S. Nawar’s research while affiliated with National Research Institute of Astronomy and Geophysics and other places

Photoelectric observations of night sky brightness (NSB) at different zenith distances and azimuths, covering all the sky, at the Egyptian Kottamia Astronomical observatory (KAO) site of coordinates ϕ = 29° 55.9′ N and λ = 31° 49.5′ E, were done using a fully automated photoelectric photometer (FAPP). The Bessel wide range system (UBVRI) is used for the first time to observe NSB for three consecutive nights (1–3 August, 2022) under good seeing conditions after the moon sets. The deduced results were taken in photons and converted into mag/arcsec². The average zenith sky brightness for U, B, V, R and I filters are found to be 20.49, 20.38, 19.41, 18.60 and 17.94 mag/arcsec² respectively. The average color indices (U–B), (B–V), (V–R) and (R–I), at the zenith are detected to be 0.11, 0.98, 0.81 and 0.66, respectively. We plotted the isophotes of the sky brightness at KAO in U, B, V, R and I colors (filters) and determined both the average atmospheric extinction and sky transparency through these UBVRI filters. The atmospheric and other meteorological conditions were taken into our consideration during the observational nights. The results of the current study illustrate the main impact of the new cities built around KAO on the sky glow over it, and which astronomical observations are affected.

Observations of sky brightness at zenith during the total lunar eclipse of July 27th, 2018 and partial lunar eclipse of July 16th, 2019 were carried out at Kottamia Observatory site in Egypt using a Sky Quality Meter photometer. A variation of the zenith sky brightness with time was observed with average values of 20.3 and 19.44 magSQM/arcsec² at the maximum eclipse of the total and partial lunar eclipses, respectively. The visibility of stars and planets during different phases of the two eclipses are discussed. The present results of the total lunar eclipse are compared with those obtained during the total lunar eclipse of April 14th, 2014, the total solar eclipse of March 9th, 2016 and the total lunar eclipse of January 2019. We found similarities in the variation in the sky brightness during the different phases of the eclipses with respect to the different sites and atmospheric conditions.

Using a customized fully automatic photoelectric photometer, we measured the NSB in UBVRI filters at Kottamia Astronomical Observatory. The observations covered all the sky at all altitudes and azimuths with 10° steps at each. The observations were carried out on three nights of August, 1 st , 2 nd and 3 rd 2022 and were averaged. The average zenith sky brightness is 20.49, 20.7, 19.34, 18.25 and 17.04 mag/arcsec ² in UBVRI filters respectively. The average color index is U-B = - 0.21, B-V = 1.37, V-R = 1.09 and R-I = 1.21 for the zenith. The average atmospheric extinction and sky transparency of the UBVRI filters were determined for the three nights of observations. We also included the atmospheric conditions during the observational nights.

Most of the previously published results of the zenith sky twilight brightness, obtained by different investigators, are given in different units. These results have been converted to (mag/arcsec²) to make a comparison between them. The variation of the colour indices (B-V) and (B-R) of the zenith sky twilight with the sun’s depression will be studied. The present photometric results of the zenith sky twilight brightness expressed in mag/arcsec², have been obtained for the first time for blue, yellow and red colours.

This work aims for choosing preliminary sites that can be candidates for a new astronomical observatory that will replace the Kottamia observatory. The candidate sites have been selected to be investigated after proposing a few criteria. Concerning light pollution and elevation of the sites as the main factors, we chose the candidate sites depending on the zenith sky brightness and the elevation. The night sky brightness has been measured using a Sky Quality Meter at different altitude degrees and the values of the zenith sky brightness were compared to those of different observatory sites. Our night sky brightness observations were found to be in agreement with the satellite’s data and comparable to those of dark sites, with average values of 22.07, 22.03, 21.57 and 21.58 magSQM/arcsec². We also have obtained the Aerosol Optical Depth (AOD) and extinction coefficient due to the aerosol for the candidate sites using satellite data (Terra/MODIS).

The axis of zodiacal lights have been obtained in blue and yellow colors using photoelectric observations of zodiacal light. These observations have been carried out at Abu Simbel Site in Egypt, in October 1975. This site lies too near to the tropic of Cancer, at which the axis of the zodiacal light cone perpendiculars to the horizon. The results show that the plane of the zodiacal light is inclined to the normal by 1.59 degrees in blue color and 1.18 degrees in yellow color. This means that there is a slight variation in zodiacal light axis with wavelength, and the axis almost coincide with the ecliptic. The present results for blue color can be considered as the first one in the world near the tropic of Cancer.

A search for discontinuity in the sky twilight brightness at different wavelengths and different solar depressions for altitudes 0, 5, 10, 30, 50 & 70 degrees is done. It is found that the logarithmic of difference in the brightness log (I1-I2) of two similar patches lying at solar and anti-solar verticals are not suffering of any discontinuity, when plotted versus sun’s depression. Whatever, the ratio I1/I2 shows that there is discontinuity in the curves at altitudes less than or equal 30 degrees. While for altitudes 50 & 70 degrees slight patches of discontinuities have been detected. The phenomenon of discontinuities may be referred to the changes in the sky twilight brightness due to the effect of the Earth’s shadow on diffusing and luminescent layers in the upper atmosphere. Comment: 11 pages, 3 figures, 1st Arab Conference on Astronomy and Geophysics, Egypt 2008

From photoelectric observations of night sky brightness carried out at Abu-Simbel, Asaad et al. (1979) have obtained values of integrated starlight brightness at different Galactic latitudes. These data have been used in the present work to obtain the brightness and color of the integrated starlight at North and South Celestial, Ecliptic and Galactic Poles. The present values of the brightness are expressed in S10 units and mag/arcsec2. Our results have been compared with that obtained by other investigators using photometric and star counts techniques. The B-V and B-R have been calculated and the results are compared with that obtained by other investigators.

Photoelectric observations of the sky brightness along Sun’s meridian have been carried out at Salloum during the March 29, 2006 total solar eclipse. The measurements have been taken at different zenith distances along the Sun’s meridian using yellow and red wide band glass filters centered at 5500 Å and 7900 Å, respectively. The present results of the sky brightness during the total solar eclipse have been compared with that of twilight, and night sky obtained by the same instrument at Abu-Simbel and Kottamia observatory sites respectively. The variation of V R color index with zenith distance have been also studied. The visibility of planets and stars during the March 29, 2006 total solar eclipse is given.

Photoelectric observations of the sky brightness along Sun’s meridian have been carried out at Salloum during the March 29, 2006 total solar eclipse. The measurements have been taken at different zenith distances along the Sun’s meridian using yellow and red wide band glass filters centered at 5500 Å and 7900 Å, respectively. The present results of the sky brightness during the total solar eclipse have been compared with that of twilight, and night sky obtained by the same instrument at Abu-Simbel and Kottamia observatory sites respectively. The variation of V R color index with zenith distance have been also studied. The visibility of planets and stars during the March 29, 2006 total solar eclipse is given.