Astrometry for NGAO Brian Cameron, Matthew Britton, Jessica Lu, Andrea Ghez, Rich Dekany, Claire...
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Transcript of Astrometry for NGAO Brian Cameron, Matthew Britton, Jessica Lu, Andrea Ghez, Rich Dekany, Claire...
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
€
σ ph ≈ 350μasλ
2.1μm
⎛
⎝ ⎜
⎞
⎠ ⎟10m
D
⎛
⎝ ⎜
⎞
⎠ ⎟
10
SNR
⎛
⎝ ⎜
⎞
⎠ ⎟
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