1. After placing the sample roughly horizontally and adjusting the specular axis to the center of the CCD, four images were taken along the q z direction under stepwise increase of the angle of beam incidence (Da i = 0.45°) enabling the calculation of the actual a i as well as the exact sample detector distance and allowing for scanning the low q k space for peaks resulting from superstructures (Fig. 3 ). 2. Adjustment of the specular axis at one side of the CCD and coverage by a lead stripe facilitated scanning the Eur Biophys J (2011) 40:417-436 421 reciprocal space far from the specular axis yielding combinations of exposures that provide a partial overlap. With this data collection an overview of the reciprocal space was achieved in which peaks representing intrahelical distances might occur (data not shown). 3. Quantitative images including the nonattenuated specular axis (q k = 0) on one side of the CCD plate up to q k = 2.5 Å -1 were taken with accumulated exposure series of 20-600 images applying single exposure times of 0.3-5.0 s. In analysis of the chain correlation peak using the same geometry, images were taken with exposure times of 20-30 s under attenuation of the primary beam and the specular axis.