Six-colour photometry of the night sky at declination -19 between wavelengths 3900 Å and 7100 Å.
Abstract
The surface brightness of the night sky at declination-19.20 has been measured at six wavelengths ranging from 3920 to 7100 A, in a circular field of view of 4.5 square degrees with a photoelectric zenith photometer by one-year observations at Tsumeb, South West Africa, from July, 1965, until June, 1966. Mean values for all clear mconless nights of the surface brightness in units of 5io' the equivalent number of stars of spectral type A0V and 10 visual magnitude per square degree, are given in Figs. 1 and 2 at intervals of 5 min of right ascension and for wavelengths 3920, 4900, 5580, 5900, 6300, and 7100 A. The intensities given for wavelengths 3920, 5580, 5900, and 6300 A are free of contributions from the strong monochromatic nightglow emissions at wavelengths 3914, 5577, 5890, and 6300 A. The separation of the nightglow line emissions and the surface brightness components with quasi-continuous spectrum is done by measuring two times at each wavelength through two interference filters of different half-bandwidths, with accurate calibration of the filter responses to continuous-spectrum and to monochromatic intensities. A method is given to separate approximately the OH nightglow emission and the non-terrestrial component of the night sky brightness at wavelength 7100 A for which only one single filter was available, using the observations at shorter wavelengths and the mean spectrum of the night sky brightness. The surface brightness at right ascension 0h50rn, declination 190 (galactic latitude 820) for wavelength 5580 A, omitting the [0 1] 5577 A airglow emission component, is 240 Sio (A0V). This is shown to be equal within the measuring errors to the value of 218 Sio (G2V) for wavelength 5300 A found by Roach and Smith (1964) at the opposite side of the night sky at right ascension 12h50rn, declination + 20.70 (galactic latitude + 830), taking into account the mean spectrum of the night sky light. The spectrum of the surface brightness of the night sky less airglow line emissions closely resembles that of the Sun for galactic latitudes < 400 or > + 300 in the ring, in the wavelength range from 4900 A to 6300 A (Fig. 3). From Fig. 1, it may be concluded that the absolute photometry of the zodiacal light at wavelength 5300 A by Smith, Roach and Owen (1965) correctly explains 70 to 75% of the total surface brightness less airglow line emissions at wavelength 5580 A and at high galactic latitudes; about 20% are due to the stellar component, and only 5 to 10% to other sources. It is shown that the spectrum of the zodiacal light must have essentially the same energy distribution as the Sun in the wavelength range from 4900 A to 6300 A. A southern extension of the isophotes of the galactic component of the night sky brightness in the constellation Orion is implicated by the observations, the 505io isophote at wavelength 5580 A reaching as far as galactic latitude 600. The calibrations of the surface brightness of the Milky Way as measured by Elsasser and Haug (1960) and by Pannekoek and Koelbloed (1949) have been compared to the calibration of this photometry at wavelength 5580 A (Figs. 4 and 5). Key words: galactic light - zodiacal light - night sky
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- 1970
- Bibcode:
- 1970A&A.....6..155D