Download Laser guide star adaptive optics at the Keck Observatory

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Laser guide star adaptive optics at
the Keck Observatory
Adam R. Contos, Peter L. Wizinowich, Scott K.
Hartman, David Le Mignant, Christopher R. Neyman,
Paul J. Stomski, Jr., and Douglas Summers
W.M. Keck Observatory
SPIE Astronomical Telescopes and Instrumentation Conference
23 August 2002, Waikoloa, HI
Talk Outline
 Overview
 Sky Coverage for LGS and NGS Modes
 Optical Path
 New LGS AO Hardware
 System Upgrades & Issues
 Laser Guide Star
 Future
 Summary
Overview
 Traditional AO with NGS
– Bright star in close proximity ~ 1% coverage
– Sodium LGS for nearly complete coverage
• Still require an NGS, but can be much dimmer
 Keck II System Upgrades for LGS AO
–
–
–
–
Low Bandwidth Wavefront Sensor (LBWFS)
Tip/Tilt Sensor (TTS), using quad-cell APDs
Software control
Laser system (described in following talk)
 Scheduled for integration in 2003
Sky Coverage: NGS and LGS
1Bahcall
& Soneira, 1981; 2Keck Report 208, 1996.
Sky Coverage vs. NGS Limiting Magnitude
(Varying Galactic Lat. and Field of View)
22
Reference Star Magnitude (R-band)
21
20
LGS
19
18
17
16
15
14
NGS
13
12
30'' FOV @ 45º lat
11
60'' FOV @ 45º lat
10
60'' FOV @ 0º lat
9
8
0.10%
1.00%
10.00%
Sky Coverage
100.00%
Optical
Path
Highlighted AO Bench Elements:
1-Image Rotator
2-Tip/Tilt Mirror
4-Deformable Mirror
6-IR Dichroic
12-Sodium Dichroic
13-Field Steering Mirrors
14, 15-Wavefront Sensing Camera
and Focus Camera Stage
17-Beamsplitter
18-Low Bandwidth Wavefront
Sensor, Tip/Tilt Sensor & Stage
19-Acquisition Camera
LBWFS & TTS Description
 Low Bandwidth Wavefront Sensor
–
–
–
–
Shack-Hartmann lenslet array sensor
304 subapertures (240 active)
Photometrics 512x512 CCD detector
Readout every few sec. to few min.
 Tip/tilt Sensor
– Quad-lens
– Fiber-fed APDs
 Mounted on 3-axis tip/tilt stage
 Beamsplitter cube
– 90% to TTS, 10% to LBWFS
System Upgrades & Issues (1)
1
no bright NGS
nearby

LGS
2
3
4
5
laser spot moving

wavefront changing
WFS to deformable

rapidly
mirror loop
laser spots
elongated &
distorted in WFS
sodium distance
changing

recalibrate using
LBWFS centroid &
gains

move WFS (focus)
6
7
WFS to Up-Tip/Tilt
(laser pointing)
sodium in science

camera
pupil size changes 
IR transmissive
dichroic & science
filters
motorized pupil
relay optics
System Upgrades & Issues (2)
no tip/tilt info; limited
8 focus info; no 'truth' 
calibrator
9
10
Dim NGS
LGS wavefront will

drift w.r.t. NGS
LGS provides no
tip/tilt information

LGS focus will drift

11
w.r.t. NGS
LBWFS to
determine avg.
wavefront (truth)
quad cell TTS w/
APDs
LBWFS to
determine best
focus
System Upgrades & Issues (3)
LBWFS/TTS
co-located on x,y,z
TSS stage
when move TSS,
LBWFS looses
14

registration with
deformable mirror
sodium trasmissive
see excess sodium
dichroic & sodium
light in long
13

rejection filter for
exposures
TTS/LBWFS
NGS not co-located

12
w.r.t. LGS
LBWFS lenslet
mounted on x, y
stage
1st Laser Propagation (12/01)
 Laser power ~ 17 W
 FWHM ~ 1.4” (seeing ~ 1”)
 Magnitude ~ 9.5
Future
Milestones and Deliverables
First corrected images on NIRC2 with laser
LGS AO facility operable by AO experts
First LGS AO engineering science
LGS AO observing strategy decision
First shared-risk science
First light engineering science paper
LGS AO facility operable for science by OA and one expert
First queue/non-shared science
Target Date
Spring 2003
Summer 2003
Fall 2003
Fall 2003
Winter 2004
Spring 2004
Fall 2004
Winter 2005
Summary of LGS AO at Keck
Achieved first laser propagation
AO upgrades being incorporated for LGS
Integration milestones throughout 2003
– LGS AO on world’s largest optical telescope
will become incredible tool for astronomy
– First operational LGS on an 8-10 m telescope
System Flow & Control