1. High-Precision Optical
Interferometry of Faint and
Low-Contrast Targets
Requires Coherent Integration
Anders M. Jorgensen 1 ,
David Mozurkewich 2 , Henrique Schmitt 3,4
1Electrical Engineering, New Mexico Tech
2Seabrook Engineering, MD
3Naval Research Laboratory, DC
4Interferometrics, inc., MD

2. Outline
Optical Interferometry
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The problem of short exposures
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How to coherently integrate
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Some coherent integration results
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Diameter of a star
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Binary star measurements
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Imaging of a binary star
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Conclusion
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3. Optical Interferometry
Resolution limit of telescopes
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Size of stars
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Separate telescopes
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Fourier transform
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4. The SNR problem
Atmosphere
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Fringe motion
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Short exposures
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Few photons
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Average many exposures
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But how?
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5. Incoherent Averaging
V 2 averaging
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Just amplitude
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squared, forget
phase.
Triple phase
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Three telescopes,
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
one phase.
Throwing away
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phase information

6. Coherent integration
Imagine a world in which we can make
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long integrations.

7. Incoherent vs. Coherent
Improves SNR on both amplitude and
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8. How to coherently integrate
Estimate phase for
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each exposure and
shift.
Multi-wavelength
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measurements.
Enough photons in
Wavelength
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band even if not
in channel.

9. How to coherently integrate
Two ways:
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Fast real-time control loop.
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Better when read noise high: Infrared, VLTI.
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Combine in software after the fact.
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Better when atmosphere is fast: Visible, NPOI.
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10. HR 5544 V=4.5 star
2x (red end) to 10x (blue end) better
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

11. The meaning of the phase
Several components.
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Fit a model including atm and source.
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12. Binary star measurement
More phases, and better SNR. Tauri
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13. Binary star measurement
10x better dM than Armstrong (2006)
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14. Stellar Diameters
Measured as location of null in visibility
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versus wavelength. Ophichus

15. Stellar Diameters

16. Imaging
No special software needed:
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Complex visibilities. Can use standard AIPS
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package.
Tauri
Gaussian noise.
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17. Conclusions
Coherent integration works
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Demonstrated with NPOI data
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Improves SNR
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Better stellar diameters
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Better binary parameters
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Imaging in the usual way
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