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. 1998 (AASS, 127, 1-99)

•OH (near IR)

•O2 (IR+Herzberg, Chamberlain bands)

•NO2 (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 25-02-2001; Z=45º; 2 hours after Evening Twilight

0.17

of

V f

lux

0.10

of

R f

lux

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…

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

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

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>>UT3

0.13 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 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

Model by Krisciunas & Schaefer (1991)

Dark time sky brightness

obtained with FLI=0 or hm<-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 0.4-0.5 mag during a full solar cycle

Solar Flux

Penticton-Ottawa 2800 MHz

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

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.

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 suffi

cient!

Sky brightness vs. solar activity

Krisciunas 1997

Walker 1988

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?

NaI D Seasonal Effects?

Intensity of [OI]6300,6364 (Rayleigh)

Roach & Gordon 1973

Micro-auroral activity @ 300km

Searching for light pollution…

Calam

a:12

1,00

0; 2

80km

225,

000;

108

km

La Escondida; 150km

Yumbes; 23km

12km

South, 15 minutes

μVel

δCen

βCar +26º

αCru +6º

S

Photo by L. Vanzi

North, 13 minutesPhoto 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…

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.

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