Hi, Two small problems: 1. Another issue with field parameters. I can't pass them to a slice if the derived field also needs ValidateSpatial. i.e. def _EffectivePotential(field, data): center = na.array(data.get_field_parameter("disk_center")) h_vec = na.array(data.get_field_parameter("disk_vector")) omega = na.array(data.get_field_parameter("thiscore_omega")) . . blah . . add_field("EffectivePotential", function=_EffectivePotential, validators=[ValidateSpatial(1, ["x-velocity", "y-velocity", "z-velocity", "x", "y", "z"])]) p = pc.add_slice("EffectivePotential", 2, field_parameters={"disk_vector":disk_vector, "disk_center":disk_center, "thiscore_omega":core_omega[c]}) can't find its field parameters but if I change the end line of the function to: add_field("EffectivePotential", function=_EffectivePotential) everything is fine. It actually seems to work OK even without the ValidateSpatial (I have cross products in the function so pain has the potential to happen without the full data) so perhaps this isn't a problem, but I think it is a small bug? 2. For some reason, yt has decided to save all my image files in: /home/tasker/.yt/my_plugins.py_Slice_z_EffectivePotential.png This is less than intuitive. Does anyone know why and how I can stop it? Thanks! Elizabeth