  Current large-aperture cosmic microwave background (CMB) telescopes have
nearly maximized the number of detectors that can be illuminated while
maintaining diffraction-limited image quality. The polarization-sensitive
detector arrays being deployed in these telescopes in the next few years will
have roughly $10^4$ detectors. Increasing the mapping speed of future
instruments by at least an order of magnitude is important to enable precise
probes of the inflationary paradigm in the first fraction of a second after the
big bang and provide strong constraints on cosmological parameters. The CMB
community has begun planning a next generation "Stage IV" CMB project that will
be comprised of multiple telescopes with between $10^5$ - $10^6$ detectors to
pursue these goals. This paper introduces new crossed Dragone telescope and
receiver optics designs that increase the usable diffraction-limited
field-of-view, and therefore the mapping speed, by an order of magnitude
compared to the upcoming generation of large-aperture instruments. Polarization
systematics and engineering considerations are presented, including a
preliminary receiver model to demonstrate that these designs will enable high
efficiency illumination of $>10^5$ detectors in a next generation CMB
telescope.
