Introduction

The sampled visibility data in a uv plane can be converted to an image with Fourier transform. The task invert is used to produce an image or a spectral image cube based on a collection of calibrated data.

Even if one is only interested in continuum data, the multi-frequency imaging is still relevant. The SMA can simultaneously observe 24 spectral windows in each side band. Thus one needs to handle 48 spectral windows observed simultaneously. In particular, each side band can have a bandwidth of 2 GHz and a sideband separation of 10 GHz. One can get extra sensitivity and improved uv coverage by combining all the spectral windows in the imaging process. The technique of forming a single continuum image from a variety of frequencies is called ``multi-frequency synthesis'' (MFS). Miriad nicely supports the MFS feature. The MFS process is useful to SMA users in imaging the continuum dust emission from distant galaxies, planets outside the solar system, young stars in the Galactic center/disk and starburst galactic nuclei. An example of making an image from raw SMA data to final image using MFS technique is shown in the sections below. We also discuss the weighting methods in the imaging process.