Transcript LGS Lecture ppt - UCO/Lick Observatory

Lecture 12
Part 1: Laser Guide Stars, continued
Part 2: Control Systems Intro
Claire Max
Astro 289, UC Santa Cruz
February 14, 2013
Page 1
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 2
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
Page 3
Page 9
Small size: MEMS DM
Page 10
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
Page 11
Page 12
Laser guide star AO needs to use a faint
tip-tilt star to stabilize laser spot on sky
from A. Tokovinin
Page 14
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
Page 15
Tip-tilt mirror and sensor configuration
Telescope
Deformable mirror
Tip-tilt mirror
Beam splitter
Tip-tilt sensor
Wavefront sensor
Beam splitter
Imaging camera
Page 16
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?”
Page 17
Page 18
Page 19
Page 20
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)
Page 21
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
Page 22
Page 23
Page 24
New wavefront errors for laser guide
star AO
• “Cone effect”
• Tilt anisoplanatism
Page 25
“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
Page 26
Cone effect, continued
• Characterized by parameter d0
• Hardy Sect. 7.3.3 (cone effect = focal anisoplanatism)
FA2 = ( 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 27
Dependence of d0 on beacon altitude
from Hardy
• One Rayleigh beacon OK for D < 4 m at  = 1.65 micron
• One Na beacon OK for D < 10 m at  = 1.65 micron
Page 28
90 km
Cone effect for one laser guide star
“Missing” Data
Credit: Miska Le Louarn
Page 29
90 km
Multiple laser guide stars can measure
the un-sensed turbulence
Credit: Miska Le Louarn
Page 30
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
Page 31
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)
Page 32
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
s
2
TA
æ J ö
=ç
è JTA ÷ø
2
D
where q =
0.688k 2 m2
1/3
2
TA
• 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
¥
m2 = ò z dzC (z)
2
•
0
2
N
Page 33
Compare NGS and LGS performance
• From a Keck
study several
years ago
Page 34
LGS Hartmann spots are elongated
Sodium layer
Telescope
Laser projector
Image of beam as it lights up
sodium layer = elongated spot
Page 35
Elongation in the shape of the LGS
Hartmann spots
Representative
elongated Hartmann
spots
Off-axis
laser
projector
Keck pupil
Page 36
Keck: Subapertures farthest from laser
launch telescope show laser spot elongation
Image: Peter Wizinowich, Keck
Page 37
New CCD geometry for WFS being
developed to deal with spot elongation
CW Laser
Pulsed Laser
Sean Adkins, Keck
Page 38
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
Page 39
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
Page 40