Today: Atmospheric Seeing Adaptive Optics Reading: Ch. 7 (pp. 126–143), Bradt, Ch. 5.5
Astronomy 101 Lecture # 8
Contributions to “Seeing”
Image motion and blurring: Movement of the image centroid due to atmospheric turbulence.
Scintillation: Image intensity fluctuations due to atmospheric turbulence.
Astronomy 101 Lecture # 8
Ideal (noatmosphere) Case
Astronomy 101 Lecture # 8
θmin = 1.22 λ/d
d = telescope diameter
θmin
Optics Refresher
Astronomy 101 Lecture # 8
n1
n2
n1 < n2v1 > v2
Velocity of light in matter is:
where n is index of refraction.
Results: 1) Compression of wavefronts
at larger n; 2) Snell's Law:
v=cn
n1sin 1=n2sin 2
2
1
Kband imaging of the galactic center with integration time 0.3 sec. Images obtained with a Speckle Camera SHARP I at La Silla, Chile.
Image Credit: IR and Submillimeter Astronomy Group at MPE
Astronomy 101 Lecture # 8
Effects of Atmosphere: Speckles
Dependence of “Seeing” on Airmass
Astronomy 101 Lecture # 8
Fried's coherence length:
The “seeing” angle:
rc≈0.114 m
550nmcosz
0.6
c≈1' ' sec z0.6
550nm 0.4
Kband imaging of the galactic center with integration time 0.3 sec. Images obtained with a Speckle Camera SHARP I at La Silla, Chile.
Image Credit: IR and Submillimeter Astronomy Group at MPE
Astronomy 101 Lecture # 8
Effects of Atmosphere: Speckles
Scintillation
Astronomy 101 Lecture # 8
Photometric noise for point stars:
sec z = airmassd = telescope diameter (m)t = integration time (s)h = elevation above sea level (m)
msc≃0.09sec z7/4
d2 /32 t
550nm −7 /12
e−h/8000m
Astronomy 101 Lecture # 8
Image Credit: Louise Good, U Hawaii
Palomar Adaptive Optics System
Astronomy 101 Lecture # 8
An image of an mV=10 star with and without AO correction. In the equivalent of 1.3 arcsecond visible seeing a corrected Kband image with a FWHM of 0.16 arcseconds (~1.7x the diffraction limit) was achieved. The laser power was 6 watts and the high order AO wavefront sensor was running at a frame rate of 150Hz.
Complete Adaptive Optics System
Astronomy 101 Lecture # 8
Image Credit: ARC (Australian Research Council)
●Sampling System
●Wavefront Sensing
●Wavefront Correction
Astronomy 101 Lecture # 7
Image of the GC taken with a NIR adaptive optics camera on an 8.2 m VLT (ESO)Image of the GC obtained using
speckle imaging at La Silla, Chile
Image Credit: IR and Submillimeter Astronomy Group at MPE1”
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