A robotic instrument for measuring high altitude atmospheric turbulence from Dome C, Antarctica
To properly characterize the atmospheric properties of a site for a future large telescope or interferometer, it is insufficient to measure quantities, such as the full-width at half-maximum of a ste...
Optical turbulence outer scale and coherence outer scale at different astronomical sites
Here we analyse 168 optical turbulence profiles made at nine different locations worldwide by means of free flight balloons equipped with instrumentation. Optical turbulence outer scales Lo, as well a...
Ionosphericphase errors degrade high-resolution radio images below100 MHz, and theydiffer significantly from the tropospheric errorswhich dominate at high frequencies. The ionosphere is so high (~400 km) and theVLA primary beam is so wide (~0.2 rad) thatthe intersectionof the beam with the ionospheric screen is largerthan the"isoplanatic patch" size, a phase coherent region on thesky. Antenna-basedcalibration techniques developed at higherfrequencies cannot be used because ionosphericphase errors varysignificantly across the field-of-view of each antenna. This paperdescribes the "field-based calibration" technique adopted for the74 MHzVLA Low--frequency Sky Survey (VLSS) being made with the 10km"B" configuration. This technique is useful for a rangeof arraysizes but fails on baselines longer than the linearsize of theisoplanatic patch, a few 10s of km at74 MHz. Implications fordesigning larger low-frequency arrays are discussed.
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