# Units system¶

The package makes use of the astropy.units package to handle units and unit conversions. Several of the arguments of the functions and classes of Naima require Quantity instances. Defining quantities is straightforward:

from astropy import units as u

# Define scalar quantity
q1 = 3. * u.kpc

# Define array quantity using a list
q2 = [1., 2., 3.] * u.arcsec

# Define array quantity using a Numpy array
q3 = np.array([1., 2., 3.]) * u.cm ** 2 / u.g


## A note on physical types¶

Units defined through astropy.units.Unit have an associated physical type. Naima defines a few additional physical types to those defined in astropy.units. They are used internally to check that the inputs have the correct physical type and can be converted to the appropriate units. These are:

• flux: convertible to $$\mathrm{erg\,cm^{-2}\,s^{-1}}$$
• differential flux: convertible to $$\mathrm{1/(s\,cm^2\,eV)}$$
• differential power: convertible to $$\mathrm{1/(s\,eV)}$$
• differential energy: convertible to $$\mathrm{1/eV}$$