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

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

Optical Interferometry● Resolution limit of telescopes● Size of stars● Separate telescopes● Fourier transform●

The SNR problem● Atmosphere● Fringe motion● Short exposures● Few photons● Average many exposures

– But how?

Incoherent Averaging● V 2 averaging

– Just amplitude squared, forget phase.

● Triple phase– Three telescopes, one phase.

● Throwing away phase information

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

Incoherent vs. Coherent

CoherentCoherent

incoherent

incoherent

● Improves SNR on both amplitude and phase

Coherent

baseline

How to coherently integrate● Estimate phase for each exposure and shift.

● Multi-wavelength measurements.

● Enough photons in band even if not in channel.

Wavelength

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.

HR 5544 V=4.5 star● 2x (red end) to 10x (blue end) better

The meaning of the phase● Several components.● Fit a model including atm and source.

Binary star measurement● More phases, and better SNR. Tauri

Binary star measurement● 10x better dM than Armstrong (2006)

Stellar Diameters● Measured as location of null in visibi l ity versus wavelength. Ophichus

Stellar Diameters

Imaging● No special software needed:

– Complex visibil ities. Can use standard AIPS package.

– Gaussian noise.Tauri

Conclusions● Coherent integration works● Demonstrated with NPOI data● Improves SNR● Better stellar diameters● Better binary parameters● Imaging in the usual way