The Copernicus observations of the supernova remnant Cas A, previously discussed by Fabian, Zarnecki & Culhane, have been reanalysed using an algebraic reconstruction technique (ART) to investigate further the X-ray structure of the source on the scale of 3 arcmin. This structure contains certain similarities to that observed at radio wavelengths with a comparable beam size. Additional X-ray observations through a 1.7-arcmin aperture confirm the main features of the ART X-ray map. The reliability and accuracy of the reconstruction technique are examined in detail. The spatial inhomogeneity and high X-ray luminosity of Cas A may be due to the blast wave interacting with matter ejected prior to the supernova explosion. Alternatively, a ‘reverse’ shock may be heating material ejected in the supernova itself.