  The gravitational field of the sun will focus light from a distant source to
a focal point at a minimal distance of 550 Astronomical Units from the sun. A
proposed mission to this gravitational focus could use the sun as a very large
lens, allowing (in principle) a large amplification of signal from the target,
and a very high magnification. This article discusses some of the difficulties
involved in using the sun as such a gravitational telescope for a candidate
mission, that of imaging the surface of a previously-detected exoplanet. These
difficulties include the pointing and focal length, and associated high
magnification; the signal to noise ratio associated with the solar corona, and
the focal blur. In addition, a method to calculate the signal gain and
magnification is derived using the first-order deflection calculation and
classical optics, showing that the gain is finite for an on-axis source of
non-zero area.
