Schematic of the experimental geometry at the SRS, optimised for compromise between image resolution and amount of useful information per photographic plate; used to obtain depth profiles from a SiC crystal edge. Typical dimensions are a source-sample distance ~ 80m and a sample-plate distance ~ 50 mm. The geometry is nominally centered on the 213l row for l = 1.5 Å, 2θ = 100° with the crystal edge inclined at an angle of 37° to the horizontal as shown. Under such conditions the photographic plate will record a Laue type pattern of the crystal. The diagram highlights a central l = 0 reflection topograpgh of the crystal edge (darker type) in which all of the polytypes would be present. In the adjacent row for those reflections when l = 0 different polytypes would be displaced spatially as illustrated (see text for an explanation).