developing performance estimates for high precision astrometry with tmt matthias schoeck, tuan do,...

Developing Performance Estimates for High Precision Astrometry with TMT

Matthias Schoeck, Tuan Do, Brent Ellerbroek, Gilles Luc,Glen Herriot, Leo Meyer, Ryuji Suzuki, Lianqi Wang, Sylvana Yelda

AO4ELT3 Conference, Florence, Italy26-31 May 2013

1TMT.AOS.PRE.13.079.REL01AO4ELT3, Florence, 05/28/13

Presentation Outline

TMT astrometry requirements

Example astrometry science cases

Astrometry error budget– Overview– Work to date– Error budget spreadsheet by category

Summary


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Current 8-10 m telescopes: ~100-300 µas differential astrometry with adaptive optics

– Some higher-precision special cases

TMT requirement:– 50 µas differential astrometry for 100s exposure on 30” FoV in H band– Error falling as t-1/2 to a systematic floor of 10 µas– 50 µas is 10 cm at distance of Moon

Challenging constraints on all parts of opto-mechanics

Unique scientific opportunities enabled by astrometry at 10-100 µas level– We need to make sure that TMT will be able to achieve this

Astrometry with TMT

TMT.AOS.PRE.13.079.REL01AO4ELT3, Florence, 05/28/13

Example Astrometry Science Cases

50 micro-arcsecs differential astrometry in densely populated fields: General Relativity at the Galactic Center Velocities in dwarf galaxies Star forming regions: accurate determination of the Initial Mass Function

with cluster membership

2 milli-arcsecs in sparse fields, i.e., where only wavefront sensor guide

stars are available:– Magnetar proper motions to establish velocity imparted during progenitor

explosion– Binary star/planet orbits to measure stellar, compact object and planet

masses– Astrometric microlensing to measure accurate stellar masses– Gravitational lensing to probe dark matter substructures– Binary Kuiper Belt Objects


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Many error terms that are negligible for ~100 µas astrometry become important for ~10 µas astrometry

We tried to identify all effects that might influence astrometry– Currently more than 30 error terms in 5 categories

Many terms are correlated or interconnected– They cannot simply be added in quadrature

Almost every error term depends on the details of the astrometry observations

– Absolute vs. differential astrometry– Sparse vs. crowded fields– Short vs. long times scales– Differences can be qualitative, not just quantitative– We also need calibration, observing and data reduction sequences


TMT Astrometry Error Budget Overview

Lots of work done has been done, for example:– Residual atmospheric turbulence– Multi-conjugate vs. single-conjugate adaptive optics– Telescope, NFIRAOS and IRIS distortions and aberrations– Atmospheric dispersion– Pinhole grid and optical surfaces requirements– Galactic Center simulations– Focal plane errors– Top-down astrometry error budget

Analytical analyses, simulations and observations

Data reduction sequences with error propagation

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TMT Astrometry Error Budget Work to Date

Galactic Center Astrometry Simulations

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Simulations of Galactic Center Astrometry

~100,000 stars, 3,000 of which are known GC stars

20-200 second exposures

Will be bench-marked by Keck simulations

See talk by Sylvana Yelda

TMT Astrometry Error Budget

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Zoomed-in versions on next slides

Rows: error terms in 5 categories

Columns: 8 generic science cases

Right column: current status

Words: Status

Color: Reliability of error estimate

Purple: value calculated from equation

Blue: sum of error terms above

Orange: input parameters

Note: Work in Progress; values are very dependent on type of observation (by more than an order of magnitude)

By Comparison:One year Ago

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Status of the error budget

on May 1, 2012

TMT Astrometry Error BudgetReference Catalog Errors

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Differences between real and assumed properties of reference sources– Pretty much “under control”– Color & variability errors taken care of in atmospheric dispersion analysis

Note: Values are very dependent on type of observation


TMT Astrometry Error BudgetAtmospheric Refraction

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– Achromatic differential refractionDue to differences between zenith angles of different objects

– Chromatic differential refraction (dispersion)Due to wavelength dependence of index of refraction

Requires atmospheric dispersion corrector (ADC), and possibly a different ADC for each wavelength band

A posteriori corrections likely required for highest-precision astrometry

– Refraction-caused errors can be one of the dominant terms– Currently under investigation in both simulations and measurements (Subaru)


TMT Astrometry Error BudgetOther Atmospheric Residuals

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– Analysis of residual turbulence after NFIRAOS correction mostly done– Most terms depend on integration time as T-0.5

– Halo effect Seeing-limited halos of other stars cause background gradients

– Variable atmospheric effects (during exposure, e.g. Strehl, transparency):Couple with image motion to cause astrometric uncertainties

Unintuitively, this error term increases with integration time


TMT Astrometry Error BudgetResidual Turbulence Errors

Simulations to study astrometry errors due to residual wavefront errors after MCAO compensation by NFIRAOS– Geometric tip/tilt effects (<30 marcsec)– Atmospheric “speckle noise” (10-12 marcsec)

– Uncertainty in r0 estimate (1-3 μarcsec)

2 x 15 sec exposures of a 30x30 arcsec field at 50 degrees zenith– Global tip/tilt and plate scale distortion removed using field stars (-> these are

high-order residuals)

All errors found to scale as T-0.5 (previous slide is for 100s)

Errors are much larger for single-conjugate AO systems– MCAO is good for astrometry– Also: Distortions by DM11 are not a problem (see talk by Brent Ellerbroek)


TMT Astrometry Error BudgetOpto-Mechanical Errors

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– Stability of distortions and repeatability of configurations are crucial

Observing sequences (incl. calibration methods) are critical input information

– See Brent’s talk– No show stoppers found so far


TMT Astrometry Error BudgetFocal Plane Errors

– Photon noise of 24 µas is for S/N = 200 ( 100 s exposure for K = 20 point source )– Confusion not an issue for this generic science case (by definition), but can be a major

contribution in other cases (Galactic Center, dwarf galaxies, globular clusters)

Currently under investigation in Galactic Center simulations (see Sylvana’s talk)

– Total error for differential astrometry with many reference sources: 37 µas

(But note again: Values are very dependent on type of observation)

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TMT Astrometry Error Budget Summary

Lots of work done has been done, for example:– Analytical analyses– Simulations– Observations– Top-down astrometry error budget– Data reductions sequences with error propagation– Still lots to do …

No show stoppers found so far for very high-precision astrometry– Work has resulted in a couple tweaks to NFIRAOS design

Results depend very much on the science case– Need analysis and requirements for different science cases

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Acknowledgements

The TMT Project gratefully acknowledges the support of the TMT partner institutions.

They are– the Association of Canadian Universities for Research in Astronomy (ACURA),– the California Institute of Technology– the University of California– the National Astronomical Observatory of Japan– the National Astronomical Observatories and their consortium partners– And the Department of Science and Technology of India and their supported institutes.

This work was supported as well by– the Gordon and Betty Moore Foundation– the Canada Foundation for Innovation– the Ontario Ministry of Research and Innovation– the National Research Council of Canada– the Natural Sciences and Engineering Research Council of Canada– the British Columbia Knowledge Development Fund– the Association of Universities for Research in Astronomy (AURA)– and the U.S. National Science Foundation.


developing performance estimates for high precision astrometry with tmt matthias schoeck, tuan do,...

Documents

high precision astrometry

error terms

error propagation

differential astrometrysparse

galactic center velocities

tmt matthias schoeck

categoriesmany terms

dateerror budget spreadsheet