Specify the spectral distribution

In this section, the user is offered a choice of one of the following spectral distributions of an astronomical object, F(λ) (in erg/(cm² s Å) for a point source and erg/(cm² s Å arcsec²) for an extended source):

1. Upload your own spectrum file.

The user can upload file in two formats:

- FITS format, in which only wavelengths and flux are specified. To check the possibility of using your file – you can run the following script written in Python:

                    import pandas as pd

                    import numpy as np

                    from astropy.io import fits

                    source_fits = fits.getdata("filename.fits", 1)

                    source_df = pd.DataFrame(source_fits)

                    source_df.columns = ['lambda', 'FLUX']

                    flux_dict = {'FLUX': list(source_df['FLUX'].to_numpy()), 'lambda': list(source_df['lambda'].to_numpy())}

                    print(flux_dict)

- Text format: .dat, .DAT, .txt, .csv – with delimiters in the form of space or several spaces (as many as desired). The file must contain: wavelengths and flux. To check the possibility of using your file – you can run the following script written in Python:

                    import pandas as pd

                    import numpy as np

                    source_df = pd.read_csv("filename.format", sep = '\s+')

                    source_df.columns = ['lambda', 'FLUX']

                    flux_dict = {'FLUX': list(source_df['FLUX'].to_numpy()), 'lambda': list(source_df['lambda'].to_numpy())}

                    print(flux_dict)

2. Choose a spectrum of a star of a certain spectral type from the databases [Castelli and Kurucz, 2004], [Pickles, 1998] and [Kurucz, 1993], published in [Diaz, 2019].

From the stellar atmosphere models database [Castelli and Kurucz, 2004] available for selection are spectra of stars O3 – M2 spectral types with the following parameters: effective temperature 3000 – 45000 K, surface gravity log(g) = 2 – 5, metallicity log(z) = 0.

In the database [Pickles, 1998] the spectrum of a star of a certain spectral type is a combination of real spectra of different stars taken from various works. From this database, available for selection are spectra of stars O5 – M5 spectral classes with effective temperature 3000 – 40000 K and solar metallicity.

From the stellar atmospheres database [Kurucz, 1993] available for selection are spectra of stars O3 – M5 spectral types with effective temperature 3200 – 52000 K, surface gravity log(g) = [0 – 5], metallicity log(Z) ∈ [-5; +1].

3. Selection of spectral distribution of non-stellar objects:

elliptical, spiral galaxies from works [Coleman et al., 1980], [Benitez et al., 2004] and database [Diaz, 2019];
star-forming burst galaxies from [Kinney et al., 1996], [Benitez et al., 2004];
quasar from work [Zheng et al., 1997] (spectrum composed based on Hubble Space Telescope Faint Object Spectrograph survey);
Orion Nebula and planetary nebula NGC 7009 from database [Diaz, 2019];

4. Selection of spectral distribution of Hubble Space Telescope standard stars from [Bohlin et al., 2020], [Diaz, 2019].

Available for selection are composite spectra of white dwarfs, subdwarfs, and main sequence stars, including the Sun, obtained with Hubble Space Telescope spectrographs, as well as from ground-based observations.

5. Selection of other spectral distribution models:

Blackbody spectra, $F(\lambda) = \frac{2hc^2}{\lambda^5} \frac{1}{e^{\frac{hc}{\lambda kT}} - 1}$, with a given effective temperature of the object
Power-law $F(\lambda) = \lambda^{-n}$ with a given exponent $n$
Flat spectra $F(\lambda) = const$

6. Use of emission spectral line (mode "No continuum")

$$F(\lambda) = \frac{F_{line}}{\sigma\sqrt{2\pi}} \exp\left(-\frac{(\lambda - \lambda_c)^2}{2\sigma^2}\right), \quad \sigma = \frac{FWHM}{2\sqrt{2\ln 2}}$$

with given: wavelength $\lambda_c$ at the center of the line, integral flux in the line $F_{line}$, and full width at half maximum of the line, $FWHM$.

Specify the extinction and redshift

The user is also offered to account for redshift $z$, $\lambda_{obs} = (1 + z) \lambda_{rest}$ and interstellar absorption $F_\lambda = F_\lambda \cdot e^{\left( -\frac{A_V}{1.086} \right)}$, where $A_V = R_V E(B - V)$, $E(B - V)$ – given value of color excess.

To account for interstellar absorption in this version of the exposure calculator, the extinction curve in the Galaxy at $R_V = 3.1$ from work [Cardelli, Clayton, & Mathis 1989] is used. It should be noted that this curve is not universal for different directions in the Galaxy. Therefore, in subsequent versions of the exposure calculator, other interstellar absorption curves will also be used.