Telescopes & recent observational techniques
description
Transcript of Telescopes & recent observational techniques
![Page 1: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/1.jpg)
Telescopes & recent observational
techniques
ASTR 3010
Lecture 4
Chapters 3 & 6
![Page 2: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/2.jpg)
Telescope mounts
![Page 3: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/3.jpg)
Different Designs
Newtonian
Gregorian
Cassegrain
![Page 4: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/4.jpg)
Focal Planes• Prime focus = large field of view, least
number of optical elements (best imaging quality).
• Most radio telescopes
![Page 5: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/5.jpg)
Focal Planes• Prime, Newtonian, Cassegrain, Coude, Coude
![Page 6: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/6.jpg)
Coudé focus• 1m telescope at Teide Observatory on Canary Island useful to use a large instrument with the telescope
![Page 7: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/7.jpg)
Nasmyth foci + Cassegrain focus instrument selector
![Page 8: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/8.jpg)
Telescope mirror• Honeycomb design• Zerodur (zero thermal expansion glass)• Silver (99.5%) or aluminum (98.7%) coating
![Page 9: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/9.jpg)
Protected silver coating (2004-)• Especially important in mid-IR (emissivity = 1 – reflectivity)
![Page 10: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/10.jpg)
Diffraction
![Page 11: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/11.jpg)
Diffraction and Airy Pattern
€
θ =1.22 ×λD
θ :radianλ :wavelengthD:apertture diameter
![Page 12: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/12.jpg)
Atmospheric Seeing
![Page 13: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/13.jpg)
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
![Page 14: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/14.jpg)
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
![Page 15: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/15.jpg)
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
![Page 16: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/16.jpg)
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
![Page 17: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/17.jpg)
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
![Page 18: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/18.jpg)
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”
![Page 19: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/19.jpg)
Recombine 100s of short exposures to achieve the diffraction limited imaging
Speckle imaging
400 100ms exposures
reconstructed image
40sec single exposure
![Page 20: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/20.jpg)
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
![Page 21: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/21.jpg)
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
![Page 22: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/22.jpg)
Adaptive Optics
![Page 23: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/23.jpg)
Adaptive Optics
![Page 24: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/24.jpg)
Adaptive Optics observation
![Page 25: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/25.jpg)
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
![Page 26: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/26.jpg)
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
![Page 27: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/27.jpg)
Anisoplanitsm and cone effect• Different light paths b/w the reference star and others
![Page 28: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/28.jpg)
MCAO & GLAO• Multi-conjugate AO and Ground Layer AO
![Page 29: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/29.jpg)
Laser MCAO at Gemini South
![Page 30: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/30.jpg)
Single AO versus MCAO
![Page 31: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/31.jpg)
• MCAO : Best AO correction over large FOV
GLAO : improve image quality over large FOV
![Page 32: Telescopes & recent observational techniques](https://reader035.fdocuments.us/reader035/viewer/2022062501/56816415550346895dd5ca01/html5/thumbnails/32.jpg)
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