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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.

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Telescopes recent observational techniques

Telescopes & recent observational techniques

ASTR 3010

Lecture 4

Chapters 3 & 6



Different designs
Different Designs

Newtonian

Gregorian

Cassegrain


Focal planes
Focal Planes

  • Prime focus = large field of view, least number of optical elements (best imaging quality).

  • Most radio telescopes


Focal planes1
Focal Planes

  • Prime, Newtonian, Cassegrain, Coude, Coude


Coud focus
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
Nasmyth foci + Cassegrain focus  instrument selector


Telescope mirror
Telescope mirror

  • Honeycomb design

  • Zerodur (zero thermal expansion glass)

  • Silver (99.5%) or aluminum (98.7%) coating


Protected silver coating 2004
Protected silver coating (2004-)

  • Especially important in mid-IR (emissivity = 1 – reflectivity)





Astronomical seeing
Astronomical Seeing

  • In a short exposure, wavefront distortions caused by variations in refractive index in the atmosphere.

short

exposures

speckle pattern

Perfect

wavefronts

Distorted

wavefronts

Star

r0

long

exposures

seeing

disk

TrubulentAtmo.


Continue
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)

Coherent timescale (second) :

t0 = timescale of the change of turbulence

Seeing (radian)

Typically: r0=10cm, t0=10msec  FWHM=1” in the visible (0.5m)


Signature of atmospheric turbulence
Signature of Atmospheric Turbulence

Shorterexposuresallow to freezesomeatmosphericeffects

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 appearswhen the exposureisshorterthan the atmospherecoherencetime t0

1ms exposure at the focus of a 4m diameter telescope


Speckle pattern
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 limitedimaging

reconstructed image

400 100ms exposures

40sec single exposure


Mirror seeing
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


Thermal emission analysis vlt unit telescope
Thermal Emission AnalysisVLT Unit Telescope

UT3 Enclosure

  • 19 Feb. 1999

  • 0h34 Local Time

  • Wind summit: ENE, 4m/s

  • Air Temp summit: 13.8C





Conventional ao
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 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
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
Anisoplanitsm and cone effect

  • Different light paths b/w the reference star and others


Mcao glao
MCAO & GLAO

  • Multi-conjugate AO and Ground Layer AO




GLAO : improve image quality over large FOV


In summary
In summary…

Important Concepts

Important Terms

Seeing

Diffraction limit

Airy ring/pattern

Fried parameter

Atmospheric coherence time

Anisoplanitism

MCAO, GLAO

  • Telescope designs and foci

  • Atmospheric turbulence and its effects on astronomical observations

  • Speckle Imaging

  • Adaptive Optics

  • Chapter/sections covered in this lecture : 3 & 6


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