Photometry Module (sbpy.photometry)

Introduction

photometry provides routines for photometric phase curve modeling of small bodies, and for retrieving sbpy’s built-in filter bandpasses.

Disk-integrated Phase Function Models

Disk-integrated phase function refers to the brightness or reflectance of an asteroid, often measured in magnitude, with respect to phase angle. The IAU adopted a number of models to describe the phase function of asteroids, including the two-parameter H, G system (Bowell et al. 1989), the three-parameter H, G1, G2, and the two-parameter H, G12 system derived from the three-parameter system (Muinonen et al. 2010). The H, G12 system was further revised by Penttilä et al. (2016). photometry provides classes for all four models HG, HG1G2, HG12, and HG12_Pen16, respectively.

The photometric model class can be initialized with the default parameters, by supplying the model parameters as either dimensionless numbers or Quantity:

>>> import astropy.units as u
>>> from sbpy.photometry import HG, HG1G2, HG12, HG12_Pen16
>>> m = HG()
>>> print(m)
Model: HG
Inputs: ('x',)
Outputs: ('y',)
Model set size: 1
Parameters:
     H   G
    --- ---
    8.0 0.4

>>> m = HG(H = 3.34 * u.mag, G = 0.12, radius = 460 * u.km, wfb = 'V')
>>> print(m)
Model: HG
Inputs: ('x',)
Outputs: ('y',)
Model set size: 1
Parameters:
     H    G
    mag
    ---- ----
    3.34 0.12

The calculations that involve conversion between magnitude and reflectance requires valid object size and wfb (wavelength/frequency/band) parameter to be set for the photometric model. The corresponding solar flux at the wfb of the photometric model object has to be available through calib system, or set by set, which works with context management with syntax.

Calculate geometric albedo, Bond albedo, and phase integral:

>>> import astropy.units as u
>>> from sbpy.calib import solar_fluxd
>>> solar_fluxd.set({'V': -26.77 * u.mag})
<ScienceState solar_fluxd: {'V': <Quantity -26.77 mag>}>
>>> print(m.geomalb)  
0.09557298727307795
>>> print(m.bondalb)  
0.03482207291799989
>>> print(m.phaseint)  
0.3643505755292945

Note that the current version of astropy.modeling.Model doesn’t support astropy.units.MagUnit instance as model parameters. For now one has to use the dimensionless magnitude mag in the phase function parameter, and manually set solar flux in order for the conversion between magnitude and reflectance to work.

The model class can also be initialized by a subclass of sbpy’s DataClass, such as Phys, as long as it contains the model parameters:

>>> from sbpy.data import Phys
>>> phys = Phys.from_sbdb('Ceres')
>>> m = HG.from_phys(phys)
INFO: Model initialized for 1 Ceres (A801 AA). [sbpy.photometry.core]
>>> print(m)                          
Model: HG
Inputs: ('x',)
Outputs: ('y',)
Model set size: 1
Parameters:
     H    G
    mag
    ---- ----
    3.31 0.12

Note that in this case, model set is not supported. Only one model can be initialized with the first set of valid parameters in the input DataClass.

To fit a photometric model, one may follow the standard procedure defined in astropy.modeling submodule, first initializing a model, then using one of the fitter classes defined in astropy.modeling.fitting submodule, such as SLSQPLSQFitter. Note that the HG1G2 model requires that the G1 and G2 parameters are bounded within [0, 1], as well as an inequality constraint, 0 <= 1 - G1 - G2 <= 1. These constraints are implemented in sbpy via the bounds parameter of Parameter and the ineqcons parameter of Model. Some fitters, such as astropy.modeling.LevMarLSQFitter, do not support constrained fit via the ineqcons parameter, though.

>>> import numpy as np
>>> import astropy.units as u
>>> from astropy.modeling.fitting import SLSQPLSQFitter
>>> # generate data to be fitted
>>> model1 = HG(3.34 * u .mag, 0.12)
>>> alpha = np.linspace(0, 40, 20) * u.deg
>>> mag = model1(alpha) + (np.random.rand(20)*0.2 - 0.1) * u.mag
>>> # fit new model
>>> fitter = SLSQPLSQFitter()
>>> model2 = HG()
>>> model2 = fitter(model2, alpha, mag, verblevel=0)

Alternatively, one may use the class method from_obs to initialize a model directly from an Obs object by fitting the data contained therein.

>>> # use class method .from_obs
>>> from astropy.modeling.fitting import SLSQPLSQFitter
>>> fitter = SLSQPLSQFitter()
>>> from sbpy.data import Obs
>>> obs = Obs.from_dict({'alpha': alpha, 'mag': mag})
>>> model3 = HG12.from_obs(obs, fitter, 'mag', verblevel=0)

One can also initialize a model set from multiple columns in the input Obs object if it contains more than one columns of brightness measurements. The columns to be fitted are specified by a keyward argument fields. By default, the column 'mag' will be fitted.

>>> # Initialize model set
>>> model4 = HG(5.2 * u.mag, 0.18)
>>> mag4 = model4(alpha) + (np.random.rand(20)*0.2 - 0.1) * u.mag
>>> fitter = SLSQPLSQFitter()
>>> obs = Obs.from_dict({'alpha': alpha, 'mag': mag, 'mag1': mag4})
>>> model5 = HG.from_obs(obs, fitter, fields=['mag', 'mag1'], verblevel=0)

Filter Bandpasses

A few filter bandpasses are included with sbpy for internal tests and your convenience. The function bandpass will return the filter transmission as a SpectralElement (requires synphot):

>>> from sbpy.photometry import bandpass
>>> bp = bandpass('Cousins R')
>>> print(bp.avgwave())    
6499.914781904409 Angstrom

For other bandpasses, obtain the photon-counting relative spectral response curves as a two-column file. If the first column is wavelength in Angstroms, and the second is the response, then read the file with:

>>> from synphot import SpectralElement             
>>> bp = SpectralElement.from_file('filename.txt')  

See synphot.spectrum.SpectralElement for other options and file format details.

Reference/API

sbpy Module for small body photometry

Functions

bandpass(name)

Retrieve bandpass transmission spectrum from sbpy.

Classes

DiskIntegratedPhaseFunc(*args[, radius, wfb])	Base class for disk-integrated phase function model
HG(*args, **kwargs)	HG photometric phase model (Bowell et al. 1989).
HG12([H, G12])	HG12 photometric phase model (Muinonen et al. 2010).
HG12_Pen16(*args, **kwargs)	Revised H, G12 model by Penttil"a et al. (2016).
HG1G2(*args, **kwargs)	HG1G2 photometric phase model (Muinonen et al. 2010).
InvalidPhaseFunctionWarning
LinearPhaseFunc(H, S, **kwargs)	Linear phase function model

Class Inheritance Diagram