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5 - Dealing with the atmosphere (What the atmosphere does (Corrupts phase…
5 - Dealing with the atmosphere
What the atmosphere does
Corrupts phase and amplitude of incoming signals
Corruption is different for different telescopes/aperture
Corruption changes with time (scales ms to mins)
Corruption varies with position (<size of tel. for optical)
Sources:water vapour...,ionosphere (L radio)
How bad is the problem
Phase
The shorter the wavelength, the more rapid the phase fluctuation and the harder the problem becomes
Optical:fired parameter r0 - length scale of fluctuations
Timescale=r0/wind velocity
Amplitude
mm wavelengths: amplitude drops precipitously with atmospheric water vapour column
One approch:closure phase
Phase error on baseline
add phases around triangle
Maths
BUT: requires bright source (S:N in one coherence time)
COAST
Self calibration
Development of closure techniques which implicitly preserves closure phase and amplitude
Use an arbitrary model to determine gi and gj for an arbitrary model and then replace the observed visiblities with maths
Shocking but it works;relies on errors separating by telescope
Phase calibration
Require a<size of isoplanactic patch
T< coherence time of atmosphere a this wavelength
Can nod back and forth, or have target and calibrator in same FOV
Phase calibrator must be
bright (S:N in reasonable time/atmospheric coh.time
Close (same corruption)
If isoplanetic patch is small the calibrator may not exist
Signal to noise
Maths
In practice rarey get to this value
Optical need photons in coherance vol taking throughout into account