Dear Tassos, I'm interested in your third point. Do you have any explanation for why 0.5-1 degrees oscillation gave better data? Purely due to the fact that the crystals survived longer and thus yielded higher redundancy data, or also other parameters? Also do anyone know where the threshold lies for when not to use fine phi slicing on the PILATUS? ie, at what level of diffraction would one need to increase the exposure (and oscillation in order to still get redundant data)?
We'll be in a similar position in the coming weeks with data collection using PILATUS detectors, and would like to maximize the potential data quality from our weak diffracting crystals. Any input on this would be greatly appreciated! Cheers, Ronnie Berntsson On Nov 5, 2010, at 16:16, Anastassis Perrakis wrote: > three additional points: > > 1. > >> OTOH, if "The diffraction is quite weak", one may be limited by counting >> statistics. This also cannot be overcome by processing. > > As JIm suggests above then, maybe you should look if the 15% Rmerge is almost > reasonable given the specific I/sigI at low resolution? > > > 2. If there is one thing I do not like in XDS, is that there is no (or I have > failed to find) statistics of I/sigI and Rmerge as function of image. > Have a look at the SCALA output. Maybe some images are bad? > > 3. making too fine slices of too weak diffraction images ends up with either > too weak counting statistics or inability to 'lock' the refinement. > we did that for one crystal form, collecting 0.1, 0.2, 0.35, 0.5, 0.7, 1.0 > from various crystals (with the same dose per degree, at SLS using a PILATUS, > mosaicity 0.4-0.6) in an attempt to get better Se signal. We miserably failed > to get any useful signal at the end, but learned that for these very weak > diffracting plates (submicron) collecting 0.5-1.0 degrees was actually giving > at the end better data. > > A.
