solar telescopes Solar Physics course lecture 5 Feb Frans Snik BBL 707

Transcription

1 Solar Physics course lecture 5 Feb Frans Snik BBL 707 f.snik@astro.uu.nl

2 solar vs. nighttime telescopes solar constant: 1.37 kw/m 2 destroys optics creates seeing

3 solar vs. nighttime telescopes 0.5m DOT 1m SST nighttime telescopes: 4-10 m 4m WHT solar telescopes: m

4 fighting local seeing (3.1.3) SST vacuum telescope VTT DST

5 fighting local seeing THEMIS Helium-filled telescope SOLIS

6 fighting local seeing DOT open telescope VTT

7 fighting local seeing active cooling mirror cooling heat stop cooling light path cooling

8 fighting local seeing (3.1.1) SOHO TRACE space /rocket telescope also UV, X-ray no atmospheric extinction no aerosol scattering coronagraph in situ solar wind measurements

9 focal length (3.2.3, 3.2.4) high resolution: d = 2 f r / sol AU (3.21) large f large telescope building to accommodate >50 m of beam length or large f eff re-imaging + field stop steerable telescope

10 telescope types ( ) heliostat

11 telescope types coelostat Sonnenborgh

12 telescope types image rotation

13 telescope types equatorial (RA-dec) IRSOL

14 telescope types altazimuth DST

15 focus positions prime focus (heat stop) (Cassegrain) Gregorian Nasmyth Coudé image rotation & (constant) instrumental polarization?

16 focus positions THEMIS

17 focus positions reflecting and cooled field stop enlargement lens and coma corrector beamsplitter with transport lens parabolic primary G band camera Hα camera Ca II H camera Ba II camera Hα Lyot filter Ba II Lyot filter + polarization modulation pupil stop refocusing - and telecentric lens dichroic splitters (schematic) correction lenses for each channel (except G band) DOT

18 aberrations (3.2.3) spherical use parabolae, high F/# coma meniscus lenses, small field astigmatism cylindrical optics, bend chromatic use mirrors, achromats

19 coronagraph (3.6.3) corona is x fainter than the disk scattered/diffracted light is a huge issue coronographic mask Lyot stop in pupil plane to kill diffracted light off-axis design clean optics

20 options fixed vs. steerable open vs. closed tube reflective vs. refractive on-axis vs. off-axis field size (aberrations) focus position image rotation instrumental polarization mechanical difficulty

21 new designs Gregor (1.5 m) ATST (4 m)

22 site selection (3.1.3) no jet streams tropics high volcano in ocean stable trade winds and atmospheric stratification (low inversion layer) in a lake less ground layer seeing still need high tower to decrease influence of ground layer seeing

23 site selection Roque de los Muchachos La Palma Big Bear lake, CA Haleakala, Maui, HI Udaipur, India

24 seeing (3.1.2) blurring image motion image distortion due to atmospheric pressure/temperature fluctuations n 1 P T / / P T 0 0 (3.1)

25 PSFs and MTFs I( x, y) = I0( x', y' ) PSF PSF: point spread function : convolution (3.2) MTF: modulation transfer function = F(PSF) MTF = MTF MTF total telescope seeing (3.4)

26 PSFs and MTFs diffraction limited: PSF telescope = Airy function (3.6) fully sampled seeing: PSF seeing =Gaussian(s 0 ) (3.9) s 0 : ~angular resolution good seeing: s 0 <1

27 Fried parameter Kolmogorov turbulence spectrum of n: r 0 (Fried parameter): characteristic coherence scale of turbulent air pockets in the light path PSF total is seeing-dominated for D>r 0 τ r 0 V wind ~10 ms

28 seeing monitors scintillation (ζ=zenith angle) ground layer seeing σ I 1/5 1/5 cos ζ < h > r 0 5/ 6 differential image motion of limb Longintudinal and Transverse between two apertures Σ 2 L, Σ 2 T 2 5/3 λ r0

29 compensating seeing adaptive optics post processing

30 adaptive optics (3.1.4) focusing tip-tilt deformable mirror

31 adaptive optics Shack-Hartmann wavefront sensor isoplanatic patch

32 post processing (3.1.5) frame selection ( lucky imaging ) rubbersheeting speckle reconstruction phase diversity (MO)MFBD

33 speckle reconstruction 2 O = I S t t 2 2 < S n 2 > needs to be determined =speckle transfer function in Fourier space =f(r 0 (t), PSF telescope ) (3.16) also need Fourier phase from cross-spectrum I ( f ) I *( f + f ) = O( f ) O*( f + f ) S( f ) S *( f + f ) real! (3.17)

34 speckle reconstruction 100 samplings of max. 20 ms within solar change time ( ) still need reasonable seeing

35 speckle reconstruction

36 speckle reconstruction

37 phase diversity =wavefront measurement ( ) two images of same object at known different focal positions allow for two local samplings of the PSF object + aberrations

38 exercises list pros and cons of the different telescope types design the ultimate solar telescope 3.1 formulate the image (de)rotation properties of a Dove prism (3.2)