Hi I'd read Jim Pflugrath's 1999 paper in Acta D for a discussion on fine phi slicing - in general, if memory serves me correctly, he suggested using an oscillation angle ~0.5x the mosaic spread.
I think there may be issues with collecting data too finely with a Pilatus, even in shutterless mode. I don't know where the problems arise (can't be shutter/rotation axis synchronisation), but it seems to be the normal thing in XDS (which should have no problems with fine phi-slicing) to use the "PATCH_SHUTTER_PROBLEM=TRUE" that Martin Hallberg suggested, which looks a bit like a fudge to me (but I expect Kay to correct me on that!). The important thing is to treat each data collection as a scientific experiment, and to treat each crystal as an individual. Use the strategy tools (e.g. BEST, DNA, Edna) to work out how to get the best data from each crystal - remember that the data collection itself is the last experimental step. Any of the issues mentioned earlier in this thread will be exacerbated by very short exposure times - so it may be an idea to attenuate the beam and spend five minutes on a data collection rather than one.... On 5 Nov 2010, at 15:57, Ronnie Berntsson wrote: > 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. > Harry -- Dr Harry Powell, MRC Laboratory of Molecular Biology, MRC Centre, Hills Road, Cambridge, CB2 0QH
