  The Nancy Grace Roman Space Telescope (Roman), under development by NASA,
will investigate possible causes for the phenomenon of dark energy and detect
and characterize extra-solar planets. The 2.4 m space telescope has two main
instruments: a wide-field, infra-red imager and a coronagraph. The coronagraph
instrument (CGI) is a technology demonstrator designed to help bridge the gap
between the current state-of-the-art space and ground instruments and future
high-contrast space coronagraphs that will be capable of detecting and
characterizing Earth-like planets in the habitable zones of other stars. Using
adaptive optics, including two high-density deformable mirrors and low- and
high-order wavefront sensing and control, CGI is designed to suppress the star
light by up to 9 orders of magnitude, potentially enabling the direct detection
and characterization of Jupiter-class exoplanets. Contrast is the measure of
starlight suppression, and high contrast is the chief virtue of a coronagraph.
But it is not the only important characteristic: contrast must be balanced
against acceptance of planet light. The remaining unsuppressed starlight must
also have a stable morphology to allow further estimation and subtraction. To
achieve all these goals in the presence of the disturbance and radiation
environment of space, the coronagraph must be designed and fabricated as a
highly optimized system. The CGI error budget is the top level tool used to
guide the optimization, enabling trades of various competing errors. The error
budget is based on an analytical model which enables rapid calculation and
tracking of performance for the numerous and diverse questions that arise in
the system engineering process. In this paper we outline the coronagraph system
engineering approach and the error budget.
