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

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

6. Coudé focus 1m telescope at Teide Observatory on Canary Island 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 Honeycomb design Zerodur (zero thermal expansion glass) Zerodur (zero thermal expansion glass) Silver (99.5%) or aluminum (98.7%) coating Silver (99.5%) or aluminum (98.7%) coating

9. Protected silver coating (2004-) Especially important in mid-IR (emissivity = 1 – reflectivity) 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. In a short exposure, wavefront distortions caused by variations in refractive index in the atmosphere. Star Perfect wavefronts Trubulent Atmo. Distorted wavefronts short exposures long exposures speckle pattern seeing disk r0r0

14. Continue r 0 = coherent length  typical size of air packet. For a superb seeing: r 0 ~20cm, poor seeing r 0 ~1cm r 0 = coherent length  typical size of air packet. For a superb seeing: r 0 ~20cm, poor seeing r 0 ~1cm Seeing disk = averaged speckle patterns over long exposure. Seeing disk = averaged speckle patterns over long exposure. Seeing disk size = Full width half maximum of the long exposure image. Seeing disk size = Full width half maximum of the long exposure image. Half maximum FWHM

15. Fried parameter (r 0 ): size of a typical lump of uniform air in the turbulent atmosphere (meter) Seeing (radian) Typically: r 0 =10cm, t 0 =10msec  FWHM=1” in the visible (0.5  m) Coherent timescale (second) : t 0 = timescale of the change of turbulence Atmospheric Turbulence

16. 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 Signature of Atmospheric Turbulence

17. A Speckle structure appears when the exposure is shorter than the atmosphere coherence time t 0 1ms exposure at the focus of a 4m diameter telescope Shorter exposures than t 0  speckle imaging

18. Speckle pattern Very short (< 10 msec) exposures of a star 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” 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. Recombine 100s of short exposures to achieve the diffraction limited imaging Speckle imaging 400 100ms exposures reconstructed image 40sec single exposure

20. 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. 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

22. Adaptive Optics

24. Adaptive Optics observation

25. Conventional AO AO performance can be measured by Strehl ratio AO performance can be measured by Strehl ratio I PSF is peak intensity of an actual image, I Airy 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. AO corrected field is within an isoplanatic angle from the guide star. isoplanatic angle is typically 5-6 arcsec at near-IR (~2micron) 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. 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 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. 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 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 Different light paths b/w the reference star and others

28. MCAO & GLAO Multi-conjugate AO and Ground Layer AO 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 MCAO : Best AO correction over large FOV GLAO : improve image quality over large FOV

32. 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