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Atmospheric Effects and Other Observational Issues AS3100 Lab. Astronomi Dasar 1 Prodi Astronomi 2007/2008 B. Dermawan.

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Presentation on theme: "Atmospheric Effects and Other Observational Issues AS3100 Lab. Astronomi Dasar 1 Prodi Astronomi 2007/2008 B. Dermawan."— Presentation transcript:

1 Atmospheric Effects and Other Observational Issues AS3100 Lab. Astronomi Dasar 1 Prodi Astronomi 2007/2008 B. Dermawan

2 While convenient for breathing, the atmosphere is the single biggest annoyance for observing… Issues of particular importance: scattering, absorption, refraction, atmospheric turbulence …which impact: integration time, resolution, altitude (airmass) range at which one can observe, telescope site selection

3 Scattering Key factor: size of scattering particles (d) Rayleigh scattering: d <<  proportional to -4 Mie scattering: d   weak dependence Non-selective scattering: d >>  wavelength independence

4 Rayleigh Scattering Scattering by air molecules Strongly wavelength dependence Observable effects: - Blue sky - Red sunrise and sunset

5 Mie Scattering Scattering by aerosols and dust Weakly wavelength dependent Strongly directional – stronger and more intense in the forward direction Observable effects: - White light for clouds and fog - Halo around street light

6 Zodiacal Light  Mie scattering in UV, optical, NIR - Solar system Dust - Mostly particles larger than 10  m  Thermal emission in FIR Left: zodiacal light at Mauna Kea, Right: photo by Jack Newton,

7 Comparison of Rayleigh & Mie Scattering

8 Absorption (1) Radiant energy are absorbed and reradiated at a different wavelength Primary culprits: molecular rotational and vibrational bands (H 2 0, C0 2, O 3, O 2 ) Net result: atmospheric “windows” that are observable from the ground: optical, radio, narrow windows in IR Exact transparency dependent upon atmospheric conditions: most importantly water vapor content

9 Absorption (2) Also referred as atmospheric extinction Typical value at zenith for selected observatories (in mag): Kittpeak: (U  0.5; B  0.25; V  0.15) Mitaka: (B  0.44; V  0.27; R  0.24; I  0.12) Rule of thumb: A 0.1 mag change is ~10% Decent approximation for  m < 0.5

10 Refraction  1 n 1  2 n 2 In an astronomical context, the incident medium is the vacuum ( n 1 = 1) and the refractive medium is the atmosphere. For air, n 2  , which means that the angle of refraction is close, but not quite the same, as the angle of incident. Most importantly, the index of refraction of air is a function of temperature and pressure – integrated along the path of the light ray through the atmosphere It is also a function of wavelength Snell’s Law: n 1 sin  1 = n 2 sin  2

11 Refraction: examples flatteneddeformed, flattened, distorted The Sun is above horizon: Light rays from the low Sun will refract more closer to the horizon The Sun is low above horizon: The optical path of light is very long. The atmosphere usually has a layered structure: different temp. gradient and pressure. Inferior and superior mirages are responsible for the distortions Why is the bottom of the Sun redder?

12 Refraction: observational impacts Telescope pointing Changes the direction in which a telescope must be pointed (usually a minor effect), with the amount being dependent upon zenith angle Different refraction for different wavelengths  Dispersion is the variation in refraction as a function of wavelength  elongated  Can be problematic for astrometry (usually minor) and spectroscopy, where some wavelengths of interest may not even pass through the slit of spectrograph

13 Refraction: observational impacts Scintillation and seeing  The atmosphere is not quiescent planar medium, but rather is characterized by turbulence and spatial variability in the temp. and pressure.  Because the index of refraction varies with temp. and pressure, light passing along different paths will refract differently.  Rapid fluctuation  scintillation  changes the apparent positions and sizes of stars  When time averaged into an image on the detector  seeing

14 Why do stars twinkle? The Earth’s atmosphere is clumpy, so that different air pockets produces different images of a single point-like star. Because the atmosphere is always windy and changing, the number and position of images is always changing, with the result that the stars appear to twinkle. In reality, the above time-lapse sequence occurs ten times faster. Why don’t planets twinkle?

15 Airmass (1) When observing a star, the total atmospheric absorption depends upon the column density of gas along the line of sight. Away from zenith, the column density of material increases, and hence so does the absorption. First approx.: planar atmosphere (plane parallel) x = h sec z (descent for z <= 60°) For z > 60°: x = sec z – (sec z -1) – (sec z -1) 2 – (sec z -1) 3

16 Airmass (2) For z = 60 ° the planar model gives x = 2. For many observing programs, this is about the maximum airmass Other drawbacks: Increased scattering Increased reddening (color- dependent extinction) Degraded image quality in terms of seeing Grater light pollution from scattered city light

17 Light Pollution

18 Optimal Telescope Location  Dry: - Low water vapor content to minimize absorption - High number of cloudless night  High altitude: Minimize atmospheric column density  Isolated (but accessible): Minimize light and air pollutions  Minimal atmospheric turbulence: Best possible seeing … or else put the telescope in space …


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