Conventions & gotchas

A few conventions cause silent mistakes if you get them wrong. Read them before fitting real data. The repository also ships a fuller misuse guide in GOTCHAS.md.

Metallicity is log10 of ABSOLUTE Z

The parameter Z (and ssp_lgmet) is log10(Z) in absolute units, not log10(Z/Z☉). Solar is roughly -1.85 (Z☉ ≈ 0.014), not 0.0.

The FSPS grid spans roughly [-4.0, -1.4]. Values outside it are silently clamped to the nearest grid edge, so a "solar" guess of Z = 0.0 is off the grid and gets clamped.

Common mistake

A prior like Uniform(low=-2.5, high=0.2) puts most of its mass off the grid. Use something like ClippedNormal(mean=-2.0, low=-4.0, high=-1.4). If you are unsure of your grid bounds, print them with print(float(csp.zmet.min()), float(csp.zmet.max())), or call csp.check_param_ranges(theta) to warn about out-of-grid values.

Lookback time increases with index (index 0 = today)

lookback_time element 0 is the present; the last element is the oldest bin, near the age of the universe. The sfh array is indexed the same way. The old decreasing convention (lookback = T_univ - t_grid) is rejected at construction with a ValueError, so do not reintroduce it, and do not reverse arrays "to be safe".

Units

Quantity	Unit
Model wavelength grid (csp.wave)	Å, vacuum, rest frame
Spectrum pixel wavelengths (data)	Å, vacuum, observed frame (model is redshifted onto them)
Lines wavelengths / mask_lines centres	Å, vacuum, rest frame (redshifted internally)
Model spectra	F_ν (per unit frequency)
Broadband fluxes	AB maggies
Emission-line fluxes	erg s⁻¹ cm⁻²
Stellar mass	logmass = log10(M⋆/M☉)

The forward model is evaluated at unit mass and scaled by 10**logmass.

SFH mass normalisation

The logsfr_ratios_to_sfh transform normalises the SFH so the trapezoidal integral of SFR over the lookback grid equals 1 M☉, and logmass sets the amplitude. Use the provided transform rather than hand-rolling it. Getting this wrong biases logmass by many dex.

theta is a dict, so typos are silently ignored

A mistyped key (logmas for logmass, dust2 for diffuse_tau_kc, and so on) is simply not read, so the parameter takes its default. predict() and get_spectrum_components() emit a warning listing unrecognised keys at trace time, at no cost to the hot path. Model-level free parameters like logsfr_ratios are registered automatically and do not warn.

FSPS at runtime

With add_neb=True or add_dust_emission=True, the forward model reads CLOUDY nebular grids and Draine & Li templates from $SPS_HOME. FSPS must be installed and $SPS_HOME set. See Installation.