Specialized Quantities¶

Beyond astropy.units.Quantity, astropy provides further specialized quantities. astropy-xarray captures this using the units.class attribute.

import xarray as xr
from astropy import units as u

Logarithmic Units and Quantities¶

Astropy provides a generic LogQuantity and other various log scale specialized classes and units:

\[scalar = A * log_{B}(\frac{V}{V_0})\]

Name	Equiv SI Unit	Scale Factor	Log Base	0 Reference
Magnitude (absolute)	Unitless	-2.5	10	Apparent brightness at 10pc away
Dex	Unitless	1	10	1
Decibels (power)	Unitless	10	10	Variable
Decibels (amplitude)	Unitless	20	10	Variable
Magnitude (apparent)	Unitless	-2.5	10	Usually Vega or AB from Earth
Magnitude (AB)	Jy	-2.5	10	3631 Jy

Custom Unit References¶

import types

import astropy.units
import astropy.units as u
import numpy as np
from astropy.units import Dex, Equivalency, Magnitude, Quantity


# extend registry
def solar():
    solar = types.ModuleType("solar")

    # Solar relative metalicity
    astropy.units.def_unit(
        ["solH", "Xsun"],
        format={"latex": "X_{\\odot}", "unicode": "X☉"},
        namespace=solar.__dict__,
    )
    astropy.units.def_unit(
        ["solHe", "Ysun"],
        format={"latex": "Y_{\\odot}", "unicode": "Y☉"},
        namespace=solar.__dict__,
    )
    astropy.units.def_unit(
        ["solMetal", "Zsun"],
        format={"latex": "Z_{\\odot}", "unicode": "Z☉"},
        namespace=solar.__dict__,
    )

    return solar


if not hasattr(u, "solar"):
    u.solar = solar()
    u.add_enabled_units(u.solar)
    u.__dict__.update(u.get_current_unit_registry().registry)