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Good bye, blue sky. UBVRI Night Sky Brightness at ESO-Paranal during sunspot maximum F. Patat - ESO. Photo by Leo[p]ardo Vanzi-ESO. The components of the sky background. Extra Terrestrial Zodiacal light (solar spectrum); Milky Way (diffuse stellar continuum); Faint stars and galaxyes;.
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UBVRI Night Sky Brightness at ESO-Paranal during sunspot maximum F. Patat - ESO Photo by Leo[p]ardo Vanzi-ESO
The components of the sky background • Extra Terrestrial • Zodiacal light (solar spectrum); • Milky Way (diffuse stellar continuum); • Faint stars and galaxyes; • Terrestrial • Night glow (pseudo-continuum, emission lines); • Micro-Aurora (emission lines); • Artificial light (emission lines, weak continuum);
for more details see The Light of the Night Sky Gordon & Roach, 1973 The 1997 reference of diffuse night sky brightness Leinert et al. 1998 (AASS, 127, 1-99)
OH (near IR) • O2 (IR+Herzberg, Chamberlain bands) • NO2 (pseudocont.) • Na (seas. variation); • Hg, Na lines • Weak continuum
Zodiacal Light; Diffuse Milky Way light; Faint stars and galaxies [OI]6300,6364 (300km) N 5200 (258km)
0.10 of R flux 0.17 of V flux FORS1+G150I 25-02-2001; Z=45º; 2 hours after Evening Twilight
Typical night sky brightness surveys • Small telescopes (20-30cm); • Photoelectric photometer; • Several arcmin diaphragm; • Small number of nights; • Interactive procedure; • Inclusion of bright (V>13) stars; A different approach?
Paranal UBVRI Night Sky Brightness Survey • Totally automatic, CCD based; • 4439 FORS1 frames analysed (April 2000 – September 2001); • 3883 (88%) suitable frames on 174 different nights; • Measurements logged with astronomical and ambient data (ASM); • No diaphragm and faint stars problems; VERY large telescope…
Typical background count rates expected for FORS1 (SR) during dark time
Rejecting bad areas: The Δ-test But see Patat, 2002a
Airmass effect (Garstang 1989) Van Rhjin Layer The optical pathlength is given by: If f is the fraction of total sky brightn. generated by the airglow, we have: Earth’s surface and therefore:
re-darkening Expected effects
A few real examples… f=0.7
A: Rain; B: M1 re-aluminisation; C: UT1>>UT3 Photometric Calibration 0.13 mag yr-1
-30º<β<+30º |b|>10º
Zodiacal Light Contribution 1sbu=10-9 erg s-1 s-2Å-1 sr-1 0.5 mag in B @ |λ-λ0|=90º from |β|>60º to β=0º (0.15 mag in I)
Scattered Moonlight contribution • Target elevation • Moon elevation • Moon FLI • Target moon angular distance • Extinction coefficient Dark time sky brightness obtained with FLI=0 or hm<-18º Model by Krisciunas & Schaefer (1991)
Solar Flux Penticton-Ottawa 2800 MHz • Rayleigh (1928) pointed out the dependency of [OI]5577Å intensity from sunspot number; • Walker (1988) confirmed this finding for broad band photometry, with a variation of 0.4-0.5 mag during a full solar cycle
Dark time sky brightness @ ESO-Paranal • Dark Time Criteria • Airmass X≤1.4 • |b|>10º; • Δttwi>1 hour; • FLI=0 or hm≤-18º; • |λ-λ0|≥90º (ZL bias)
Dark time zenith night sky brightness measured at various observatories mag arcsec-2 Mattila et al. 1996; Pilachowski et al. 1989; Walker 1987, 1988; Leinert et al. 1998; Krisciunas 1997.
? 0.04+/-0.01 mag hour-1 FORS1 Data
Examples of short time scale fluctuations COUNTER EXAMPLE
Testing KS91 moon-brightness model Moon age is not sufficient! ETCs!
Walker 1988 Krisciunas 1997 Sky brightness vs. solar activity Δm≈0.5-0.6 mag !
Daily Averages Even though the solar flux density range is comparable to that of full solar cycle, the dependency is much weaker (0.24 mag on a full cycle). Unpredictability… Time scales of physical processes?
Intensity of [OI]6300,6364 (Rayleigh) Roach & Gordon 1973
Calama:121,000; 280km 225,000; 108km La Escondida; 150km 12km Yumbes; 23km
S βCar +26º μVel αCru +6º δCen South, 15 minutes Photo by L. Vanzi
N 2Aur +28º αAur +18º βCam +5º North, 13 minutes Photo by L. Vanzi
βGem ecliptic αGem Jupiter +16º αLeo +5º N Az=74.5º 01:45 before sunrise
No azimuthal dependency in our UBVRI data (h>20º); • No traces of NaI, HgI emission lines; • No traces of broad components in NaI D (high pressure lamps) in UVES spectra (Hanuschik et al. 2003, in prep.) Paranal’s sky health is excellent! We probably would like to keep it… Dedicated monitoring during tech. nights?
Observing @ high airmass is bad because… • Sky gets brighter; • Extinction gets higher; • Seeing gets worse: This, together with KS moon light brightness can be included in the ETC for now-casting during SM. s=s0X0.6 If we combine together all these effects, this is what we get:
If you are interested in more details (which I doubt), have a look to Patat 2002b.