# An easy approach to compute object counts from a H-Alpha Emission PN?

How can I approach computing the object counts (photons/sec) from a H-alpha emission planetary nebula or a diffuse h-alpha emitting object in general? I need it for an exposure time calculator mini project

I did find an example here: wfc3_ihb.book (stsci.edu)
Section 9.93 Example 3: Imaging an HII region in M83 in Hα
What is the exposure time needed to obtain a signal-to-noise of 10 for an HII
region in M83 in Hα which has a diameter of 2" and a flux Fλ in Hα of 5×10–16
ergs/cm2/s?
Although specific for HST … it is what I need to understand.
How do they get the Fλ value? Can I get it for an object of interest from a vizier catalog? I did search ESO Catalogue of Galactic Planetary Nebulae (Acker+, 1992) on Vizier and found Flux in H-alpha.
If getting the Flux from Vizier is not correct, should I use synphot to make a model spectrum?
Would something like this work?

``````# Retrieve Object H-alpha Flux from VizieR in erg/s/cm²/Å else do a conversion for the whatever flux units you find
h_alpha_flux = your_h_alpha_flux * units.erg / (u.s * u.cm**2 * u.Angstrom)

# Define H-alpha Wavelength
h_alpha_wavelength = 656.3 * u.nm

# Create filter for H-alpha
Hydrogen_filter = SpectralElement(Box1D, amplitude=1, x_0=6563, width=30)

# Create Source Spectrum for H-alpha
spectrum = SourceSpectrum(ConstFlux1D, wave=h_alpha_wavelength, flux=h_alpha_flux)

# Convolve Source Spectrum with Hydrogen_filter
observed_spectrum = Observation(spectrum, Hydrogen_filter)

# Integrate Over Wavelength to Calculate Photon Count Rate
integrated_flux = observed_spectrum.integrate(flux_unit=units.FLAM)
``````

Thanks

I went another route:

1. Get the spectra of a nebula
2. Get the spectra of the sky
3. Make a h-alpha observation with a hydrogen filter
4. Output the flux density for object and sky

Source

``````spN = SourceSpectrum.from_file('nebulaSpectra.csv')
spN.plot(flux_unit='flam',left=4000, right=9700)

# Create filter for H-alpha
Hydrogen_filter = SpectralElement(Box1D, amplitude=1, x_0=6563, width=30)
observe_N = Observation(spN, Hydrogen_filter)

fluxN = observe_N.effstim(flux_unit='flam')
# Convert flux density to W/m²/nm using spectral density
fluxN_in_Wmnm = fluxG.to(u.W / (u.m**2 * u.nm), u.spectral_density(Hydrogen_filter))
``````

Sky

``````spSky = SourceSpectrum.from_file('spectrum_data.txt')
spSky.plot(flux_unit='flam',left=4000, right=9000)

# Create filter for H-alpha
Hydrogen_filter = SpectralElement(Box1D, amplitude=1, x_0=6563, width=30)
observe_H2 = Observation(spSky, Hydrogen_filter)

flux2 = observe_H2.effstim(flux_unit='flam')
# Convert flux density to W/m²/nm using spectral density
flux3_in_Wmnm = flux2.to(u.W / (u.m**2 * u.nm), u.spectral_density(Hydrogen_filter))
``````

For source I get flux = 4.174644e−17 W/m^2/nm
For sky I get flux = 1.1655725e−19 W/m^2/nm

Does this approach make sense?