high-precision optical interferometry of faint and low-contrast targets requires coherent...
TRANSCRIPT
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