  High-contrast imaging enabled by a starshade in formation flight with a space
telescope can provide a near-term pathway to search for and characterize
temperate and small planets of nearby stars. NASA's Starshade Technology
Development Activity to TRL5 (S5) is rapidly maturing the required technologies
to the point at which starshades could be integrated into potential future
missions. Here we reappraise the noise budget of starshade-enabled exoplanet
imaging to incorporate the experimentally demonstrated optical performance of
the starshade and its optical edge. Our analyses of stray light sources -
including the leakage through micrometeoroid damage and the reflection of
bright celestial bodies - indicate that sunlight scattered by the optical edge
(i.e., the solar glint) is by far the dominant stray light. With telescope and
observation parameters that approximately correspond to Starshade Rendezvous
with Roman and HabEx, we find that the dominating noise source would be
exozodiacal light for characterizing a temperate and Earth-sized planet around
Sun-like and earlier stars and the solar glint for later-type stars. Further
reducing the brightness of solar glint by a factor of 10 with a coating would
prevent it from becoming the dominant noise for both Roman and HabEx. With an
instrument contrast of 1E-10, the residual starlight is not a dominant noise;
and increasing the contrast level by a factor 10 would not lead to any
appreciable change in the expected science performance. If unbiased calibration
of the background to the photon-noise limit can be achieved, Starshade
Rendezvous with Roman could provide nearly photon-limited spectroscopy of
temperate and Earth-sized planets of F, G, and K stars <4 parsecs away, and
HabEx could extend this capability to many more stars <8 parsecs. (Abridged)
