average brightness of moonlight as a function of lunar phase, in mag/(arcsec)2 for V and B

II.3 Atmospheric background radiation

Main contributions to atmospheric sky background at dark site: • twilight at sunset/sunrise • moonlight • airglow and aurora • thermal emission

It is sometimes convenient to express astronomical surface brightness in ‘S10 units’:

S10 = equivalent number of 10th

magnitude stars per (deg)2

twilight

sky brightness in zenith at ! = 550 nm (in S10 and mag/(arcsec)

2) between sunset and ‘end of astronomical twilight’ (hSUN = -18 °) :

Ex.: resolution human eye: ~1arcmin ! at hSUN =0° sky brightness per res. element is –1.7mag, i.e. brighter than brightest stars

moonlight

••

Q. : can you explain this difference ?

without Moon, Zodiacal light

Q. : why this change ?

airglow and aurora

airglow (nightglow) is line-emission that arises in the upper atmosphere from photo-ionization/dissociation and photo-chemical reactions driven by UV sunlight

most reactions involve O/O2/O3, N/N2, Na, H2O, OH: nightglow is strongly variable, both spatially and in time Ex.: all-sky 6300 Å maps made at Haleakala (Hawaii), 15min intervals

figures from Roach & Gordon: ‘The Light of the Night Sky’ (1973)

many different reactions contribute to nightglow:

tables from Roach & Gordon: ‘The Light of the Night Sky’ (1973)

nightglow lines can be very bright (OI 5577 Å, OH bands in near-IR ) ! stay away from them !

Airglow spectrum La Palma

OH rotational-vibrational lines

||

|

| : street lamps

Benn & Ellison (1998)

Ingham (1962)O2 lines & Zodiacal Light

atmospheric radiation quantities are frequently expressed in Rayleigh units: at given !, 1 R " 4# · (surface brightness B) with B in units of (106 quanta) · cm-2 · s-1 · sterad-1

relation to S10: 1 R·Å-1 = 227 S10 (vis)

near-IR nightglow is dominated by OH emission (rotation-vibration bands)

a forest of emission lines, but between these lines the sky is very dark and the lines are narrow ! higher spectral resolution (R > 104) helps !

example of OH lines in H-band:

Table + fig. From Maihara et al. PASP 105, 940,1993

Principal upper atmosphere emissions (Roach & Gordon 1973)

aurora is transient emission driven by high-energy particles from the Sun mostly a polar phenomenon ! usually unimportant for observatories at latitudes < 40°

when active, aurora can be much brighter than nightglow most prominent auroral lines: OI 5577 Å, 6300 Å, H$ 6563 Å

thermal atmospheric emission

Teff(atmosphere) % 250 K ! peak at ! % 12 µm, but note: atmosphere is not a black body !

Kirchhoff: in thermal equilibrium emissivity = absorptivity ! IR sky emissivity is ‘mirror image’ of IR atmospheric transmission curve

this hits us twice: &atm. high ! large fraction of source photons removed and high sky background

Sea level

RA

DIA

NC

E W

cm

-2 s

r-1 µ

-1

for ! > 2.5 µm thermal radiation from the atmosphere becomes the dominant sky background (hence: the ‘thermal infrared’)

consequence for ground-based observations in the thermal IR: nearly all astronomical sources are very faint w.r.t. sky background

solution: differential measurements by means of ‘chopping + nodding’

VLT + VISIR Q-band spectroscopy medium resolution: R %1500

Example:FRAME TYPE 1: SKY+STAR no chop-subtraction !star invisible w.r.t. high sky level

FRAME TYPE 2: STAR ONLY sky removed by subtracting chopped/nodded frames

3 chopped/nodded spectra of star HD4128 (F18µ = 25 Jy) strong absoptions due to sky lines, but good S/N (> 100) in clean windows

18.2 µm

!

18.6

slit length 32.5”

more noisy horizontal bands: drop in S/N at strong sky line clusters

NB: all ‘raw’ data, no flat-fielding

A B

A’ B’nod-pos. 1 !

