Transcript cameron_ngao.ppt

Astrometry for NGAO
Brian Cameron, Matthew Britton, Jessica Lu,
Andrea Ghez, Rich Dekany, Claire Max, and
Chris Neyman
NGAO Meeting #6
April 25, 2007
The Limits… In Principle
• Photon Noise
 ph
  10m  10 
 350as
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



2.1m  D SNR 
– AO helps with SNR and achieving D
• 
Confusion
– Important for densest fields
2/14
The Limits… In Practice
Systematics…
• Physical optics
– Geometric distortions
• An atmosphere
– Refraction
– Anisoplanatism
All are ~10 mas, but can be reduced
3/14
Geometrical Distortion:
Pin Hole Mask
•
NIRC2 suffers from
distortion.
•
Characterize with pin
hole slit mask
– Polynomial fit
– Residuals ~0.1 pixels
(1-4 mas)
•
Stable within the errors
over the last ~6
months.
•
Don’t dither!
– Doesn’t help for
heterogeneous data
sets
4/14
State of the Art: HST
• Distortion from huge, welldithered, rotated data set
• Understood at <0.01 pixel
(<0.3 mas)
– Both WFPC2 and ACS
• Key’s to success
– PSF modeling
– Stability
• On-sky experiment planned
for summer 2007
Anderson & King 2003
5/14
Differential Tilt
• Stars separated by some
angle sample different
turbulence.
• Elongation of the PSF
toward the TT reference.
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
 DT ~ 20 mas (/20”)(5m/D)6/7
• Averaged down by
integration.
6/14
Differential Tilt
• Vector separation
between 21” binary
– 100s of 1.4 sec
exposures
• RMS ~20 mas
– anisotropic
• Uses a PSF prediction
technique
– OTF measured from
guide star
– Propagated off-axis with
ATF (calculated with help
of DIMM/MASS)
Britton 2006
7/14
Differential Atmospheric Refraction
• DAR contributes ~5 mas
every 10” at transit.
• Can we detect?
– Sort of…
8/14
Differential Atmospheric Refraction
• 12 mas @ zenith angle of
45 degrees, separation of
30” and T ~ 5000 K
• Noise from correction:
– Model Uncertainties
– Uncertainties in 7
parameters.
Parameter Uncertainty
Noise
(uas)
Ground
Temp.
3K
50
Pressure
8 mb
50
Zenith
Angle
36”
10
Separation 30 mas
10
Relative
Humidity
10%
--
Relative
Stellar
Temp.
100-1700 K
10
Gubler & Tytler 1998
9/14
Confusion
• Inner few mas of the
Galaxy is limited by
confusion
– Goal < 0.1 mas
• Trade study
(although noisy)
shows ~140nm
WFE required.
10/14
Scientific Applications
• Galactic Center
– GR, young stars, high
velocity stars
• ‘Isolated’ point sources
– Galactic Plane targets
• Proper Motions
11/14
Distortion Corrected
• Compute Allan
variance for
diagnostics
– variance after
averaging over
various timescales
• Recover ~ 1/sqrt(t)
dependence until
the next floor
– ~1 mas for isolated
point source data.
12/14
GC Astrometric Precision
13/14
Recommendations
•
•
•
•
•
Turbulence monitoring
PSF monitoring camera
Distortion solutions and monitoring
Atmospheric dispersion corrector (?)
System with < 170nm WFE to mitigate
GC confusion
14/14
More DAR
15/14
Field Dependent Errors
16/14