Post on 21-Dec-2015
Astrometry for NGAO
Brian Cameron, Matthew Britton, Jessica Lu, Andrea Ghez, Rich Dekany, Claire Max, and
Chris Neyman
NGAO Meeting #6
April 25, 2007
2/14
The Limits… In Principle
• Photon Noise
– AO helps with SNR and achieving D
• Confusion– Important for densest fields
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σ ph ≈ 350μasλ
2.1μm
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⎠ ⎟10m
D
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⎠ ⎟
10
SNR
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3/14
The Limits… In Practice
Systematics…• Physical optics
– Geometric distortions
• An atmosphere– Refraction – Anisoplanatism
All are ~10 mas, but can be reduced
4/14
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
5/14
State of the Art: HST
• Distortion from huge, well-dithered, 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
6/14
Differential Tilt• Stars separated by some
angle sample different turbulence.
• Elongation of the PSF toward the TT reference.
σDT ~ 20 mas (/20”)(5m/D)6/7
• Averaged down by integration.
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
7/14
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
8/14
Differential Atmospheric Refraction
• DAR contributes ~5 mas every 10” at transit.
• Can we detect?– Sort of…
9/14
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.
3 K 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
10/14
Confusion
• Inner few mas of the Galaxy is limited by confusion– Goal < 0.1 mas
• Trade study (although noisy) shows ~140nm WFE required.
11/14
Scientific Applications
• Galactic Center– GR, young stars, high
velocity stars
• ‘Isolated’ point sources– Galactic Plane targets
• Proper Motions
12/14
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.
14/14
Recommendations
• Turbulence monitoring
• PSF monitoring camera
• Distortion solutions and monitoring
• Atmospheric dispersion corrector (?)
• System with < 170nm WFE to mitigate GC confusion