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Atmospheric Dispersion Correction for ELT instruments

Atmospheric Dispersion Correction for ELT instruments. David Henry, Eli Atad, Tim Hawarden UKATC, Edinburgh Nicholas Devaney, Alexander Goncharov, Chris Dainty Applied Optics group, NUI, Galway. OBJECTIVES OF STUDY Identify and attempt to quantify all effects

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Atmospheric Dispersion Correction for ELT instruments

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  1. Atmospheric Dispersion Correction for ELT instruments David Henry, Eli Atad, Tim Hawarden UKATC, Edinburgh Nicholas Devaney, Alexander Goncharov, Chris Dainty Applied Optics group, NUI, Galway Ringberg 26-29 July 2005

  2. OBJECTIVES OF STUDY Identify and attempt to quantify all effects of Atmospheric Dispersion (AD) which might affect the performance of a European ELT For each effect, examine implications and possible methods of correction Produce “demonstration-of-possibility” ADC designs for proposed 50-m (“Euro50”) and 100-m (“OWL”) ELT designs Ringberg 26-29 July 2005

  3. EFFECTS OF ATMOSPHERIC DISPERSION(Terminology: where different from ours, that of Hardy, 1998, “Adaptive Optics for Astronomical Telescopes” (OUP) p 322, is given in parentheses) • Refractive Angular Dispersion: familiar ZD-dependent effect, requires “classical” ADC correction (see later). • Refractive chromatic seeing (Chromatic path-length error). (Wallner, 1977: JOSA 67, 407: hereinafter W77) • Refractive chromatic anisoplanatism (Dispersion displacement error) (W77) • Refractive multispectral error (Hardy 1998:hereinafter H98) • Diffractive Effects (e.g. Hogge & Butts, 1982: JOSA, 72, 606) Ringberg 26-29 July 2005

  4. PROBLEMS ON ELTS? • All these effects except angular dispersion are considered negligible on current AO systems (near-IR on D ≤ 10m). • BUT what about ELTs ? They will want AO: • At (some) visible wavelengths; • Over significant FOVs (1'-2' MCAO, 10' GLAO); • Giving Strehl Ratios > 0.7 in some cases • Diffraction limit @ λ = 1μm~ 0."1/D, so  FWHM = 1mas for D=100m Very easy to have problems on an ELT! Ringberg 26-29 July 2005

  5. Atmospheric dispersionon 100m ELT … on Mauna Kea (4196m), AD in units of the diffraction limit λ/D, for bandwidth R = λ/Δλ = 5 Ringberg 26-29 July 2005

  6. EFFECTS OF ATMOSPHERIC DISPERSION: (1) Refractive Angular Dispersion Familiar as “Atmospheric Dispersion” We have produced ADC designs for • 50-m f/13 “Euro50” • 100-m f/6 “OWL” Ringberg 26-29 July 2005

  7. R R’f 8 Euro50: Linear Atmospheric Dispersion Compensator Avila et al., 1997: Proc SPIE, 2871,1135; Owner-Petersen & Goncharov, 2004: Proc SPIE, 5489, 507 f 4 Identical opposing prisms displace dispersed images back into coincidence Ringberg 26-29 July 2005

  8. Dispersion Correction with SF5 Glass • Converging beam causes aberrations: • Corrected by deformation of M2. At z=45º: • Z3= -57 m • Z4= 0.003 m • Z6= 0.22 m • Z7= -1.27 m • (Wedge separation = 1275 mm) Ringberg 26-29 July 2005

  9. Euro50 linear ADC: J band, Z=45  Circle = Airy disc Ringberg 26-29 July 2005

  10. Euro50 linear ADC in Jband, Z=45  Nominal diffraction limit (S = 0.8) SF5 Glass Ringberg 26-29 July 2005

  11. ADC for Euro50: conclusions • The size of optics (D1.2m) limits FoV to 4' • Deformable M2 must correct for SA, C & AS • Rotating Prisms ADC better for NIR • Linear ADC better in the visible • Performance Summary Ringberg 26-29 July 2005

  12. ADC for OWL: visible band, collimated beam DETAILS • 500mm beam diameter • Three glasses (Ohara): • LAH80 • S-LAH64 • S-BSM93 • All plane surfaces, collimated beam • No monochromatic aberrations  Good images Ringberg 26-29 July 2005

  13. Collimated Beam Design • Residual AD after correction • R=5 bandwidth • Note plots of l/D for D=100m Ringberg 26-29 July 2005

