1 / 32

Telescopes & recent observational techniques

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.

gamba
Download Presentation

Telescopes & recent observational techniques

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Telescopes & recent observational techniques ASTR 3010 Lecture 4 Chapters 3 & 6

  2. Telescope mounts

  3. Different Designs Newtonian Gregorian Cassegrain

  4. Focal Planes • Prime focus = large field of view, least number of optical elements (best imaging quality). • Most radio telescopes

  5. Focal Planes • Prime, Newtonian, Cassegrain, Coude, Coude

  6. Coudé focus • 1m telescope at Teide Observatory on Canary Island  useful to use a large instrument with the telescope

  7. Nasmyth foci + Cassegrain focus  instrument selector

  8. Telescope mirror • Honeycomb design • Zerodur (zero thermal expansion glass) • Silver (99.5%) or aluminum (98.7%) coating

  9. Protected silver coating (2004-) • Especially important in mid-IR (emissivity = 1 – reflectivity)

  10. Diffraction

  11. Diffraction and Airy Pattern

  12. Atmospheric Seeing

  13. 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.

  14. 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

  15. 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)

  16. 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

  17. 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

  18. 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”

  19. Speckle imaging Recombine 100s of short exposures to achieve the diffraction limitedimaging reconstructed image 400 100ms exposures 40sec single exposure

  20. 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 AnalysisVLT Unit Telescope UT3 Enclosure • 19 Feb. 1999 • 0h34 Local Time • Wind summit: ENE, 4m/s • Air Temp summit: 13.8C

  22. Adaptive Optics

  23. Adaptive Optics

  24. Adaptive Optics observation

  25. 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.

  26. 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

  27. Anisoplanitsm and cone effect • Different light paths b/w the reference star and others

  28. MCAO & GLAO • Multi-conjugate AO and Ground Layer AO

  29. Laser MCAO at Gemini South

  30. Single AO versus MCAO

  31. MCAO : Best AO correction over large FOV GLAO : improve image quality over large FOV

  32. 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

More Related