Transform properties determined by Fourier kernel:

exp( i ) = cos + i sin

Fourier Transform of a real function (eg. sky brightness) is Hermitian (conjugated on reflection):

Real <==> Real Even + Imag Odd
Imaginary <==> Real Odd + Imag Even

Real & Even <==> Real & Even
Real & Odd <==> Imag & Odd

Even <==> Even
Odd <==> Odd

The sky brightness distribution is assumed to be real, and thus its transform should be Hermitian

At any given time the complex visibility on a given baseline is measured, and its complex conjugate is used as the reflected baseline visibility, guaranteeing a real Image Plane