Flux and magnitude conversion

In this tutorial we will use the F115W filter of JWST/NIRCam to convert a magnitude into a flux and vice versa. The magnitudes are defined in the Vega System with the magnitude of Vega set to 0.03 for each filter.

Getting started

We start by importing species and initiating the database.

import species
species.SpeciesInit()

Initiating species v0.5.3... [DONE]
Creating species_config.ini... [DONE]
Database: /Users/tomasstolker/applications/species/docs/tutorials/species_database.hdf5
Data folder: /Users/tomasstolker/applications/species/docs/tutorials/data
Working folder: /Users/tomasstolker/applications/species/docs/tutorials
Grid interpolation method: linear
Creating species_database.hdf5... [DONE]
Creating data folder... [DONE]

<species.core.init.SpeciesInit at 0x10d1e2f50>

Magnitude to flux

We now create an instance of SyntheticPhotometry with the filter name as listed by the SVO Filter Profile Service.

synphot = species.SyntheticPhotometry('JWST/NIRCam.F115W')

Adding filter: JWST/NIRCam.F115W... [DONE]

To covert from apparent magnitude to flux, we use the magnitude_to_flux method. The zero-point flux can be provided but is otherwise calculated from a flux-calibration Vega spectrum. Both the filter profile and the Vega spectrum will be downloaded and stored into the database. Let’s calculate the flux for a magnitude of 15 +/- 0.2.

flux, error = synphot.magnitude_to_flux(15., error=0.2, zp_flux=None)
print(f'Flux (W m-2 um-1) = {flux:.2e} +/- {error:.2e}')

Downloading Vega spectrum (270 kB)... [DONE]
Adding Vega spectrum... [DONE]
Flux (W m-2 um-1) = 4.08e-15 +/- 7.56e-16

Flux to magnitude

To convert backwards from flux to magnitude, we use the flux_to_magnitude method. When the parallax (in mas) or distance (in pc) is provide, both the apparent and absolute magnitude are calculated. The uncertainty on the parallax/distance is propagated into the uncertainty of the absolute magnitude.

app_mag, abs_mag = synphot.flux_to_magnitude(flux, error=error, parallax=(10., 1.))
print(f'Apparent magnitude = {app_mag[0]:.2f} +/- {app_mag[1]:.2f}')
print(f'Absolute magnitude = {abs_mag[0]:.2f} +/- {abs_mag[1]:.2f}')

Apparent magnitude = 15.00 +/- 0.20
Absolute magnitude = 10.00 +/- 0.30

As expected, we obtain again the magnitude that we started out with.

Zero point flux

Finally, let’s see what the zero-point flux (i.e. the flux of Vega) is for the JWST/NIRCam.F115W filter.

zp_flux = synphot.zero_point()
print(f'Zero point flux (W m-2 um-1) = {zp_flux:.2e}')

Zero point flux (W m-2 um-1) = 3.97e-09

This flux is indeed very similar to the zero point that is provided on the website of the SVO Filter Profile Service: 3.99e-10 erg cm\(^{-2}\) s\(^{-1}\) A\(^{-1}\).