Conclusion:

Wide open, and until f/2.8 it presents a lot of comma. At f/4.0 the image is good in all the field, although it begins to demonstrate longitudinal chromatic aberration (as being an achromatic lens only). At f/5.6 the image is almost perfect to my pleasure. With the vigneting passes something similar, becoming very little evident in the two last pics.

For astrophotography work, due to its good behavior and relative rapidity, I choose f/4.0 to work with this lens.