Left: WISE two-color composite – red: 22 μm, green: 12 μm – of a particular active echoing region. The Cassiopeia A supernova remnant is visible at the lower border of the image. Light echoes are clearly seen as reddish filamentary structures distributed all over the field of view. Right: two-epoch difference image from a Spitzer-MIPS 24 μm (2006) image and the WISE 22 μm image. Cirrus regions cancel out to a smooth background, while echoes can be identified as white (2010) or black (2006) structures. Saturated stars leave residuals owing to their complex outer PSF; a WISE latent image (marked “L”) and a ghost (“G”) of Cas A can be seen as white structures.