
This pull request implements a simple model for the spatially varying CTF in tilted images. The model is described in