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

€

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)

€

FWHM(λ ) = 0.98λr0∝ λ−0.2

€

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.

€

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