A moderate sized mission to use bright X‐ray sources as physics labs is described. A combination of microchannel plate optics and a 32×32 pixel microcalorimeter would allow the successor to theRossi XTEto explore new domains of spectroscopic timing in a MIDEX class mission. With ∼10 times the area and ∼100 times the spectral resolution of the PCA (and 10 times that of silicon detectors) such a mission would be able to explore spectral line shifts in magnetars, neutron stars and black holes. Adding a polarimeter would create a powerful and unique capability. Such a mission would make numerous tests of basic physics, both quantum electrodynamics/chromodynamics (QED/QCD) and general relativity. © 2004 American Institute of Physics