PAF application on a large spherical reflector to survey 100 times faster than the FAST

In this paper, we describe an application of a Phased Array Feed (PAF) on a large spherical reflector, as a cost effective approach to realize a radio telescope with large collecting area and Field of View (FoV). Considering the neutral spherical reflector of the FAST telescope, calculations show that a PAF consisting of
$85 \times 85$
elements with a spacing of 12cm could collect about 80% of the energy intercepted by a spherical aperture with a diameter of 300m at 1.4GHz, which is slightly higher than the efficiency of FAST telescope. And a FoV of
$2 \times 2=4$
square degree is achievable if a 20% gain reduction at the edge is acceptable. The fixed spherical surface makes the design and construction of the reflector much easier and less expensive than the active main reflector of FAST. The number of elements of the PAF is far beyond that of the current PAFs being developed for paraboloid reflector. Engineering challenges of and the prospective measures to cope with this large number may need to be investigated.