Plot scattering cross section

[jtgasparik]

Function and associated pytest-mpl to plot the differential scattering cross section of a particle integrated over the solid angle of light collection, $\Delta C_{\rm sca}$, computed from computed parameters $\tau$ and $\sigma$.

[Copilot]

Pull request overview

This PR adds a new visualization helper to plot the differential scattering cross section, (\Delta C_{\rm sca}), derived from the Moteki & Kondo (\tau)/(\sigma) fit outputs, and introduces a pytest-mpl image-comparison test to validate the plot output.

Changes:

• Added plot_scattering_cross_section(...) to compute and plot (\Delta C_{\rm sca}(t)) using the normalized incident irradiance profile.
• Added a new pytest-mpl test (test_plot_scattering_cross_section) that runs the tau/sigma workflow and snapshots the resulting figure.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 2 comments.

File	Description
pysp2/util/normalized_derivative_method.py	Adds plot_scattering_cross_section to compute and plot the derived scattering cross section from Moteki & Kondo outputs.
tests/test_vis.py	Adds a pytest-mpl image regression test that exercises the new plotting function end-to-end.

Comments suppressed due to low confidence (2)

pysp2/util/normalized_derivative_method.py:1358

• The time-axis conversion for time_units='s' is inverted: the code multiplies by 1e6 (microseconds) while labeling seconds. This makes the x-axis values incorrect and also forces the incident-irradiance evaluation to use inconsistent units.

    if time_units == "us":
        t_plot = t
        x_label = r"Time ($\rm \mu$s)"
    elif time_units == "s":
        t_plot = t * 1e6

pysp2/util/normalized_derivative_method.py:1285

• Docstring defaults/units don’t match the actual signature: calibration_constant defaults to 1.22e-17/2.44, and h is treated as a bin width in microseconds (consistent with compute_normalized_incident_irradiance_moteki_kondo() and plot_incident_irradiance), not seconds.

    calibration_constant : float, default 1.22e-17  # (m²/2.44 mV)
        Scale factor applied to S / (I/I0).
        Default value from Moteki & Kondo (2008).
    h : float, default 0.4e-6
        Sampling interval in seconds.

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[rcjackson]

Copilot gave good suggestions this time. I have nothing else to add! Merging after tests pass.