  (abridged) The Atacama Large Aperture Submillimeter Telescope (AtLAST)
project aims to build a 50-m-class submm telescope with $>1^\circ$ field of
view, high in the Atacama Desert, providing fast and detailed mapping of the
mm/submm sky. It will thus serve as a strong complement to existing facilities
such as ALMA. ALMA's small field of view ($<15^{\prime\prime}$ at 350 GHz)
limits its mapping speed for large surveys. Instead, a single dish with a large
field of view such as the AtLAST concept can host large multi-element
instruments that can more efficiently map large portions of the sky. Small
aperture survey instruments (typically much smaller than $<3\times$ the size of
an interferometric array element) can mitigate this somewhat but lack the
resolution for accurate recovery of source location and have small collecting
areas. Furthermore, small aperture survey instruments do not provide sufficient
overlap in the spatial scales they sample to provide a complete reconstruction
of extended sources (i.e.\ the zero-spacing information is incomplete in
$u,v$-space.) The heterodyne instrumentation for the AtLAST telescope that we
consider here will take advantage of extensive developments in the past decade
improving the performance and pixel count of heterodyne focal plane arrays.
Such instrumentation, with higher pixel counts, has alredy begun to take
advantage of integration in the focal planes to increase packaging efficiency
over simply stacking modular mixer blocks in the focal plane. We extrapolate
from the current state-of-the-art to present concept first-generation
heterodyne designs for AtLAST.
