In 1D mode every scan revisits the same line. In 2D mode successive scans step across the map, so the same total number of scans gives less integration time per spatial pixel.

The two units explained. MJy sr⁻¹ √s describes noise in surface brightness and is the natural unit for resolved or extended emission. mJy √s describes noise in flux density for an unresolved source. An unresolved source does not have to be physically point-like: it only needs to be smaller on the sky than the telescope beam, so the instrument cannot resolve structure across it and the source is measured as one beam-integrated flux. The two descriptions are related by the beam solid angle, S_ν = I_ν Ω_beam. For a Gaussian beam, Ω_beam = 1.1331 θ_FWHM². With the default 33.56″ beam, 1 MJy sr⁻¹ = 30.0 mJy per beam, so 10 MJy sr⁻¹ √s and 300 mJy √s are the same physical detector sensitivity. The numerical values are different because one is per steradian and the other is integrated over one beam. The detector noise and desired sensitivity do not need to use the same unit system. The calculator converts each input independently through the beam solid angle before comparing them. For example, with this beam, a detector NEFD of 300 mJy √s can be paired directly with a desired map sensitivity of 0.1 MJy sr⁻¹; the calculator converts 300 mJy √s to the equivalent 10 MJy sr⁻¹ √s internally. Likewise, a detector NEI of 10 MJy sr⁻¹ √s can be paired with a 3 mJy target. These two examples describe the same physical detector noise and target depth.