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High-Precision Optical Interferometry of Faint and
Low-Contrast Targets Requires Coherent Integration
Anders M. Jorgensen1, David Mozurkewich2, Henrique Schmitt3,4
1Electrical Engineering, New Mexico Tech2Seabrook Engineering, MD3Naval Research Laboratory, DC4Interferometrics, inc., MD
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Outline● Optical Interferometry● The problem of short exposures● How to coherently integrate● Some coherent integration results
– Diameter of a star– Binary star measurements– Imaging of a binary star
● Conclusion
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Optical Interferometry● Resolution limit of telescopes● Size of stars● Separate telescopes● Fourier transform●
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The SNR problem● Atmosphere● Fringe motion● Short exposures● Few photons● Average many exposures
– But how?
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Incoherent Averaging● V 2 averaging
– Just amplitude squared, forget phase.
● Triple phase– Three telescopes, one phase.
● Throwing away phase information
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Coherent integration● Imagine a world in which we can make long integrations.
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Incoherent vs. Coherent
CoherentCoherent
incoherent
incoherent
● Improves SNR on both amplitude and phase
Coherent
baseline
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How to coherently integrate● Estimate phase for each exposure and shift.
● Multi-wavelength measurements.
● Enough photons in band even if not in channel.
Wavelength
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How to coherently integrate● Two ways:
– Fast real-time control loop.● Better when read noise high: Infrared, VLTI.
– Combine in software after the fact.● Better when atmosphere is fast: Visible, NPOI.
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HR 5544 V=4.5 star● 2x (red end) to 10x (blue end) better
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The meaning of the phase● Several components.● Fit a model including atm and source.
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Binary star measurement● More phases, and better SNR. Tauri
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Binary star measurement● 10x better dM than Armstrong (2006)
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Stellar Diameters● Measured as location of null in visibi l ity versus wavelength. Ophichus
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Stellar Diameters
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Imaging● No special software needed:
– Complex visibil ities. Can use standard AIPS package.
– Gaussian noise.Tauri
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Conclusions● Coherent integration works● Demonstrated with NPOI data● Improves SNR● Better stellar diameters● Better binary parameters● Imaging in the usual way