Add Hapke photometric model

[jianyangli]

This is a request for

• a new feature
• an enhancement to existing sbpy functionality
• somethings else: [explain here]

The requested changes will be implemented by

• me
• the sbpy developers

High-level concept

Add Hapke photometric model to sbpy.photometry submodule.

Explain the relevance to sbpy

The Hapke photometric model is in the original development plan for sbpy.

Proposal details

The Hapke photometric model will be implemented in the sbpy.photometry.hapke submodule. It will include both disk-resolved model and disk-integrated model. Given the complicated nature and various versions of the Hapke model, the design of the module is still TBD.

Example (pseudo-)code

An example API of the disk-resolved photometric model could be as follows:

from sbpy.photometry.hapke import Hapke

# generate model
model = Hapke(ssa=0.1, g=-0.2, theta=20*u.deg, B0=1, h=0.01)
# calculate bidirectional reflectance
bdr = model.bdr(i, e, a)
# calculate RADF
radf = model.radf(i, e, a)
# geometric albedo
alb_g = model.geomalb
# Bond albedo
alb_b = model.bondalb