When do Cs values change?

Hi @stephan!

If extracted particles(from same data collection set) were split in two groups and homogeneous refinement run with optimization of CTF parameters and afterwards the set combined again for homogeneous refinement again with optimization of CTF parameters, would that be a valid reason for the Cs values to change?
Global CTF refinement and Fit Spherical Aberration are suppose to refine the Cs value, right?


Hi @AndreGraca,

If the particles were indeed split into two separate groups with different exposure group ID’s, then homogeneous refinement with “Fit Spherical Aberration” enabled would refine the spherical aberration independently for each group, potentially allowing the cs_mm value to differ across the two groups. However, within each group, cs_mm should still be constant.

Are you noticing that this is not the case, or otherwise encountering any errors/unexpected results after combining the two groups?


Hi @mmclean

In the beggining the particle stack as one exposure ID, but I assume that when two particle stacks are split (let’s say after ab initio class separation) and 3D refined separetly (where CTF corrections were enabled) and joined later for a common 3D refinement, it is as if the particle stacks have different origins regarding CTF corrections. This was the case when I got the error

“AssertionError during CTF Refinement job: Field ctf/cs_nm is not constant in group 0”

If this is the expected behaviour, then I think we can close this topic.


Hi @AndreGraca ,

Yes – in order to have the desired behaviour, you must import particles from each exposure group with different exposure group IDs