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Lecture 12 Part 1: Laser Guide Stars, continued Part 2: Control Systems Intro

Lecture 12 Part 1: Laser Guide Stars, continued Part 2: Control Systems Intro. Claire Max Astro 289, UC Santa Cruz February 14, 2013. Outline of laser guide star topics. Why are laser guide stars needed? Principles of laser scattering in the atmosphere

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Lecture 12 Part 1: Laser Guide Stars, continued Part 2: Control Systems Intro

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  1. Lecture 12Part 1: Laser Guide Stars, continuedPart 2: Control Systems Intro Claire Max Astro 289, UC Santa Cruz February 14, 2013

  2. Outline of laser guide star topics • Why are laser guide stars needed? • Principles of laser scattering in the atmosphere • What is the sodium layer? How does it behave? • Physics of sodium atom excitation • Lasers used in astronomical laser guide star AO • Wavefront errors for laser guide star AO

  3. First, a digression on Robo-AO System • Palomar 60” telescope, Christoph Baranec PI (Caltech) • Fully robotic AO system and Rayleigh laser guide star • LGS is range gated – 650 m at 10 km • Makes guide star with mV~9

  4. Small size: MEMS DM

  5. Potential issues with robotic LGS system • FAA: must avoid laser shining on airplanes • Robo-AO has UV laser, not an issue • FAA says it’s fine • Space Command: must avoid laser shining on spacecraft • Submit target lists to Space Command several days ahead of time • Robo-AO has Target of Opportunity mission (don’t know in advance where targets are) • Also has survey mission: many potential targets • Novel solution – see next slide

  6. Laser guide star AO needs to use a faint tip-tilt star to stabilize laser spot on sky from A. Tokovinin

  7. Effective isoplanatic angle for image motion: “isokinetic angle” • Image motion is due to low order modes of turbulence • Measurement is integrated over whole telescope aperture, so only modes with the largest wavelengths contribute (others are averaged out) • Low order modes change more slowly in both time and in angle on the sky • “Isokinetic angle” • Analogue of isoplanatic angle, but for tip-tilt only • Typical values in infrared: of order 1 arc min

  8. Tip-tilt mirror and sensor configuration Telescope Deformable mirror Tip-tilt mirror Beam splitter Wavefront sensor Tip-tilt sensor Beam splitter Imaging camera

  9. Tip-tilt correction determines LGS sky coverage fraction • Trade-off between the low probability of high quality TT correction (bright nearby TT stars) and broad area coverage at lower performance (dimmer TT stars and farther away) • Use statistics on number of stars per square degree to determine whether a bright enough star will be within tilt anisoplanatic angle • There is no absolute “sky coverage fraction.” • Rather, you can ask “statistically, over what fraction of the sky am I likely to obtain a tip-tilt correction better than xxx milli-arc-sec?”

  10. Infrared versus optical tip-tilt sensing • Until now, all tip-tilt sensing has been done using visible light • Visible-light CCDs had lower read noise, read out faster than infrared arrays • This is changing rapidly: much better IR arrays • Keck NGAO, TMT NFIRAOS, other AO systems plan to use infrared tip-tilt sensing • Advantage: higher sky coverage • There are many more low-mass stars (faint, red) than high-mass stars (bright in visible wavelengths)

  11. Tip-tilt sensing at K band gives much higher sky coverage • TRICK is new IR tip-tilt sensor for Keck 1 (Caltech + Keck) Existing visible TT sensor New IR tip-tilt sensor, K band

  12. New wavefront errors for laser guide star AO • “Cone effect” • Tilt anisoplanatism

  13. “Cone effect” or “focal anisoplanatism” for laser guide stars • Two contributions: • Unsensed turbulence above height of guide star • Geometrical effect of unsampled turbulence at edge of pupil from A. Tokovinin

  14. Cone effect, continued • Characterized by parameter d0 • Hardy Sect. 7.3.3 (cone effect = focal anisoplanatism) σFA2= ( D / d0)5/3 • Typical sizes of d0 ~ a few meters to 20 meters • Cone effect gets worse fast, as telescopes get larger • Remedy will be to use multiple guide stars

  15. Dependence of d0 on beacon altitude • One Rayleigh beacon OK for D < 4 m at λ = 1.65 micron • One Na beacon OK for D < 10 m at λ = 1.65 micron from Hardy

  16. Cone effect for one laser guide star 90 km “Missing” Data Credit: Miska Le Louarn

  17. Multiple laser guide stars can measure the un-sensed turbulence 90 km Credit: Miska Le Louarn

  18. Tilt anisoplanatism: residual TT errors if TT star is too far away • See Hardy section 7.4 (reading for next Tuesday) • Need separate tip-tilt star because laser (up and down thru atmosphere) moves differently on the sky than a “real” star

  19. Effects of laser guide star on overall AO error budget • The good news: • Laser is brighter than your average natural guide star • Reduces measurement error • Can point it right at your target • Reduces high-order anisoplanatism • The bad news: • Still have tilt anisoplanatism • New: focus anisoplanatism • Laser spot larger than NGS (lower SNR for high-order aberrations)

  20. Residual tip-tilt error due to tip-tilt anisoplanatism • Hardy sections 7.4.2 – 7.4.4 • Small angle approximation: for field angles < D/40,000 • Angle θTA is the angle between the target and the TT star such that the wavefront phase error due to tilt anisoplanatism is 1 radian and

  21. Compare NGS and LGS performance • From a Keck study several years ago

  22. LGS Hartmann spots are elongated Sodium layer Laser projector Telescope Image of beam as it lights up sodium layer = elongated spot

  23. Elongation in the shape of the LGS Hartmann spots Representative elongated Hartmann spots Off-axis laser projector Keck pupil

  24. Keck: Subapertures farthest from laser launch telescope show laser spot elongation Image: Peter Wizinowich, Keck

  25. New CCD geometry for WFS being developed to deal with spot elongation CW Laser Pulsed Laser Sean Adkins, Keck

  26. Polar Coordinate Detector • CCD optimized for LGS AO wavefront sensing on an Extremely Large Telescope (ELT) • Allows good sampling of a CW LGS image along the elongation axis • Allows tracking of a pulsed LGS image • Rectangular “pixel islands” • Major axis of rectangle aligned with axis of elongation

  27. Laser guide star topics we’ve discussed • Why are laser guide stars needed? • Principles of laser scattering in the atmosphere • What is the sodium layer? How does it behave? • Physics of sodium atom excitation • Lasers used in astronomical laser guide star AO • Digression on Robo-AO system • Wavefront errors for laser guide star AO

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