good bye, blue sky. ubvri night sky brightness at eso-paranal during sunspot maximum f. patat - eso...
TRANSCRIPT
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Good bye, blue sky
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UBVRI Night Sky Brightness at ESO-Paranal
during sunspot maximum
F. Patat - ESO
Photo by Leo[p]ardo Vanzi-ESO
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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);
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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)
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•OH (near IR)
•O2 (IR+Herzberg, Chamberlain bands)
•NO2 (pseudocont.)
•Na (seas. variation);
•Hg, Na lines
•Weak continuum
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[OI]6300,6364 (300km)
N 5200 (258km)
Zodiacal Light; Diffuse Milky Way light; Faint stars and galaxies
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FORS1+G150I 25-02-2001; Z=45º; 2 hours after Evening Twilight
0.17
of
V f
lux
0.10
of
R f
lux
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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?
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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…
Filter ft(%) nt ns fs(%)
U 1.8 204 68 33.3
B 11.3 479 434 90.6
V 17.3 845 673 79.6
R 27.1 1128 1055 93.5
I 42.5 1783 1653 92.7
4439 3883 87.5
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Passband Count Rate t3
(e- px-1 s-1) (s)
U 0.5 714
B 3.8 94
V 15.8 23
R 26.7 13
I 32.1 11
Typical background count rates expected for FORS1 (SR) during dark time
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One has to deal with a large variety of cases…
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But see Patat, 2002a
Rejecting bad areas: The Δ-test
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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)
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Expected effects
re-darkening
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A few real examples…
f=0.7
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Photometric Calibration
A: Rain; B: M1 re-aluminisation; C: UT1>>UT3
0.13 mag yr-1
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Alt-Az Telescope Pointings Distribution
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|b|>
10º
-30º
<β<+
30º
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1sbu=10-9 erg s-1 s-2 Å-1 sr-1
Zodiacal Light Contribution
0.5 mag in B
@ |λ-λ0|=90º from
|β|>60º to β=0º
(0.15 mag in I)
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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 hm<-18º
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•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
Solar Flux
Penticton-Ottawa 2800 MHz
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Dark Time Criteria
•Airmass X≤1.4
•|b|>10º;
•Δttwi>1 hour;
•FLI=0 or hm≤-18º;
•|λ-λ0|≥90º (ZL bias)
Filter Sky Br. σ Min Max N Δmzl
U 22.28 0.22 21.89 22.61 39 0.18
B 22.64 0.18 22.19 23.02 180 0.28
V 21.61 0.20 20.99 22.10 296 0.18
R 20.87 0.19 20.38 21.45 463 0.16
I 19.71 0.25 19.08 20.53 580 0.07
Dark time sky brightness @ ESO-Paranal
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Site Year S10.7cm U B V R I
MJy
La Silla 1978 1.5 - 22.8 21.7 20.8 19.5
Kitt Peak 1987 0.9 - 22.9 21.9 - -
CTIO 1987-8 0.9 22.0 22.7 21.8 20.9 19.9
Calar Alto 1990 2.0 22.2 22.6 21.5 20.6 18.7
La Palma 1994-6 0.8 22.0 22.7 21.9 21.0 20.0
Mauna Kea 1995-6 0.8 - 22.8 21.9 - -
Paranal 2000-1 1.8 22.3 22.6 21.6 20.9 19.7
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.
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Zodiacal Light bias in FORS1 data
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The Walker-Effect Revisited
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FORS1 Data
?0.04+/-0.01 mag hour-1
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Examples of short time scale fluctuations
COUNTER EXAMPLE
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Testing KS91 moon-brightness model
ETCs!Moon age is
not suffi
cient!
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Sky brightness vs. solar activity
Krisciunas 1997
Walker 1988
Δm≈0.5-0.6 mag !
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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?
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NaI D Seasonal Effects?
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Intensity of [OI]6300,6364 (Rayleigh)
Roach & Gordon 1973
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Micro-auroral activity @ 300km
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Searching for light pollution…
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Calam
a:12
1,00
0; 2
80km
225,
000;
108
km
La Escondida; 150km
Yumbes; 23km
12km
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South, 15 minutes
μVel
δCen
βCar +26º
αCru +6º
S
Photo by L. Vanzi
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North, 13 minutesPhoto by L. Vanzi
αAur +18º
βCam +5º
2Aur +28º
N
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01:45 before sunrise
αGem
Jupiter +16º
αLeo +5º
βGem
Az=74.5ºN
ecliptic
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• 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…
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Observing @ high airmass is bad because…
• Sky gets brighter;• Extinction gets higher;• Seeing gets worse: s=s0X
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.
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If you are interested in more details (which I doubt), have a look to Patat 2002b.