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Can small beat the big?. A. N. Ramaprakash On behalf of Robo -AO collaboration partners (IUCAA and Caltech). Can small beat the big?. Transition decade for astronomy Can small beat the big ?. Downtime. On-Sky Efficiency - PowerPoint PPT Presentation
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IUCAA
Can small beat the big?
A. N. RamaprakashOn behalf of Robo-AO collaboration partners (IUCAA and Caltech)
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Can small beat the big? Transition decade for astronomy
Can small beat the big?
18/09/2012 A. N. Ramaprakash, IUCAA 2
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Downtime On-Sky Efficiency
Fraction of the available time which the telescope spends in collecting useful astronomical and calibration data
Factors Transmission Weather Technical downtime Scheduled Maintenance Commissioning
Large observatories are expensive to operate Keck Telescopes – 175 USD per minute VLTs – 125 Euros per minute
Minimize the above factors Currently about 70% - 80% efficiency is typical Often not clear what exactly this means
18/09/2012 A. N. Ramaprakash, IUCAA 3
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Overheads, Mitigation Overheads
Identification and Acquisition of target and guide star
Instrument and telescope set up time Readout, pre-processing & quick look Calibration observations
Mitigation Ease of acquisition Ease of use Stable systems Observatory calibrations Pipelines
18/09/2012 A. N. Ramaprakash, IUCAA 4
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Mitigation Multiplexing
Wide field, Mosaics Multi-band, MOS, IFU Telescopes
Observing Modes Automation and Queue Schedules Service Observing Adaptive queue schedules Remote observing
18/09/2012 A. N. Ramaprakash, IUCAA 5
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Adaptive Optics Merits
Sensitivity (S) - inverse of the time needed to reach a desired SNR
For faint point sources observed under background limited conditions
where D is the telescope diameter Enhanced angular resolution
1/D
18/09/2012 A. N. Ramaprakash, IUCAA 6
422
22
)/(D
SNRDFDFStrehlS
bg
star
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Most of the light within w=/2d;d=spacing of actuators
How good is your adaptive optics? Diffraction Limited Performance
Measured FWHM of PSF = FWHM of Airy disk (1.03 /D)
different from Rayleigh criterion for resolving power
Strehl Ratio (S) – ratio of the peak intensity of the measured PSF to the theoretical maximum (< 1 by definition)
FWHM can reach diffraction limit even when Strehl is very small
How good depends on what you want to do:
High resolution - Strehl of 10-15% is often enough; Also for slit spectroscopy
Detect faint objects - Strehl of 40% may not be enough as the central core might contain only a fraction of the total light
Point Source Sensitivity (PSS)
18/09/2012 A. N. Ramaprakash, IUCAA 7
S is only 0.23Normalized
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Simple AO approach - Limitations Guide star availability
0 = 0.31 r0 / <V> Closed loop at 100Hz to 1KHz Need a R=12-15 star typically
Small isoplanatic angle θ0 = 0.31 r0 / <h> 5" -30" for 1rad2 mean square
wavefront error [Strehl Ratio~exp(-2) ~30%]
18/09/2012 A. N. Ramaprakash, IUCAA 8
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Theoretical faint limits Rlim=17 for Strehl Ratio~30% at K
(2.2um) Rlim=13.1 for Strehl Ratio~30% at R
(0.6um) r0 λ6/5
r0 at 2μm is 5.9 times what it is at 0.5 μm
D(r)=6.88(r/r0)5/3
Larger phase error at shorter wavelengths demands more photons for correction
Also needs more number of correction subsections (~D/r0)2
AO guide star faintness limit
18/09/2012 A. N. Ramaprakash, IUCAA 9
For a given desired Strehl Ratio, r0 and λ definethe faintest star that can be used for AO, irrespective of the telescope size.
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Laser Guide Stars
18/09/2012 A. N. Ramaprakash, IUCAA
R~17 for tip-tilt NGS in LGS systems
Isokinetic angle larger than isoplanatic angle
About 87% of the power is in the lowest two modes, tip and tilt 10
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Adaptive Optics - Limitations Expensive
Only large telescopes can afford good AO systems
Setting up overheads Large telescopes use AO only
sparingly
18/09/2012 A. N. Ramaprakash, IUCAA 11
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Laser Guide Stars - Limitations Cone effect
Light paths from the distant star and the laser guide star are different
Dmax~2.91 θ0 <H> ~8m for sodium
LGS ~3m for Rayleigh
LGS Natural guide stars
for tip-tilt compensation
18/09/2012 A. N. Ramaprakash, IUCAA 12
ROBO-AO
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ROBO-AOAdaptive optics for small and medium telescopes
18/09/2012 A. N. Ramaprakash, IUCAA 13
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Acknowledgements
Partially funded by the National Science Foundation.
