  In conventional tomographic reconstruction, the pre-processing step includes
flat-field correction, where each sample projection on the detector is divided
by a reference image taken without the sample. When using coherent X-rays as
probe, this approach overlooks the phase component of the illumination field
(probe), leading to artifacts in phase-retrieved projection images, which are
then propagated to the reconstructed 3D sample representation. The problem
intensifies in nano-holotomography with focusing optics, that due to various
imperfections creates high-frequency components in the probe function. Here, we
present a new iterative reconstruction scheme for holotomography,
simultaneously retrieving the complex-valued probe function. Implemented on
GPUs, this algorithm results in 3D reconstruction resolving twice thinner
layers in a 3D ALD standard sample measured using nano-holotomography.
