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 (see configuration file).

Getting started

We start by importing species and initiating the database.

from species import SpeciesInit
from species.phot.syn_phot import SyntheticPhotometry

SpeciesInit()

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species
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Version: 0.10.3.dev11
Working folder: /Users/tomasstolker/applications/species/docs/tutorials

Creating species_config.ini... [DONE]
Creating species_database.hdf5... [DONE]
Creating data folder... [DONE]

Configuration settings:
   - Database: species_database.hdf5
   - Data folder: data
   - Magnitude of Vega: 0.03

Multiprocessing: mpi4py installed
Process number 1 out of 1...

<species.core.species_init.SpeciesInit at 0x109fe30e0>

Magnitude to flux

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

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

Downloading data from 'https://archive.stsci.edu/hlsps/reference-atlases/cdbs/current_calspec/alpha_lyr_stis_011.fits' to file '/Users/tomasstolker/applications/species/docs/tutorials/data/alpha_lyr_stis_011.fits'.

100%|████████████████████████████████████████| 288k/288k [00:00<00:00, 219MB/s]

Adding spectrum: Vega
Reference: Bohlin et al. 2014, PASP, 126
URL: https://ui.adsabs.harvard.edu/abs/2014PASP..126..711B

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)
print(f'Flux (W m-2 um-1) = {flux:.2e} +/- {error:.2e}')

Flux (W m-2 um-1) = 4.04e-15 +/- 7.48e-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. The value is stored by the zero_point attribute of the SyntheticPhotometry object.

print(f'Zero point flux (W m-2 um-1) = {synphot.zero_point:.5e}')

Zero point flux (W m-2 um-1) = 3.92908e-09

This flux is indeed very similar to the zero point that is provided on the website of the SVO Filter Profile Service: 3.92904e-10 erg cm\(^{-2}\) s\(^{-1}\) A\(^{-1}\).