Photometry system calibration

Select standards

Observe and grab standards fields in your filters. 30 FITS files of each field in one filter will be enough. You should observe different standard fields on different zenith angles (or same field on different zenith angles) to obtain good fit of coefficient of extinction K.

Preferred are Landold Equatorial Standard but script works with Gaia EDR3 and Johnson-Kron-Cousins standards Catalogue that contain other standard fields such as M67 for example [1].

Calibration process is based on papers [2], [3], [4].

All FITS files should be organized in folders by filter. For example you will have V folder with all FITS files captured in V band and B folder where all FITS fileas are captured in B band.

Prepare config

Example of config file is provided with scripts

[Stars_Stand]
K = 0.35           # coefficient of extinction if Band. Can be None (will be calculated) or value
Filter = V         # Band
snr = 1.0          # min star SNR
max_m_fit = 14.0   # max V_MAG for process on FITS files
max_m_calc = 13.0  # max MAG for calculations
r_max_val = 0.25   # delete stars with R2 more that this
dark_frame = path\to\dark\frame  # Path to dark frame
dark_stable = 0    # If no dark frame than you can subtract some value
FoV = 1.5          # Field of View in degrees (radius)

[APERTURE]
r_ap = 5
an_in = 10
an_out = 14

[astrometry.net]
scale_lower = 2.2         # lower limit arcsec/pix
scale_upper = 2.6         # upper limit arcsec/pix
api_key = xxxxxxxxxxx     # Astrometry.net api key

[SITE]
Name = OBS_NAME
lat = 48.000
lon = 22.000
h = 231.000

Config file should be named config_stars.ini and created in directory with FITS files (in each dir, if you have many dirs for different filters).

Run calibration

Similar to sat_phot.py system calibration scripts can be run in same ways. The only difference here is that we have two scripts they are stored in sys_calibrate folder:

• stars_process.py path/to/fits/files

  Run to process all FITS files. It will produce ref_stars_cat.json and logs. Only parameter that must be given is path to folder with FITS files.

• stars_calc_params.py path/to/ref_stars_cat.json

  Calculate system parameters and build the graphs. System parameters will appear in stdout and in log file. As the parameter set the path to ref_stars_cat.json file.

If you wish to recalculate sys parameters (Z_x, C_x, K_x) with other parameters, for example change max MAG of selected stars, you can do it without processing FITS files all data are already stored in ref_stars_cat.json file.

References

Calibration process is based on papers:

[1]

Pancino, E., “The Gaia EDR3 view of Johnson-Kron-Cousins standard stars: the curated Landolt and Stetson collections”, Astronomy and Astrophysics, vol. 664, Art. no. A109, EDP, 2022. DOI.

[2]

Hu, S. M., Han, S. H., Guo, D. F., & Du, J. J. (2014). The photometric system of the One-meter Telescope at Weihai Observatory of Shandong University. Research in Astronomy and Astrophysics, 14(6), 719. DOI

[3]

Huang, F., Li, J. Z., Wang, X. F., Shang, R. C., Zhang, T. M., Hu, J. Y., … & Jiang, X. J. (2012). The photometric system of the Tsinghua-NAOC 80-cm telescope at NAOC Xinglong Observatory. Research in Astronomy and Astrophysics, 12(11), 1585. DOI

[4]

Kinoshita, D., Chen, C. W., Lin, H. C., Lin, Z. Y., Huang, K. Y., Chang, Y. S., & Chen, W. P. (2005). Characteristics and performance of the CCD photometric system at Lulin observatory. Chinese Journal of Astronomy and Astrophysics, 5(3), 315. DOI