  14. Glasses: Schott LLF6HT, SF14, Ohara S-TIM22 Waveband 0.5 – 1.8μm 700mm diameter 60 – 100mm thick ADC for OWL: visible band, converging beam,rotating design using 3 glasses Ringberg 26-29 July 2005

  15. 30 m-arcsec 60° 15° 45° ZD = 0° 30° Performance of 3 glass converging-beam design R Band On axis 30" off axis Box side 30 mas 60" off axis Ringberg 26-29 July 2005

  16. Box side 30 mas On axis 15° 45° ZD = 0° 30° 30" off axis • Promising • performance: • ZD ≤ 45° • FoV≤ 1' • R, I & J Bands 60" off axis Ringberg 26-29 July 2005

  17. ADC for OWL: NIR band, converging beam,using curved ZnS, ZnSe prisms Throughput Ringberg 26-29 July 2005

  18. Performance of NIR design: ZD = 0° Circle = Airy disc Field centre 30" off axis 60" off axis Ringberg 26-29 July 2005

  19. Performance of NIR design: ZD = 60° Circle = Airy disc Field centre 30" off axis 60" off axis Ringberg 26-29 July 2005

  20. Performance of NIR design: ZD = 60° Diffraction limit Ringberg 26-29 July 2005

  21. OWL NIR ADC: Performance summary RMS spots(mas) Ringberg 26-29 July 2005

  22. Control of ADCs on ELTs • Open loop: control by model N(z,p,T, RH) • Several models, eg. Owens (1967); Ciddor (1996) • Disagreement: Owens-Ciddor = 3mas [0.4- 0.8μm, ZD=45°] • Accuracy of Ciddor model is ~5.10-8, ≡ 4mas [0.4-0.8 μm @ 45°] • Need closed-loop control?? • Measure B, R (say) image positions behind ADC • Feedback to ADC control • Field effects (2nd order): correct by optical model Ringberg 26-29 July 2005

  23. EFFECTS OF ATMOSPHERIC DISPERSION (2) • Refractive chromatic seeing aka: • Dispersive seeing (Russell et al., 2004: Proc. SPIE, 5382,684;) • Chromatic (path-length) error (W77) Ringberg 26-29 July 2005

  24. Chromatic Seeing • Optical Path Length differs for rays of different λ, even at ZD = 0° • Achromatic AO can’t correct perfectly over a finite waveband. Residual wave-front variance (W77) is σ2ch= ε2(λ)σ2u where σ2u = pre-AO (uncorrected) wavefront variance, ε(λ) = scale factor for AD = λ0 • ns(λ) - ns(λ0) λ ns(λ0) - 1 Integrating over “visible” ~0.33 to ~0.72 μm  ε2(λ) ≈ 10-4 (W77) • For Kolmogorov turbulence (infinite outer scale length L0), σ2u = 1.03 (D/r0)5/3(Δ1 in Table IV of Noll, 1976: JOSA, 66, 207) and we may estimate the Strehl ratio (Maréchal approximation): Sch= exp(-σ2ch). For D=100m, r0 = 20cm, σ2ch= 3.2 rad2: S~0.04 Ringberg 26-29 July 2005

  25. Sch= 0.04! Is this a showstopper? Fortunately, it seems not…. Ringberg 26-29 July 2005

  26. 1.0 D = 30m 0.8 D = 50m 0.6 Strehl Ratio 0.4 D = 100m • Infinite L0 (Kolmogorov turbulence) • λmeas= 0.589μm • r0 = 0.2m 0.2 0.0 2.0 2.5 1.0 1.5 0.0 0.5 Observation wavelength (μm) Strehl ratios as functions of observation wavelength for infinite L0(Owner-Petersen &Goncharov, 2004) Ringberg 26-29 July 2005

  27. The Outer Scale of turbulence (L0) to the rescue! Ringberg 26-29 July 2005

  28. Coefficient of total variance σ2chfor infinite L0 Coefficient of σ2ch for finite L0 100 10 1 L0/Dtel Effect on wavefront variance σ2ch of reducing L0: • ZJ = coefficient of summed Zernike terms of order > J Dramatic reduction for L0 < 10Dtel (Adapted from Winker, 1991: JOSA-A, 8, 1568) Ringberg 26-29 July 2005

  29. So what is a typical (L0) value? Ringberg 26-29 July 2005

  30. Outer Scale length L0 at Palomar • 10 nights in Sept 2001 • Measured with Palomar GSM, validated by: • Palomar AO system PALAO in DIMM mode, • Analysis of Zernikes from PALAO • Palomar Test-bed Interferometer PTI) (Ziad et al., Applied Optics, 43, 2316; 2004) Median L0 = 17.5m 1089 data points rms log[L0] = 0.24 Ringberg 26-29 July 2005

