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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 °) :
without Moon,
Zodiacal light
Ex.: resolution human eye: ~1arcmin ! at hSUN =0° sky brightness
per res. element is –1.7mag, i.e. brighter than brightest stars
moonlight
average brightness of
moonlight as a function
of lunar phase,
in mag/(arcsec)2
for V and B
Q. : why this change ?
Q. : can you explain
this difference ?
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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
nightglow is strongly variable,
most reactions involve O/O2/O3, N/N2, Na, H2O, OH:
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
Benn & Ellison (1998)
OH rotational-vibrational lines
Ingham (1962)
O2 lines & Zodiacal Light
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| : street lamps
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atmospheric radiation quantities are
frequently expressed in Rayleigh units:
at given !, 1 R " 4# · (surface brightness B)
a forest of emission lines, but between these lines with B in units of (106 quanta) · cm-2 · s-1 · sterad-1
the sky is very dark and the lines are narrow
relation to S10: 1 R·Å-1 = 227 S10 (vis)
! higher spectral resolution (R > 104) helps !
near-IR nightglow is dominated by
OH emission (rotation-vibration bands)
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
for ! > 2.5 µm thermal radiation
from the atmosphere becomes
the dominant sky background
(hence: the ‘thermal infrared’)
Teff(atmosphere) % 250 K
! peak at ! % 12 µm,
but note: atmosphere is
not a black body !
RADIANCE W cm-2 sr-1 µ-1
thermal atmospheric emission
Sea level
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
consequence for ground-based observations in
the thermal IR: nearly all astronomical sources are
very faint w.r.t. sky background
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 = 5mag): Fsky = 15000 x Fsource !
Example:
FRAME TYPE 1: SKY+STAR
no chop-subtraction
!star invisible w.r.t. high
sky level
VLT + VISIR
Q-band
spectroscopy
medium
resolution:
18.2
R %1500
µm
solution:
differential
measurements
by means of
‘chopping +
nodding’
FRAME TYPE 2: STAR ONLY
sky removed by subtracting
chopped/nodded frames
A
nod-pos. 1 !
B
6”
A’
slit length 32.5”
B’
" nod-pos. 2
!
NB: all
‘raw’ data,
no flatfielding
more noisy
horizontal
bands:
drop in S/N at
strong sky line
clusters
18.6
-
+
-
3 chopped/nodded spectra of star HD4128 (F18µ = 25 Jy)
strong absoptions due to sky lines, but good S/N (> 100)
in clean windows
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:
brightness
!"!!=30°
40°
in the range ! = 3-70 µ
thermal emission from
zodiacal dust dominates
the sky brightness
60°
brightness
Gegenschein at 180°
180°
!"!!=90°
140°
zodiacal light in the visual: scattering
note surface brightness values in S10
ecliptic latitude
compare with backgr.
of stars + galaxies:
Note scale: 1 MJy/sterad
= 2.35x10-5 Jy/arcsec2
ISO data, Leinert et al. A&A 393,1073, 2002
! (µ)
Gegenschein
Zodiacal light seen
from Earth in the visible
Zodiacal light
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
flat wavefront
scintillation:
)I + h.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
seeing:
), + e-h/H
observations confirm this: turbulence in lower layers has
highest weight for seeing (ground layer and dome seeing !)
L” = L –2h.),
a) direction changes: seeing
for small h and typical )T % 1°: ), = 2x10-6 rad = 0.4”
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
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)
seeing - a practical example
~1 arcsec
Q : can you estimate the diameter of the telescope ?