[yt-users] Creating a projection for a derived particle field

Hi yt users! Does anybody know how to create a projection for particles using another field rather than "particle_mass"? I was using this: proj = ds.proj('density', 2,data_source=disk_dd,method='sum') frb = proj.to_frb(width, res, center=[0.5,0.5,0.5]) M_star=frb[('deposit', 'young_stars_mass')].in_units('Msun') But now I would like to do this with a particle mass field that depends on the characteristic timescales of the particles rather than the default field. Cheers! José Utreras