lecture 12 part 1: laser guide stars, continued part 2: control systems intro

Lecture 12Lecture 12

Part 1: Laser Guide Stars, continuedPart 1: Laser Guide Stars, continued

Part 2: Control Systems IntroPart 2: Control Systems Intro

Claire MaxAstro 289, UC Santa Cruz

February 14, 2013

Outline of laser guide star topics 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

First, a digression on Robo-AO First, a digression on Robo-AO SystemSystem

• 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

Small size: MEMS DM

Potential issues with robotic LGS Potential issues with robotic LGS systemsystem

• 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

Laser guide star AO needs to use Laser guide star AO needs to use a faint tip-tilt star to stabilize a faint tip-tilt star to stabilize laser spot on skylaser spot on sky

from A. Tokovinin

Effective isoplanatic angle for Effective isoplanatic angle for image motion: image motion: ““isokinetic angleisokinetic 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

Tip-tilt mirror and sensor Tip-tilt mirror and sensor configurationconfiguration

Telescope

Tip-tilt mirrorDeformable mirror

Beam splitter

Beam splitter

Wavefront sensor

Imaging camera

Tip-tilt sensor

Tip-tilt correction determines LGS Tip-tilt correction determines LGS sky coverage fractionsky 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?”

Infrared versus optical tip-tilt Infrared versus optical tip-tilt sensingsensing

• 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)

Tip-tilt sensing at K band gives Tip-tilt sensing at K band gives much higher sky coveragemuch 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

New wavefront errors for laser New wavefront errors for laser guide star AOguide star AO

• “Cone effect”

• Tilt anisoplanatism

““Cone effectCone effect”” or or ““focal focal anisoplanatismanisoplanatism”” for laser guide for laser guide starsstars

• Two contributions:

– Unsensed turbulence above height of guide star

– Geometrical effect of unsampled turbulence at edge of pupil

from A. Tokovinin

Cone effect, continuedCone 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

Dependence of dDependence of d00 on beacon on beacon altitudealtitude

• 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

Cone effect for one laser guide Cone effect for one laser guide starstar

90 k

m

“Missing” Data

Credit: Miska Le Louarn

Multiple laser guide stars can Multiple laser guide stars can measure the un-sensed measure the un-sensed turbulenceturbulence

90 k

m

Credit: Miska Le Louarn

Tilt anisoplanatism: residual TT Tilt anisoplanatism: residual TT errors if TT star is too far awayerrors 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

Effects of laser guide star on Effects of laser guide star on overall AO error budgetoverall 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)

Residual tip-tilt error due to tip-Residual tip-tilt error due to tip-tilt anisoplanatismtilt 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

•

Compare NGS and LGS Compare NGS and LGS performance performance

• From a Keck study several years ago

LGS Hartmann spots are LGS Hartmann spots are elongatedelongated

Sodium layer

Laser projectorTelescope

Image of beam as it lights up sodium layer = elongated spot

Elongation in the shape of Elongation in the shape of the LGS Hartmann spotsthe LGS Hartmann spots

Off-axis laser

projector

Keck pupil

Representative elongated Hartmann

spots

Keck: Subapertures farthest from Keck: Subapertures farthest from laser launch telescope show laser laser launch telescope show laser spot elongationspot elongation

Image: Peter Wizinowich, Keck

New CCD geometry for WFS being New CCD geometry for WFS being developed to deal with spot developed to deal with spot elongationelongation

CW Laser Pulsed Laser

Sean Adkins, Keck

Polar Coordinate DetectorPolar 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

Laser guide star topics weLaser guide star topics we’’ve ve discusseddiscussed

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

lecture 12 part 1: laser guide stars, continued part 2: control systems intro

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