“RGB photometric calibration of 15 million Gaia stars”

Nicolás Cardiel, Jaime Zamorano, Josep Manel Carrasco, Eduard Masana, Salvador Bará, Rafael González, Jaime Izquierdo, Sergio Pascual, Alejandro Sánchez de Miguel2021MNRAS.507..318C
Data tables

“Participant motivation to engage in a citizen science campaign: the case of the TESS network”

Irene Celino, Gloria Re Calegari, Mario Scrocca, Jaime Zamorano and Esteban Gonzalez GuardiaJournal of science communication. Issue 06,2021

“Synthetic RGB photometry of bright stars: definition of a standard photometric system and UCM library of spectrophotometric spectra”

Nicolás Cardiel, Jaime Zamorano, Salvador Bará, Alejandro Sánchez de Miguel, Cristina Cabello, Jesús Gallego, Lucía García, Rafael González, Jaime Izquierdo, Sergio Pascual, José Robles, Ainhoa Sánchez, and Carlos Tapia 2021MNRAS.504.3730C
Appendices A and B

What is inside:

Paper 2   Cardiel et al. (2021b)

  • Extended catalog of RGB standard stars     (Data tables)
  • RGB magnitudes for ~15 million stars brighter than Gaia G = 18 mag
  • The large number of Gaia sources available in each region of the sky should guarantee high-quality RGB photometric calibrations.

Paper 1   Cardiel et al. (2021a)

  • A new library of 1346 bright spectrophotometric standards
  • A new standard RGB photometric system
  • A catalogue of reference RGB magnitudes

Synthetic RGB photometry of bright stars: the UCM catalogue

Although the use of RGB photometry has exploded in the last decades due to the advent of high-quality and inexpensive digital cameras equipped with Bayer-like color filter systems, there is surprisingly no catalogue of bright stars that can be used for calibration purposes.

We hope that the catalogue of 1346 flux calibrated stellar spectra presented here, that by itself already constitutes a library of bright spectrophotometric standards suitable for spectroscopic calibrations, and the corresponding synthetic RGB magnitudes derived from them, can be used as a reference for future work on several astronomical fields, where the collaboration of many observers equipped with high-quality digital cameras may provide data that facilitate the research advancement.

This could help to make citizen science a reality in the realm of astronomy, increasing the public’s interest and understanding of science, highlighting the fact that scientific research matters.

High-quality data using digital cameras

The advent of high-quality and relatively inexpensive digital cameras, equipped with Bayer-like color filter systems, is making citizen science a reality in the realm of astronomy. Recent market studies (Nisselson et al. 2017) estimate that by 2022 as much as 45 billion cameras will be operative worldwide.

Photo credit: Jaime Izquierdo

The Bayer filter mosaic

A Bayer filter mosaic is a color filter array for arranging RGB (Red/Green/Blue) color filters on a square grid of photosensors. Its particular arrangement of color filters is used in most single-chip digital image sensors used in digital cameras, camcorders, and scanners to create a color image. Although these filters are different from those typically employed in optical studies by professional astronomers, the relative similarity of the R, G, and B channels across camera models, and their expected stability in the foreseeable future (as far as cameras continue to be developed for human vision-driven applications) are two key features enabling the scientific usefulness of these devices.

The calibration problem

Since RGB filter are different from those typically employed in optical studies by professional astronomers, a standard catalogue of bright stars that could be employed for calibration purposes was still missing. Our work has focused on helping to solve this problem, by creating a library of 1346 model spectra, able to reproduce high-quality historical photometric data.

The historical 13-color photometric data of bright stars

Photometric data of superb quality was gathered by Johnson & Mitchell (1975) in 13 photometric bands (the so called C13 system) for most bright stars in the sky. Similar measurements were obtained later by Schuster (1976) and Bravo Alfaro et al. (1997) for some additional, but fainter, stars. The spectral sensitivity curves of the 13 filters are shown in the figure. The name of each filter indicates its approximate effective wavelength. Just for illustration, we are also displaying the atmospheric telluric absorption (upper gray line) computed with the help of the ESO SKYCALC tool (Moehler et al. 2014).

The historical 13-color photometric data of bright stars

Photometric data of superb quality was gathered by Johnson & Mitchell (1975) in 13 photometric bands (the so called C13 system) for most bright stars in the sky. Similar measurements were obtained later by Schuster (1976) and Bravo Alfaro et al. (1997) for some additional, but fainter, stars. The spectral sensitivity curves of the 13 filters are shown in the figure. The name of each filter indicates its approximate effective wavelength. Just for illustration, we are also displaying the atmospheric telluric absorption (upper gray line) computed with the help of the ESO SKYCALC tool (Moehler et al. 2014).

