Title: Adaptive Optics and Optical Interferometry or How I Learned to Stop Worrying and Love the Atmosphere
1Adaptive Optics andOptical InterferometryorHow
I Learned to Stop Worryingand Love the
Atmosphere
- Brian Kern
- Observational Astronomy
- 10/25/00
2Brief summary
- Diffraction limit vs. atmospheric limit
- Science goals vs. spatial scale
- Adaptive Optics principles
- Interferometry principles
- Recent results
3Diffraction limit
- Limit to spatial resolution set by diameter of
optics - Fundamental limit you cant simply zoom in
- For 10-m telescope, in visible light (l 0.5
mm), l/D 0.010 arcsecl/D 0.045 arcsec for l
2.2 mm
1.2 l/D
4Atmospheric limit
- Air has patches of different T, which gives
different r, and therefore different indices of
refration n.
T ?r ?n ? diverging lens
T ? r ? n ? converging lens
5Atmospheric limit - wavefront
- Think of phase changes in wavefront - advancing
and retarding wavefronts
0-
Phase map
6Atmospheric limit - seeing disk
- Atmosphere creates seeing disk, 1 arcsec
- Compare to 0.010 arcsec at l0.5 mm, 0.045 arcsec
at l2.2 mm - Keck 10m telescope no better than 4 telescope
- Features smaller than 1 arcsec lost in the blur
- Seeing is site-dependent and time-dependent
7Atmospheric limit - motivation
- Hubble Space Telescope unaffected by atmosphere
- Diffraction-limited resolution, D2.4 m
- We can achieve 4x better resolution with a 10-m
telescope
8Atmospheric limit - motivation
9Science goals
10Science goals
11Adaptive Optics - overview
- Correct aberrated wavefront using deformable
mirror - Mirror takes shape opposite to wavefront
distortion - Must measure aberrations to know how to make
correction - Can use natural guide star or laser guide star
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14Adaptive Optics - requirements
- Atmosphere sets spatial scale of correction
- r0 is coherence length (Frieds parameter)
- r0 10 cm for 1 arcsec seeing in visible (0.5
mm) light - r0 ? l6/5 r0 60 cm for l2.2 mm (IR)
- for l20 mm (mid-IR), r0 gt 8 m no need for AO
- r0 and wind speed v set time scale of correction
- v 10 m/s, so r0 /v t 10 ms
- So we need (D/r0)2 actuators, making
corrections every t seconds - for l0.5 mm, D 10 m, (D/r0)2 104, t 10 ms
- for l2.2 mm, D 10 m, (D/r0)2 250, t 60 ms
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16Adaptive Optics - wavefront sensing
- Guide star is necessary to determine corrections
- Hartmann wavefront sensor is most common way to
determine aberrations - Wavefront sensor looks at image of individual r0
sub-apertures - Position of single sub-aperture image tells you
slope of wavefront - Connect slopes to determine wavefront shape
17Adaptive Optics - isoplanatism
- To look at anything other than guide star, you
look through a different line-of-sight - For a large off-axis angle, corrections are
different for guide star and science object - Isoplanatic angle qiso is angle where corrections
stop being valid - Angle qisoh/r0
- For h10 km, l0.5 mm, qiso2 arcsec
l2.2 mm, qiso12 arcsec
18Adaptive Optics - natural guide stars
- Corrections need to be measured for each
r0-diameter patch in time t - For accurate corrections, need 100 photons per
sub-aperture per t - Magnitude limit is V 9
K 14 - Need stars to be within qiso of science objects
- Sky coverage 310-4 for l0.5 mm
0.01 for l2.2 mm
19Adaptive Optics - laser guide stars
- High atmosphere (90 km) has layer of sodium from
meteors - Tune laser to sodium spectral line, laser makes
artificial guide star 90 km up - Point it anywhere you want
- Single wavelength doesnt interfere with science
observation - Still need tip/tilt from natural guide star, but
can be farther away and much fainter (1
correction for whole telescope)
20Adaptive Optics - results
21Adaptive Optics - results
22Adaptive Optics - results
NGC 7469
23Interferometry - Youngs double-slit
- Youngs double-slit experiment
d
Path lengths equalphase difference
0ºconstructive interference
Path lengths differ by l/2phase difference
180ºdestructive interference
l/d
Intensity
0
24Interferometry - Two objects
- Two objects give same interference pattern,
shifted by position of object
(l/d)/2
25Interferometry - Michelson
- Michelson put double-slit on top of Mount Wilson
100 - vary baseline d to find Dx(l/d)/2, where
fringes disappear
d
26Interferometry - atmosphere
- Atmosphere adds random phase errors to two slits
27Interferometry - visibility
- Atmosphere affects two stars the same combined
interference pattern is shifted, but not changed - modulation is unaffected by atmosphere
- Define visibility V (Imax - Imin) / (Imax
Imin) - V ranges from 0 to 1
V1
V0.5
V0
28Interferometry - detection
- Atmospheric phase differences shift pattern
around - Place detector at zero-point, let atmosphere
shift pattern back and forth across detector - Time series of detected intensity gives
visibility - Use slit sizes r0, detector intensity changes
every t - Stars must be within qiso of each other
I
Imax
Imin
t
29Interferometry - visibility maps
- 2-dimensional map of baseline vectors is (u,v)
plane - Map of visibilities in (u,v) plane is (u,v) map
- Short baselines correspond to large angular
separations, long baselines correspond to small
angular separations
30Interferometry - bigger baselines
- Apertures can be completely disconnected from
each other - Extending baselines to hundreds of meters
resolves features at l/d 0.0003 arcsec for
l0.5 mm, d350 m
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32Interferometry - delay lines
- When apertures are not carried by a single
telescope, they need a path length
compensation - The delay lines take up lots of space
Path length difference
Delay line
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34Interferometry - phase information
- Letting atmosphere shift modulation pattern
around eliminates phase information - In order to get phase information, phase needs to
be stabilized with respect to atmospheric
distortions - Can use double-star feed, where phase is locked
to a star, and a science target can be observed
in full phase
35Interferometry - large apertures
- In order to use aperture much larger than r0, its
distortions have to be flattened - Need AO on all large apertures before they can be
interfered
36Interferometry - space
- No atmospheric distortions in space
- Spacecraft control (vibrations, positions) must
be controlled to picometer precision
37Interferometry - facilities
NAME tel aperture baseline CHARA Center
for High-Angular Resolution Astronomy 6 1.0 350CO
AST Cambridge Optical Aperture Synthesis
Tel. 5 0.40 20GI2T Grand Intérferomètre à 2
Télescopes 2 1.5 65 IOTA Infrared Optical
Telescope Array 2 0.40 38 ISI Infrared Spatial
Interferometer 2 1.6 85 MIRA-I Mitaka Infrared
Array 2 0.25 4 NPOI Navy Prototype Optical
Interferometer 3 0.12 35 PTI Palomar Testbed
Interferometer 3 0.40 110SUSI Sydney University
Stellar Interferometer 2 0.14 640 Keck K1-K2 2
10.0 60 Keck Auxiliary array upgrade 4 1.8 140
LBT Large Binocular Telescope 2 8.4 23 VIMA VLT
Interferometer Main Array 4 8.0 130 VISA VLT
Interferometer Sub-Array 4 1.8 202
38Interferometry - results
Capella
Sep 28 1995
Sep 13 1995