adding docs

docs/dust_extinction/choose_model.rst

@@ -183,8 +183,11 @@ used to fit measured extinction curves. These models allow features in the
 extinction curve to be measured (e.g., 2175 A bump or 10 micron silicate
 feature). The :class:`~dust_extinction.shapes.P92` is the most
 general as it covers the a very broad wavelength range. The
-:class:`~dust_extinction.shapes.FM90` model has been extensively used,
-but only covers the UV wavelength range. The
+:class:`~dust_extinction.shapes.FM90` model covers the UV wavelength range
+and has been extensively shown to fit all known UV extinction curves. 
+The :class:`~dust_extinction.shapes.FM90_B3` model provides a variant
+of the FM90 model that uses B3 instead of C3 as B3 = explicit 2175 A 
+bump height = C3/gamma^2.
 :class:`~dust_extinction.shapes.G21` model focuses on the NIR/MIR
 wavelength range from 1-40 micron.
 
@@ -194,6 +197,8 @@ wavelength range from 1-40 micron.
 +============+==============+==================+===================+
 | FM90 | 3.13 - 11.0 | 0.0912 - 0.32 | 6 |
 +------------+--------------+------------------+-------------------+
+| FM90_B3 | 3.13 - 11.0 | 0.0912 - 0.32 | 6 |
++------------+--------------+------------------+-------------------+
 | P92 | 0.001 - 1000 | 0.001 - 1000 | 19 (24 possible) |
 +------------+--------------+------------------+-------------------+
 | G21 | 0.025 - 1 | 1 - 40 | 10 |

docs/dust_extinction/model_flavors.rst

@@ -492,6 +492,9 @@ Shape fitting models
  These models are used to fit the detailed shape of dust extinction curves.
  The FM90 (Fitzpatrick & Mass 1990) model uses 6 parameters to fit the
  shape of the ultraviolet extinction.
+ Note there are two forms of the FM90 model, FM90 that implements the model
+ as published and FM90_B3 that B3 = C3/gamma^2 as the explicit amplitude of
+ the 2175 A bump (easier to interpret).
  The P92 (Pei 1992) uses 19 parameters to fit the shape of the X-ray to
  far-infrared extinction.
  The G21 (Gordon et al. 2021) models uses 10 parameters to fit the shape

dust_extinction/tests/helpers.py

@@ -13,7 +13,7 @@
  D22,
  G23,
 )
-from dust_extinction.shapes import FM90, P92, G21
+from dust_extinction.shapes import FM90, FM90_B3, P92, G21
 from dust_extinction.averages import (
  RL85_MWGC,
  RRP89_MWGC,
@@ -34,7 +34,7 @@
 param_ave_models_Rv = [CCM89, O94, F99, F04, VCG04, GCC09, M14, F19, D22, G23]
 param_ave_models_Rv_fA = [G16]
 param_ave_models = param_ave_models_Rv + param_ave_models_Rv_fA
-shape_models = [FM90, P92, G21]
+shape_models = [FM90, FM90_B3, P92, G21]
 ave_models = [
  RL85_MWGC,
  RRP89_MWGC,