BDT11 optIR 2 - Rijksuniversiteit Groningenpeletier/BDT11_optIR_2.pdf · 2011. 10. 6. · Haleakala...
Transcript of BDT11 optIR 2 - Rijksuniversiteit Groningenpeletier/BDT11_optIR_2.pdf · 2011. 10. 6. · Haleakala...
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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
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Q. : can you explain this difference ?
without Moon, Zodiacal light
Q. : why this change ?
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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
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)
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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
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| : street lamps
Benn & Ellison (1998)
Ingham (1962)O2 lines & Zodiacal Light
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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
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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
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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
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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°
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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
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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