The VLT Interferometer - ESO

7.16. THE ASTRONOMICAL WEATHER STATION 141
This then leads to the need to include the following measurements in the AV./S:
• Fringe Lifetime (to): It is suggested that this be done by means of a
Young's dual aperture fringe experiment on a nearby star (possibly the
same one used for the ro measurements). One would measure the displacement
of the white light interference fringe with respect to the intensity
centroid of the combined star images. Assuming two apertures
50 cm apart one expects an RMS fringe position variation of 1.6, 0.9
and 0.5 fringes for To = 10, 20 and 40 cm respectively. Aperture diameters
of 10 cm, spectral bandwidths of 89 nm centered at 550 nm, a
systems efficiency of 5% and exposure times of 5 msec give as a limiting
magnitude for 0.1 fringe position accuracy of 2.5. It should be possible
to increase the spectral bandwidth and to lower the fringe position accuracy
requirements so that this limiting magnitude estimate is pessimistic.
Notwithstanding that it appears adequate for a to estimate.
• Isoplanatic Patch Radius (8): [Roddier et al 1986] give a way to determine
8 from the ro and u; measurements (u; = variance of the log
amplitude fluctuations). It fortunately just happens that 8 is directly
proportional to (T ou;.4 )-1 with the proportionality factor (.365 x wavelength)
being a function only of wavelength. This is probably the best
way of determining 8.
• Outer scale of turbulence: It is not clear how to measure this, except for
using the VLTI itself
• Seismic activity: It is important to have a measure of both low and high
frequency seismic activity since these affect the pathlength adjustments
and fringe contrast.