Fitting a model to the historical 13C data

Since the historical 13C data only provided 13 fluxes for each star, a full spectrum, computed using the Stellar Atmosphere Models by Castelli & Kurucz (2003), have been individually fitted, providing an excellent model of optical spectral energy distribution. The figure on the left illustrates several examples of spectrum fits to stars with different effective temperatures.

The models look real!

We have checked that the fitted models do actually agree very well with real available spectra of 39 bright stars observed by Kiehling (1987). Some examples are shown in the figure, where the thin black lines are Kiehling’s flux calibrated spectra, whereas the thick blue/orange lines correspond to the stellar models, fitted to the C13 data (red symbols).

Fitting a model to the historical 13C data

Since the historical 13C data only provided 13 fluxes for each star, a full spectrum, computed using the Stellar Atmosphere Models by Castelli & Kurucz (2003), have been individually fitted, providing an excellent model of optical spectral energy distribution. The figure below illustrates several examples of spectrum fits to stars with different effective temperatures.

The models look real!

We have checked that the fitted models do actually agree very well with real available spectra of 39 bright stars observed by Kiehling (1987). Some examples are shown in the figure, where the thin black lines are Kiehling’s flux calibrated spectra, whereas the thick blue/orange lines correspond to the stellar models, fitted to the C13 data (red symbols).

Defining a standard RGB system

In order to generate a catalogue of reference RGB calibrating stars, it is important to properly define the spectral sensitivity curves of the 3 filters. For that purpose we have used the response curves measured by Jiang et al. (2013) for 28 cameras: Canon 1DMarkIII, Canon 20D, Canon 300D, Canon 40D, Canon 500D, Canon 50D, Canon 5DMarkII, Canon 600D, Canon 60D, Hasselblad H2, Nikon D3X, Nikon D200, Nikon D3, Nikon D300s, Nikon D40, Nikon D50, Nikon D5100, Nikon D700, Nikon D80, Nikon D90, Nokia N900, Olympus E-PL2, Pentax K-5, Pentax Q, Point Grey Grasshopper 50S5C, Point Grey Grasshopper2 14S5C, Phase One, and SONY NEX-5N. The individual curves corresponding to each camera (thin lines in the figure) have been averaged by computing the median values at each sampled wavelength (thick lines). These are median curves that we have adopted to define the standard RGB photometric system, and their spectral sensitivity values are listed in the dropdown menus.

3990.0  0.000000e+00
4000.0  1.504283e-02
4100.0  1.039736e-01
4200.0  4.892935e-01
4300.0  7.202255e-01
4400.0  8.216436e-01
4500.0  9.308637e-01
4600.0  1.000000e+00
4700.0  9.802917e-01
4800.0  9.275882e-01
4900.0  7.807393e-01
5000.0  6.143757e-01
5100.0  4.338580e-01
5200.0  2.491595e-01
5300.0  1.594246e-01
5400.0  9.478547e-02
5500.0  5.672213e-02
5600.0  2.732143e-02
5700.0  1.661261e-02
5800.0  1.141002e-02
5900.0  8.477845e-03
6000.0  4.891583e-03
6100.0  3.422961e-03
6200.0  2.965799e-03
6300.0  3.285335e-03
6400.0  3.795944e-03
6500.0  5.100955e-03
6600.0  5.076481e-03
6700.0  3.265961e-03
6800.0  1.129443e-03
6900.0  5.126654e-04
7000.0  2.948293e-04
7100.0  1.016809e-04
7200.0  6.157581e-05
7210.0  0.000000e+00

3990.0  0.000000e+00
4000.0  3.084000e-03
4100.0  1.135700e-02
4200.0  3.887250e-02
4300.0  5.753950e-02
4400.0  7.919100e-02
4500.0  1.006650e-01
4600.0  1.360232e-01
4700.0  2.571178e-01
4800.0  3.809050e-01
4900.0  4.251800e-01
5000.0  6.113000e-01
5100.0  7.933000e-01
5200.0  9.033850e-01
5300.0  1.000000e+00
5400.0  9.064100e-01
5500.0  8.807233e-01
5600.0  7.437300e-01
5700.0  6.428150e-01
5800.0  4.597650e-01
5900.0  3.175050e-01
6000.0  1.819950e-01
6100.0  8.972300e-02
6200.0  4.853900e-02
6300.0  3.160450e-02
6400.0  2.298700e-02
6500.0  1.676700e-02
6600.0  1.128295e-02
6700.0  8.819030e-03
6800.0  4.677050e-03
6900.0  1.687100e-03
7000.0  7.490450e-04
7100.0  3.076779e-04
7200.0  1.487700e-04
7210.0  0.000000e+00

