Hi YT users, I would like to make a (spherical and/or cylindrical) profile of the velocity dispersion. In order to do so, I need to compute the mean velocity (BulkVelocity) for a given shell (or ring), and pass this value to newly defined velocity dispersion field, and then make a profile for the velocity dispersion field. Is there a particular way in yt to handle this? Junhwan P.S.: A few weeks ago I posted a similar question and Matt gave me a following hint. @derived_field(name = "deltaLocalCircularVelocity") def delta_LocalCircularVelocity(field, data): fn, x, cv = data.get_field_parameter("local_circular_velocity") cv0 = na.interp(data[fn].ravel(), x, cv) cv0 = cv0.reshape(data[fn].shape) delta_cv = cv0 - data["LocalCircularVelocity"] return delta_cv You'll need to do something like this on the data object from which you are getting your data: sp.set_field_parameter("local_circular_velocity", (fn, prof[fn], prof["LocalCircularVelocity"])) However, it does not work for me and it is also complicate to understand and modify it. -- -------------------------------------------------------------- Jun-Hwan Choi, Ph.D. Department of Physics and Astronomy, University of Kentucky Tel: (859) 897-6737 Fax: (859) 323-2846 Email: jhchoi@pa.uky.edu URL: http://www.pa.uky.edu/~jhchoi --------------------------------------------------------------