MEM-VIS and Visibilities

What are visibilities?

• The visibility of a point source:
  
• The function analogous to the modulation profile is the sequence of terms V_j on a circle in the (u,v) plane:
  
• The V values have both amplitude and phase

Visibilities offer certain advantages over countrate profiles:

• Visibilities are hardware independent
• And they are in inertial coordinates.
• Visibility profiles can be added together (count profiles can't).
• The tools of radio astronomy are available for use with visibilities.

Disadvantages

• One disadvantage of visibilities is that they have to be computed from the countrate profiles.
      - but this is relatively straightforward,
      - and the computation time is very small compared to the imaging time.
• Another disavantage is that they involve complex arithmetic,
      - but astronomers are usually used to this
      - and the software does all the dirty work