View
33
Download
0
Category
Preview:
DESCRIPTION
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 - PowerPoint PPT Presentation
Citation preview
Page 1
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
Page 2
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
Page 3
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
Page 11
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
Page 14
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
Page 15
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
Page 16
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
Page 17
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?”
Page 21
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)
Page 22
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
Page 25
New wavefront errors for laser New wavefront errors for laser guide star AOguide star AO
• “Cone effect”
• Tilt anisoplanatism
Page 26
““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
Page 27
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
Page 28
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
Page 29
Cone effect for one laser guide Cone effect for one laser guide starstar
90 k
m
“Missing” Data
Credit: Miska Le Louarn
Page 30
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
Page 31
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
Page 32
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)
Page 33
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
•
Page 34
Compare NGS and LGS Compare NGS and LGS performance performance
• From a Keck study several years ago
Page 35
LGS Hartmann spots are LGS Hartmann spots are elongatedelongated
Sodium layer
Laser projectorTelescope
Image of beam as it lights up sodium layer = elongated spot
Page 36
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
Page 37
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
Page 38
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
Page 39
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
Page 40
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
Recommended