3990.0  0.000000e+00
4000.0  3.496976e-03
4100.0  1.038916e-02
4200.0  2.382697e-02
4300.0  2.776679e-02
4400.0  1.800560e-02
4500.0  1.803599e-02
4600.0  2.186831e-02
4700.0  2.991320e-02
4800.0  3.396205e-02
4900.0  4.018768e-02
5000.0  4.308463e-02
5100.0  6.251524e-02
5200.0  1.111756e-01
5300.0  1.419566e-01
5400.0  8.498971e-02
5500.0  4.781299e-02
5600.0  5.215874e-02
5700.0  1.533735e-01
5800.0  6.503433e-01
5900.0  1.000000e+00
6000.0  9.353758e-01
6100.0  8.337379e-01
6200.0  6.858826e-01
6300.0  5.929939e-01
6400.0  4.600072e-01
6500.0  3.717754e-01
6600.0  2.205769e-01
6700.0  1.200198e-01
6800.0  3.715120e-02
6900.0  1.260716e-02
7000.0  3.716883e-03
7100.0  1.210464e-03
7200.0  5.449463e-04
7210.0  0.000000e+00
Download the 3 filters data

Another basic ingredient in the definition of a photometric system is the adoption of a reference spectral energy distribution to define the zero point of the magnitude system. Although for many years astronomers have employed the spectrum of Vega for such a reference, magnitude scales using fixed flux densities per unit frequency (AB system) or per unit wavelength (ST system) are becoming more frequent (see comparison of the 3 reference spectra in the figure). In this work we have adopted the reference corresponding to the AB system.

0
Stars
0
Sensitivity curves by Jiang et al
0
Filters by Johnson & Michell
0
New synthetic filters

Table with standard B, G and R magnitudes for 1346 stars

Our final sample comprises 1346 stars, well spread in the sky, whose RGB magnitudes in the standard system are provided in the tables below. In addition, we also facilitate the fitted stellar model for each star, which can be employed as spectrophotometric calibrators for spectroscopic studies.

If you click on the following image a table as show below will open in another tab. The first column in the table below provides a link to the Simbad database, the second one a link to a PDF plot with the best CK04 fit to the C13 color photometry, on the third you will find a binary FITS table with the corresponding spectrum. If you follow the ASCII link, you will get a file with the same spectrum.

Appendices A and B: Comparison with CK04 and transformations to the standard RGB system

Download the table here:

Download the full table here:

Download all the spectra below:

About the authors

Nicolás Cardiel (UCM, IPARCOS)

PhD. Astrophysics

Professional astronomer and professor at UCM.

Jaime Zamorano (UCM, IPARCOS)

PhD. Astrophysics

Professional astronomer and astrophysics professor at UCM.

Salvador Bará (USC)

PhD. Physics

Associate profesor at Universidade de Santiago de Compostela.

Alejandro Sánchez de Miguel (UCM, UEC, IAA)

PhD. Astrophysics

PhD. Astrophysics. Postdoc researcher at Exeter University.

Cristina Cabello (UCM, IPARCOS)

Astrophysics master

Physics graduate. Predoctoral researcher at Universidad Complutense de Madrid.

Jesús Gallego (UCM, IPARCOS)

PhD. Physics

GUAIX Director and astrophysics professor at UCM.

Lucía García (UCM, IPARCOS)

Physics degree

Physics graduate with media communications expertise.

Rafael González (UCM)

Telecommunication Engineer

Laboratory Technician at LICA.

Jaime Izquierdo (UCM)

PhD. Astrophysics

Astronomer, astrophotographer and honorary collaborator at the Department of Astrophysics.

Sergio Pascual (UCM, IPARCOS)

PhD. Astrophysics

Astrophysicist and Computer Engineer. Assistant professor at UCM.

José Robles (UCM)

Nuclear Science master

Physics graduate. Predoctoral researcher at Universidad Complutense de Madrid.

Ainhoa Sánchez (UCM)

Research and Innovation Management master

Project manager of the GUAIX Group at the UCM.

Carlos Tapia (UCM)

Optical engineer

Optical Engineer at Escribano Mechanical & Engineering

Contact

Nicolás Cardiel:
cardiel@ucm.es