  Cassegrain designs can be used to build thin lenses. We analyze the
relationships between system thickness and aperture sizes of the two mirrors as
well as FoV size. Our analysis shows that decrease in lens thickness imposes
tight constraint on the aperture and FoV size. To mitigate this limitation, we
propose to fill the gaps between the primary and the secondary with high index
material. The Gassegrain optics cuts the track length into half and high index
material reduces ray angle and height, consequently the incident ray angle can
be increased, i.e., the FoV angle is extended. Defining telephoto ratio as the
ratio of lens thickness to focal length, we achieve telephoto ratios as small
as 0.43 for a visible Cassegrain thin lens and 1.20 for an infrared Cassegrain
thin lens. To achieve an arbitrary FoV coverage, we present an strategy by
integrating multiple thin lenses on one plane with each unit covering a
different FoV region. To avoid physically tilting each unit, we propose beam
steering with metasurface. By image stitching, we obtain wide FoV images.
