  The Astronomy and Astrophysics Decadal Survey's highest recommended space
mission was a Wide-Field Infrared Survey Telescope (WFIRST) to efficiently
conduct three kinds of studies: dark energy surveys, exoplanet surveys, and
guest surveys. In this paper we illustrate four potential WFIRST payloads that
accomplish these objectives and that fully utilize optical and technical
advances made since the community input to the Decadal Survey. These
improvements, developed by our group, are: unobscured 1.3 or 1.5 m apertures;
simultaneous dual focal lengths delivering pixel scales of 0.18" for imaging
and 0.38" or 0.45" for slitless spectroscopy; the use of a prism in converging
light for slitless spectroscopy; and payload features that allow up to 270
days/year observing the Galactic Bulge. These factors combine to allow WFIRST
payloads that provide improved survey rates compared to previous mission
concepts. In this report we perform direct comparisons of survey speeds for
constant survey depth using our optical and exposure-time tools previously
developed for JDEM. We further compare these four alternative WFIRST
configurations to JDEM-Omega and to the European Space Agency's Euclid mission,
and to an alternative Euclid configuration making use of the lessons learned
here that delivers performance approaching that of WFIRST. We find that the
unobstructed pupil is a major benefit to weak lensing owing to its tighter
point spread function, improved signal to noise, and higher resolved galaxy
count. Using two simultaneous plate scales in a fully focal system is practical
and simplifies the optical train, and the use of a prism in converging light
offers improved throughput compared to a grism. We find that a 45 degree outer
baffle cutoff angle, combined with fully articulated solar panels and K-band
antenna, substantially increase the exoplanet yield. Presented at the 217th AAS
conference.
