telescopes & recent observational techniques
DESCRIPTION
Telescopes & recent observational techniques. ASTR 3010 Lecture 4 Chapters 3 & 6. Telescope mounts. Different Designs. Newtonian. Gregorian. Cassegrain. Focal Planes. Prime focus = large field of view, least number of optical elements (best imaging quality). Most radio telescopes. - PowerPoint PPT PresentationTRANSCRIPT
Telescopes & recent observational
techniques
ASTR 3010
Lecture 4
Chapters 3 & 6
Telescope mounts
Different Designs
Newtonian
Gregorian
Cassegrain
Focal Planes• Prime focus = large field of view, least
number of optical elements (best imaging quality).
• Most radio telescopes
Focal Planes• Prime, Newtonian, Cassegrain, Coude, Coude
Coudé focus• 1m telescope at Teide Observatory on Canary Island useful to use a large instrument with the telescope
Nasmyth foci + Cassegrain focus instrument selector
Telescope mirror• Honeycomb design• Zerodur (zero thermal expansion glass)• Silver (99.5%) or aluminum (98.7%) coating
Protected silver coating (2004-)• Especially important in mid-IR (emissivity = 1 – reflectivity)
Diffraction
Diffraction and Airy Pattern
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θ =1.22 ×λD
θ :radianλ :wavelengthD:apertture diameter
Atmospheric Seeing
Astronomical Seeing
• In a short exposure, wavefront distortions caused by variations in refractive index in the atmosphere.
Star
Perfect wavefronts
Trubulent Atmo.
Distortedwavefronts
shortexposures
longexposures
speckle pattern
seeing disk
r0
Continue
• r0 = coherent length typical size of air packet. For a superb seeing: r0~20cm, poor seeing r0~1cm
• Seeing disk = averaged speckle patterns over long exposure.• Seeing disk size = Full width half maximum of the long exposure image.
Half maximum
FWHM
Fried parameter (r0): size of a typical lump of uniform air in the turbulent atmosphere (meter)
Seeing (radian)
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FWHM(λ ) = 0.98λr0∝ λ−0.2
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r0(λ ) = 0.4232Πλ
⎛ ⎝ ⎜
⎞ ⎠ ⎟2
sec(ς) Cn2(h)dh
0
∞
∫ ⎡
⎣ ⎢
⎤
⎦ ⎥−3
5
∝ λ−6 / 5
Typically: r0=10cm, t0=10msec FWHM=1” in the visible (0.5m)
Coherent timescale (second) :
t0 = timescale of the change of turbulence
Atmospheric Turbulence
Shorter exposures allow to freeze some atmospheric effectsand reveal the spatial structure of the wavefront corrugation
Sequential 5sec exposure images in the K band on the ESO 3.6m telescope
Signature of Atmospheric Turbulence
A Speckle structure appears when the exposure is shorter than the atmosphere coherence time t0
1ms exposure at the focus of a 4m diameter telescope
Shorter exposures than t0 speckle imaging
Speckle pattern• Very short (< 10 msec)
exposures of a star
• If you shift these images so that you align the brightest spot always on the same position and add all these shifted images, you can get a greatly improved image which is close to the diffraction limit. This technique is known as “Speckle Interferometry”
Recombine 100s of short exposures to achieve the diffraction limited imaging
Speckle imaging
400 100ms exposures
reconstructed image
40sec single exposure
Mirror Seeing
When a mirror is warmer that the air in an undisturbed enclosure, a convective equilibrium (full cascade) is reached after 10-15mn. The limit on the convective cell size is set by the mirror diameter
21
Thermal Emission Analysis
VLT Unit Telescope
UT3 Enclosure• 19 Feb. 1999• 0h34 Local Time• Wind summit: ENE,
4m/s• Air Temp summit:
13.8C
*>15.0°C
*<1.8°C
2.0
4.0
6.0
8.0
10.0
12.0
14.0
Adaptive Optics
Adaptive Optics
Adaptive Optics observation
Conventional AO• AO performance can be measured by Strehl ratio
IPSF is peak intensity of an actual image, IAiry is the peak intensity of the Airy patternPerfect AO will have a Strehl ratio of 1.0.
• AO corrected field is within an isoplanatic angle from the guide star.• isoplanatic angle is typically 5-6 arcsec at near-IR (~2micron) • Chance of having a suitable guide star (natural guide star) close to your
science target is slim.
• Artificial guide star created by a laser laser guide star (LGS) AO• Still, AO corrected field is within the radius of an isoplanatic angle from
your laser spot.
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RS = IPSF /IAiry
Natural Guide Star (NGS) and Laser Guide Star (LGS)• NGS : using nearby bright stars to
your science target• Make an artificial guide star close
to your science target
Anisoplanitsm and cone effect• Different light paths b/w the reference star and others
MCAO & GLAO• Multi-conjugate AO and Ground Layer AO
Laser MCAO at Gemini South
Single AO versus MCAO
• MCAO : Best AO correction over large FOV
GLAO : improve image quality over large FOV
In summary…
Important Concepts• Telescope designs and foci
• Atmospheric turbulence and its effects on astronomical observations
• Speckle Imaging• Adaptive Optics
Important Terms• Seeing• Diffraction limit• Airy ring/pattern• Fried parameter• Atmospheric coherence time• Anisoplanitism• MCAO, GLAO
Chapter/sections covered in this lecture : 3 & 6