More example usage#

Notebooks#

check notebooks in the examples directory in the repository.

You can also run these online

Basic example#

This example reads a spectrum from a fits file and calculate some photometry using units to the calculations.

from astropy.io import fits
from pyphot import unit
import pyphot

wavelength, flux = fits.getdata("foo.fits").T
wavelength  = wavelength * unit['AA']
flux = flux * unit['erg/s/cm**2/AA']
lib = pyphot.get_library()
f = lib['HST_WFC3_F110W']
mag = -2.5 * np.log10(f.get_flux(wavelength, flux)) - f.Vega_zero_mag

Unit conversions#

Pyphot contains a simple unit package based on pint. (One day maybe astropy) It allows handling wavelength and flux units very easily.

from pyphot import Vega

vega = Vega()
wavelength_angstrom = vega.wavelength
wavelength_nm = vega.wavelength.to('nm')
flux_flam = vega.flux
flux_other = flux_flam.to('W / (m**2 * angstrom)')

Number of photons through a given passband#

The following examples gives the number of photons that a passband would collect for the Sun at 10 pc.

from pyphot import Sun, unit
with Sun(distance=10 * unit['pc']) as v:
    print(vband.get_Nphotons(v.wavelength, v.flux))

11.985932608649655 photon / angstrom

Defining your own passband#

Let’s suppose you’re interested in defining bandpasses (here tophat functions) in Pyphot to determine the flux/magnitude in regions. This may be useful to determine the S/N ratio or integrated flux over a given wavelength range.

from pyphot import (unit, Filter)

wave = [4499, 4500, 4700, 4701] * unit['AA']
transmit = [0., 1., 1., 0.]
tophat = Filter(wave, transmit, name='tophat', dtype='photon', unit='Angstrom')
flux_tophat = tophat.get_flux(wavelength, flux)

Tip

The unit in the filter declaration is redundant with the unit on wave in this example.
the transmission is unitless. Its scaling factor will not affect the flux/magnitude calculations as these are implicitly normalizing the passband (see the equations) but will affect the number of photon \(photons/s/cm^2\)

Dust attenuated SED modeling#

The associated notebook can be found online. It requires a bit more than just Pyphot, but it demonstrates how to integrate Pyphot into larger projects.

Flux density units#

What are the expected units on the spectra?

This main version of Pyphot assumes that the spectral density is in the units of the wavelength of the spectrum. Also it expects that the flux is provided in flam, i.e., \(erg/s/cm^2/\unicode{x212B}\).

Conversions can be done using pyphot.unit, however it may be tedious sometimes.

if you use Astropy, you can use it to convert flux densities from frequencies to flam

from astropy import units as u

flux = u.Quantity(spec['flux']).to(pyphot.Unit('flam'),
        equivalencies=u.spectral_density(u.Quantity(spec['wave']))
        )

Check the examples associated with this new version online in the examples directory in the repository.