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

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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.

13/14

GC Astrometric Precision

14/14

Recommendations

• Turbulence monitoring

• PSF monitoring camera

• Distortion solutions and monitoring

• Atmospheric dispersion corrector (?)

• System with < 170nm WFE to mitigate GC confusion

15/14

More DAR

16/14

Field Dependent Errors