**Question:** Suppose there is a diffraction screen (DS)，a lot of small holes scattering on it, a bundle of parallel rays are incident on the DS, we will get a diffraction pattern on a receiving screen (RS) behind DS (there is a lens between DS and RS). So theoretically, are the “diffraction pattern” (noted above) and the “dirty map” (got from a VLBI observation) the same thing or any difference between them? – Hongmin

**Answer**: The “dirty image” in a radio interferometric image (whether it be acquired using a long baseline or connected interferometer) is the convolution of the spatial coherence function and the sampling function of your interferometer. The spatial coherence function is the correlation of the intensity of the object being measured over the field of view of the object. So, your characterization of a diffraction pattern being equivalent to the dirty image is roughly correct, if you include the correlation of that diffraction pattern over the source and the sampling properties of the interferometer.

*Jeff Mangum*