Choice of oscillation range

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The choice of oscillation range reflects a compromise between conflicting targets: preventing reflection overlap at high resolution (see table at end of IDXREF.LP), optimizing the signal-to-noise ratio, minimizing the disk space, and minimizing the readout time (less important with CCD and pixel detectors). It is also influenced by the type of detector used.

The following aspects need to be considered when choosing the oscillation range:

  1. crystal mosaicity: choosing the oscillation range less than (crystal mosaicity / 2) will not further increase the signal-to-noise ratio
  2. the maximum oscillation range to prevent angular overlap can be calculated from cell parameters and crystal orientation, and the mosaicity. Specifically, the mosaicity must be subtracted from the value given in the table at the end of IDXREF.LP (if the mosaicity is bigger than the given value, of course a positive value must be chosen, but there will then be overlaps).

A reasonable default value for crystals that diffract to 2A resolution or less, have a mosaicity of 0.3° or less and longest (primitive!) unit cell axis of 150A or less is, in my opinion:

  • around 0.5° for CCD detectors
  • around 0.1° for the Pilatus detector