  31. So what is a typical (L0) value?Evidently, a few tens of metres.. Ringberg 26-29 July 2005

  32. So what is a typical (L0) value?Evidently, a few tens of metres..And its effect? Ringberg 26-29 July 2005

  33. Strehl ratios as functions of observation wavelength for finite (but unusually large?) L0 1.0 Dtel = 30m, 50m and 100m 0.8 0.6 Strehl Ratio 0.4 • L0 = 100m • λmeas= 0.589μm • r0 = 0.2m 0.2 0.0 2.0 2.5 1.0 1.5 0.0 0.5 Observation wavelength (μm) Ringberg 26-29 July 2005

  34. So what is a typical (L0) value?Evidently, a few tens of metres..And its effect?Problem goes away! Ringberg 26-29 July 2005

  35. EFFECTS OF ATMOSPHERIC DISPERSION: (3) Refractive chromatic anisoplanatism aka: • Refraction-induced error(W77) • Chromatic errors (refraction) (Roddier & Roddier, 1986: Proc SPIE, 628, 298) Ringberg 26-29 July 2005

  36. Refractive chromatic anisoplanatism • Different λs coincident at the top of atmosphere, but displaced relative to each other by dispersion • On reaching telescope: • Position of pupil depends on wavelength, so … • AO system DM out of register with wavefront errors at wavelengths ≠ sensing/ central λ • For ZD > 0, AO performance impaired at wavelengths different from the sensing or correction wavelength, even if band is centered at same λ Ringberg 26-29 July 2005

  37. Telescope aperture Refracted rays b0 Refractive chromatic anisoplanatism Integral over band being corrected Ringberg 26-29 July 2005

  38. Results for: • Bandwidth • 0.4 – 0.9μm • Mauna Kea • atmosphere • NB: “typical” r0 • (seeing = 1."0): • 0.4 μm ≈ 0.06m • 0.9 μm ≈ 0.2m • So pupil shift • will produce • significant • anisoplanatism Pupil Shift Δb0 (m) Zenith Distance (degrees) Ringberg 26-29 July 2005

  39. Refractive chromatic anisoplanatism0=0.7 m (observing); s=0.8 m (sensing) SR(at 0.7m) vs zenith angle for correction at visible wavelengths Hufnagel-Valley turbulence profile at Mauna Kea (r0=30 cm); Owen’s dispersion formula with T=0 deg C; 20% H; 615mb P) Is this a showstopper ??? Ringberg 26-29 July 2005

  40. 1.00 0.75 0.50 0.25 0.00 0 30 45 60 15 ZD (degrees) Refractive chromatic anisoplanatism Strehl ratio at λ = 0.55 m • 1= 0.4 m; 2=0.75 m • Hufnagel-Valley CN2 profile on Mauna Kea • r0=10 cm at λ=0.55 m Ringberg 26-29 July 2005

  41. EFFECTS OF ATMOSPHERIC DISPERSION: (4) Refractive multispectral error (H98) • This is closely related to both chromatic anisoplanatism and to dispersive seeing. • From H98 it appears not to be a threat, except at short λsensand large ZD. • NEEDS FURTHER INVESTIGATION…. ! Ringberg 26-29 July 2005

  42. Ringberg 26-29 July 2005

  43. EFFECTS OF ATMOSPHERIC DISPERSION: (5) Diffractive effects (Hogge & Butts 1982: JOSA,72, 606) • Diffraction at a turbulent screen followed by propagation will lead to scintillation, which will depend on λ. • Initial estimates suggest this is not a threat (pupil displacements of order 1mm) but NEEDS FURTHER INVESTIGATION! Ringberg 26-29 July 2005

  44. Diffractive effects λ1=0.5 μm Area of interest looks like SR~0.9 Ringberg 26-29 July 2005

  45. Further modelling required to evaluate effects of AD on ELTs: • Include finite outer scale in Wallner’s analysis of refractive chromatic anisoplanatism (1984) • Include outer scale in refractive chromatic anisoplanatism • Examine diffraction effects • Calculate refractive chromatic anisoplanatism for different turbulence profiles, bandwidths … Ringberg 26-29 July 2005

  46. Present status of ADC designs • LADC and RADC visible designs for Euro50: Possible solutions for Euro50 in f/13 converging beam including active correction with telescope • ADC visible designs for OWL: needs more investigation including active correction with telescope • RADC NIR design for OWL: Solution giving DL performance Ringberg 26-29 July 2005

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