Download Life in the Universe

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
  1.22 

D
 :radian
 :wavelength
D:apertture diameter

Atmospheric Seeing
Astronomical Seeing
short
exposures
Perfect
wavefronts
Star
speckle pattern
Distorted
wavefronts
r0
Trubulent Atmo.
long
exposures
seeing
disk
• In a short exposure, wavefront distortions caused by variations in refractive
index in the atmosphere.
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

Atmospheric Turbulence
Fried parameter (r0): size of a typical lump of
uniform air in the turbulent atmosphere (meter)
3


2 
2
r0 ( )  0.423  sec( )  Cn (h)dh 
  


0

2
5
 6 / 5
Coherent timescale (second) :
t0 = timescale of the change of turbulence
Seeing (radian)
FWHM (  )  0.98

r0
 0.2

Typically: r0=10cm, t0=10msec  FWHM=1” in the visible (0.5m)
Signature of Atmospheric Turbulence
Shorter exposures allow to freeze some atmospheric effects
and reveal the spatial structure of the wavefront corrugation
Sequential 5sec exposure images in the K band on the ESO 3.6m telescope
Shorter exposures than t0  speckle imaging
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
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”
Speckle imaging
Recombine 100s of short exposures to achieve the diffraction limited imaging
reconstructed image
400 100ms exposures
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
20
Thermal Emission Analysis
VLT Unit Telescope
*>15.0°C
14.0
12.0
10.0
8.0
UT3 Enclosure
• 19 Feb. 1999
• 0h34 Local Time
• Wind summit: ENE,
4m/s
• Air Temp summit:
13.8C
6.0
4.0
2.0
*<1.8°C
21
Adaptive Optics
Adaptive Optics
Adaptive Optics observation
Conventional AO
• AO performance can be measured by Strehl ratio
RS  IPSF / IAiry
IPSF is peak intensity of an actual image, IAiry is the peak intensity of the Airy pattern
Perfect 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.
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
Important Terms
• Telescope designs and foci
•
•
•
•
•
•
•
• Atmospheric turbulence and its
effects on astronomical
observations
• Speckle Imaging
• Adaptive Optics
Seeing
Diffraction limit
Airy ring/pattern
Fried parameter
Atmospheric coherence time
Anisoplanitism
MCAO, GLAO
Chapter/sections covered in this lecture : 3 & 6