http://www.astro.caltech.edu/Robo-AO/18/09/2012 14A. N. Ramaprakash, IUCAA
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Why Robo-AO Robotic
high observing efficiency Adaptive Optics
spatial resolution set by D sensitivity set by D4
Laser Guide Star high sky coverage
Small Telescopes availability
Rayleigh economical
18/09/2012 A. N. Ramaprakash, IUCAA 15
Unique Science Capabilities
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Robo-AO testbed
18/09/2012 A. N. Ramaprakash, IUCAA 16
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Wavefront correctors Shack-Hartmann WFS
11 x 11 UVCCD39, <5e- noise at
<2kHz Micro-Electro-
Mechanical Systems (MEMS) deformable mirror
12 x 12 actuators 3.5 μm stroke
PI fast steering mirror USB electronics on
Linux Loop Rate > 1.2kHZ18/09/2012 A. N. Ramaprakash, IUCAA 17
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Science Cameras Andor EMCCD
1k2, 45”x45” FoV, Visible Fast ROI, Nyquist λ=620
InGaAs 320x240, noisy, to
1.7um H2RG detector
2k2, 2’x2’ FoV, 900nm-2.2um
Fast ROI, Nyquist λ=830nm
18/09/2012 A. N. Ramaprakash, IUCAA 18
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Robotic Control Software Fully robotic control system
Subsystems work as daemons Supervisor controls scheduling,
operations Watchdog processes
Programming intelligence is a challenge Robots are only as smart as the
people that make them! Error control and exception
handling Safety system for equipment and
staff Laser safety a priority
18/09/2012 A. N. Ramaprakash, IUCAA 19
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Robo-AO Cassegrain instrument model
18/09/2012 A. N. Ramaprakash, IUCAA 20
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UV laser at Palomar 60 inch telescope
18/09/2012 A. N. Ramaprakash, IUCAA 21
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Robo-AO on the P60 telescope
18/09/2012 A. N. Ramaprakash, IUCAA 22
Robotic Software
Adaptive Optics system +Science Instruments
RoboticTelescope(P60)
Laser box
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Robo-AO wavefront sensors Shack-Hartmann
11 x 11 subapertures
High sensitivity to UV
High-speed optical switch
Image motion (tip/tilt) From science
instruments
18/09/2012 A. N. Ramaprakash, IUCAA 23
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On-sky wavefront sensor data
18/09/2012 A. N. Ramaprakash, IUCAA 24
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Robo-AO in action
18/09/2012 A. N. Ramaprakash, IUCAA 25
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AO Imaging Capabilities – Robo-AO on P60 Diffraction-limited resolution
(mV<17) ~0.1-0.15” in the visible ~0.2-0.25” in the near-infrared
0.5+ Strehl in the near-infrared (30% sky)
Seeing improvement (100% sky) ~45” to 2’ field of view General imaging
range of filters, exposure times, observation setups
18/09/2012 A. N. Ramaprakash, IUCAA 26
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Power of Robo-AO
18/09/2012 A. N. Ramaprakash, IUCAA 27
Traditional Laser Guide Star
Adaptive Optics
Robo-AORobotic Laser Guide Star AO
Telescope diameter 3-10m 1.5-3m
Lock-on time 5-15 mins / target 0.5-1 minutesTargets per
night Tens Hundreds
Program Length Few nights Weeks+
Targets per program ~100 Thousands
Personnel1 astronomer +
6 spotters + 2 telescope
control
1 astronomer (peacefully sleeping)
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Signal to Noise Ratio Improvements
18/09/2012 A. N. Ramaprakash, IUCAA 28
Astrometric precision gains in both SNR and FWHM
Prediction: 100μas precision in around 15 minutes
(based on Cameron et al. Keck & Palomar performance)
Band SNR Compared to
1.5 m
SNR Compared to 4 m
FWHM (1” is typical)
Strehl
J 2.9X 0.4X 0.2” 50%
H 7.1X 0.98X 0.26” 70%
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Science programs (Sub)stellar, asteroid companion surveys Astrometric planet searches Rapid transient characterization Efficient discrimination of blended
binary false-positive candidates 1000’s of new lensed quasars Follow ups
Higher angular resolution Deeper Images Spectra Different bands, periods or cadence
The list goes on and on…18/09/2012 A. N. Ramaprakash, IUCAA 29
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Robo-AO enables new science Large single-image surveys
Several thousand targets, all at high-angular resolution Otherwise extremely time intensive on currently
available LGS AO systems E.g. stellar binarity surveys, searches for lensed
quasars, planetary transit follow up Rapid transient characterization
Diffraction limited images within minutes of detection of transients
Reduction of integration time for infrared photometry E.g. separation of transient events from host galaxy
Time-domain astronomy Queue supports recurrent, regularly spaced
observations of specific targets E.g. long-term, high-precision astrometric
characterization of sub-stellar companions
18/09/2012 A. N. Ramaprakash, IUCAA 30
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Robo-AO vision
18/09/2012 A. N. Ramaprakash, IUCAA 31
ROBO-AOROBO-AO
ROBO-AOROBO-AO
ROBO-AO
ROBO-AO
ROBO-AO
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Robo-AO vision
18/09/2012 A. N. Ramaprakash, IUCAA32
Deploy and demonstrate a robotic laser adaptive optics and visible/IR
science system on a 1.5m telescope
Emphasis on robustness
Make it affordable
Replicate and deploy to the world’s 1-3 m class telescopes
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Robo-AO Movies
18/09/2012 A. N. Ramaprakash, IUCAA 33
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18/09/2012 A. N. Ramaprakash, IUCAA 34
THANK YOU
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Robo-AO error budget
18/09/2012 A. N. Ramaprakash, IUCAA 35