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Night Sky Brightness Survey at ESO-Paranal During Solar Maximum

This study explores the night sky brightness at ESO-Paranal, capturing crucial data during the sunspot maximum. Utilizing CCD-based measurements from the FORS1 instrument, the survey covered various celestial components, including zodiacal light, the Milky Way, micro-auroras, and artificial light pollution. The research was conducted over significant time periods with meticulous calibration against solar flux variations. Important optical phenomena and light contributions were analyzed, providing insights into atmospheric conditions and their impact on night sky brightness.

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Night Sky Brightness Survey at ESO-Paranal During Solar Maximum

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  1. Good bye, blue sky

  2. UBVRI Night Sky Brightness at ESO-Paranal during sunspot maximum F. Patat - ESO Photo by Leo[p]ardo Vanzi-ESO

  3. 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);

  4. 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)

  5. OH (near IR) • O2 (IR+Herzberg, Chamberlain bands) • NO2 (pseudocont.) • Na (seas. variation); • Hg, Na lines • Weak continuum

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

  7. 0.10 of R flux 0.17 of V flux FORS1+G150I 25-02-2001; Z=45º; 2 hours after Evening Twilight

  8. 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?

  9. 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…

  10. Typical background count rates expected for FORS1 (SR) during dark time

  11. One has to deal with a large variety of cases…

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

  13. 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:

  14. re-darkening Expected effects

  15. A few real examples… f=0.7

  16. A: Rain; B: M1 re-aluminisation; C: UT1>>UT3 Photometric Calibration 0.13 mag yr-1

  17. Alt-Az Telescope Pointings Distribution

  18. -30º<β<+30º |b|>10º

  19. 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)

  20. 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)

  21. 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

  22. 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)

  23. 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.

  24. Zodiacal Light bias in FORS1 data

  25. The Walker-Effect Revisited

  26. ? 0.04+/-0.01 mag hour-1 FORS1 Data

  27. Examples of short time scale fluctuations COUNTER EXAMPLE

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

  29. Walker 1988 Krisciunas 1997 Sky brightness vs. solar activity Δm≈0.5-0.6 mag !

  30. 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?

  31. NaI D Seasonal Effects?

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

  33. Micro-auroral activity @ 300km

  34. Searching for light pollution…

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

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

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

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

  39. 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?

  40. 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:

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

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