" nod-pos. 2

- -+

6”

Ex.: $ Lyr at ! = 20 µ: F' = 12 Jy ! F! = 5.8 x10-14 W·m-2·µ-1

this was outside atmosphere ! on the ground: 1.6 x10-14 W·m-2·µ-1 sky radiance: % 10-5 W·cm-2·sr-1·µ-1 = 2.4 x10-12 W·m-2·µ-1·(arcsec) –2 so if (PSF = 1 arcsec (diffraction-limited 8-m telescope at 20 µ): Fsky = 150 x F($ Lyr) 120 mJy source (m20µ = 5

mag): Fsky = 15000 x Fsource !

comparison with sky background outside atmosphere

outside atmosphere the optical/IR sky brightness is determined by: • dust particles in inner solar system ! zodiacal light • integrated light of faint stars and galaxies

zodiacal light is strongly concentrated towards Sun and ecliptic plane:

zodiacal light in the visual: scattering note surface brightness values in S10

Gegenschein at 180°

compare with backgr. of stars + galaxies:

in the range ! = 3-70 µ thermal emission from zodiacal dust dominates the sky brightness

! (µ)

Note scale: 1 MJy/sterad = 2.35x10-5 Jy/arcsec2 ISO data, Leinert et al. A&A 393,1073, 2002

ecliptic latitude

bri

gh

tness

bri

gh

tness !"!!=30°

40°

60°

!"!!=90°180°

140°

Zodiacal light

Gegenschein

Zodiacal light seenfrom Earth in the visible

II.4 Atmospheric turbulence

turbulence is caused by temperature fluctuations in the convectively unstable troposphere (h < 10 km)

thermal conductivity of air is low ! )T can live long )P is smoothed out very quickly (sound velocity !) ! a) turbulence cells have ()T, )*) w.r.t. environment, but )P = 0 b) once formed, these cells can live rather long (> 1 s)

the lightpath is influenced by )* because (n-1) + * ! turbulence cells work as weak positive/negative lenses that float with wind velocity through the line of sight causing 2 effects: fluctuations in direction ! ‘seeing’ fluctuations in brightness ! ‘scintillation’

apply ideal gas law (index 0 for ambient quantities outside cell):

(n–1)/(n0 –1) = * / *0 = (PT0) / (P0T), P = P0 ! )n = -(n0-1)·(T0/T2)·)T

at sea level: T % T0 % 300K, n0 = 1.000293 (! = 550 nm) ! )n % 10-6 ·)T

at altitude h: (nh-1) = e-h/H ·(n0-1) with H = scale height atmosphere % 8.5 km

! )n = -10-6 · e-h/H · )T )T % 0.1-1 K, h < 10 km ! cells have )n % 10-6–10-7 ! direction changes in range 0.1-1 arcsec

very simple model for optical effects of turbulence cell

L” = L –2h.),

flat wavefront

scintillation: )I + h.e-h/H

seeing: ), + e-h/H

assume: atmosphere is isothermal, in hydrostatic equilibrium

take turbulence cell at height h, with diameter L and n = n0+)n in figure: )n > 0 ()T < 0) ! wavefront retarded inside cell

observations confirm this: turbulence in lower layers has highest weight for seeing (ground layer and dome seeing !)

for small h and typical )T % 1°: ), = 2x10-6 rad = 0.4”

Dtel. - L : at any time only 1 cell in beam ! PSFtelescope

unaffected, but whole image shifts

Dtel > L : image is smeared into seeing disc

without seeing a diffraction-limited telescope with aperture D >L has PSF-diam. . = 1.2 !/D (D= 10 m, ! = 0.5 µ ! . = 0.01”) with seeing: image becomes blob of rapidly moving ‘speckles’ with overall diam. ), regardless of telescope diam. (diam. speckles: 1.2 !/D)

a) direction changes: seeing

L’ = L[n0/(n0+)n)] ! ), = 2(L-L’)/L =

= 2)n / (n0+)n) % 2)n = -2x10-6 ·)T ·e-h/H

this function peaks at h = 0

~1 arcsec

Q : can you estimate the diameter of the telescope ?

seeing - a practical example