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; 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 (AASS, 127, 1-99)

OH (near IR) O 2 (IR+Herzberg, Chamberlain bands) NO 2 (pseudocont.) Na (seas. variation); Hg, Na lines Weak continuum

[OI]6300,6364 (300km) N 5200 (258km) Zodiacal Light; Diffuse Milky Way light; Faint stars and galaxies

FORS1+G150I ; Z=45º; 2 hours after Evening Twilight 0.17 of V flux 0.10 of R flux

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… Filterf t (%) ntnt nsns f s (%) U B V R I

PassbandCount Ratet3t3 (e - px -1 s -1 )(s) U B3.894 V R I Typical background count rates expected for FORS1 (SR) during dark time

One has to deal with a large variety of cases…

But see Patat, 2002a Rejecting bad areas: The Δ-test

Airmass effect The optical pathlength is given by: If f is the fraction of total sky brightn. generated by the airglow, we have: and therefore: Van Rhjin Layer Earth’s surface (Garstang 1989)

Expected effects re-darkening

A few real examples… f=0.7

Photometric Calibration A: Rain; B: M1 re-aluminisation; C: UT1>>UT mag yr -1

Alt-Az Telescope Pointings Distribution

|b|>10º -30º<β<+30º

1sbu=10 -9 erg s -1 s -2 Å -1 sr -1 Zodiacal Light Contribution 0.5 mag in |λ-λ 0 |=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 Model by Krisciunas & Schaefer (1991) Dark time sky brightness obtained with FLI=0 or h m <-18º

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 mag during a full solar cycle Solar Flux Penticton-Ottawa 2800 MHz

Dark Time Criteria Airmass X≤1.4 |b|>10º; Δt twi >1 hour; FLI=0 or h m ≤-18º; |λ-λ 0 |≥90º (ZL bias) FilterSky Br.σMinMaxNΔm zl U B V R I Dark time sky ESO-Paranal

SiteYearS 10.7cm UBVRI MJy La Silla Kitt Peak CTIO Calar Alto La Palma Mauna Kea Paranal mag arcsec -2 Dark time zenith night sky brightness measured at various observatories Mattila et al. 1996; Pilachowski et al. 1989; Walker 1987, 1988; Leinert et al. 1998; Krisciunas 1997.

Zodiacal Light bias in FORS1 data

The Walker-Effect Revisited

FORS1 Data ? 0.04+/-0.01 mag hour -1

Examples of short time scale fluctuations COUNTER EXAMPLE

Testing KS91 moon-brightness model ETCs! Moon age is not sufficient!

Sky brightness vs. solar activity Krisciunas 1997 Walker 1988 Δm≈ 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?

NaI D Seasonal Effects?

Intensity of [OI]6300,6364 (Rayleigh) Roach & Gordon 1973

Micro-auroral 300km

Searching for light pollution…

Calama:121,000; 280km 225,000; 108km La Escondida; 150km Yumbes; 23km 12km

South, 15 minutes μVel δCen βCar +26º αCru +6º S Photo by L. Vanzi

North, 13 minutes Photo by L. Vanzi αAur +18º βCam +5º 2Aur +28º N

01:45 before sunrise αGem Jupiter +16º αLeo +5º βGem Az=74.5ºN ecliptic

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.) Dedicated monitoring during tech. nights? Paranal’s sky health is excellent! We probably would like to keep it…

high airmass is bad because… Sky gets brighter; Extinction gets higher; Seeing gets worse: s=s 0 X 0.6 If we combine together all these effects, this is what we get: This, together with KS moon light brightness can be included in the ETC for now-casting during SM.

If you are interested in more details (which I doubt), have a look to Patat 2002b.