[BUG] consistent offset and lobes in residual

[nstarman]

Description

Figure out why and solve!

Expected behavior

• dependent on l, m max
• dependent on the grid

Actual behavior

• dependent on l, m max : it's not
• dependent on the grid : maybe it is?

Steps to Reproduce

# THIRD PARTY
import agama
import astropy.coordinates as coord
import astropy.units as u
import numpy as np
from astropy.visualization import quantity_support
from galpy import df as gdf
from galpy import potential as gpot

# PROJECT-SPECIFIC
from discO import (
    GaussianMeasurementErrorSampler,
    PotentialFitter,
    PotentialSampler,
    conf,
)
from discO.plugin.agama.fitter import (
    AGAMAMultipolePotentialFitter,
    AGAMAPotentialFitter,
)
from discO.core.pipeline import Pipeline
from discO.core.residual import GridResidual

quantity_support();

mass = 1e12 * u.solMass
r0 = 10 * u.kpc  # scale factor

agama.setUnits(mass=1, length=1, velocity=1)

hernquist_pot = gpot.HernquistPotential(amp=2 * mass, a=r0)
sampler = PotentialSampler(gdf.isotropicHernquistdf(hernquist_pot))
measurer = GaussianMeasurementErrorSampler(c_err=1*u.percent)  # 10% error
fitter = PotentialFitter(None, key="agama", pot_type="multipole", symmetry="s", gridsizeR=50, lmax=0, mmax=0)

_x = np.linspace(-5, 5, num=50)
x, y, z = np.meshgrid(_x, _x, _x) * u.kpc
points = coord.CartesianRepresentation(x, y, z)
residualer = GridResidual(points, original_pot=hernquist_pot)

pipe = Pipeline(sampler=sampler, fitter=fitter, residualer=residualer)
pipe

resid = pipe(n=10000, frame=coord.Galactocentric(), random=0)


from mpl_toolkits.mplot3d import axes3d
import matplotlib as mpl
import matplotlib.pyplot as plt
import matplotlib.cm as cm
import numpy as np

fig = plt.figure()
ax = fig.gca(projection='3d')

rar = np.abs(resid.represent_as(coord.SphericalRepresentation).vf_distance.flatten().value)
norm = mpl.colors.Normalize(vmin=min(rar), vmax=max(rar), clip=True)
mapper = cm.ScalarMappable(norm=norm, cmap=cm.inferno_r)

uvw = resid.vf_xyz.value * 1e13

qv = ax.quiver(
    x, y, z, *uvw,
    color=mapper.to_rgba(rar, alpha=0.1),
    length=0.1, )
ax.set_xlim(-8, 8)
ax.set_ylim(-8, 8)
ax.set_zlim(-8, 8)

plt.colorbar(qv)

plt.show()

System Details

macOS-10.16-x86_64-i386-64bit
Python 3.8.2 | packaged by conda-forge | (default, Apr 24 2020, 07:56:27)
[Clang 9.0.1 ]
astropy 4.3.dev310+g59d625df4.d20201220
astroquery 0.4.2.dev6458
Matplotlib 3.2.1
Numpy 1.18.4
Scipy 1.4.1

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