For those wondering why the Department of Energy is building a space telescope rather than focussing on nuclear things, the Department of Energy funds the SLAC Linear Accelerator centre at Stanford and it's people at that centre who have designed SNAP, a spacecraft that happens to fulfill exactly the requirements NASA put forth for JDEM.
The Dark Energy Mission is a wide-field high-resolution space telescope; a hundred million or so pixels of 0.2 arcsecond extent, and a five-foot main mirror. The idea's to survey most of the sky at about four times the resolution possible from Earth (adaptive optics, which are useful for very high-resolution imaging of very narrow fields from Earth, are not useful for these large fields).
There are two mission models: take pictures of galaxy clusters and work out the mass in them implied by the way the gravity of the foreground cluster distorts the light from background galaxies, and take pictures of lots of galaxies looking for supernovae.
It's perhaps not entirely accidental that this large-scale high-resolution survey work will produce very attractive images of the sky for outreach, a task at which the JWST replacement for Hubble, being aimed more at work in the infra-red and on the faintest and most distant objects, is not as superb as Hubble.