Dear Ronnie,
we are working with weak diffracting crystals. Many tests (and taking
into account Marcus Muellers results) showed that 0.3-0.5 of XDS
mosaicity, (very) low dose, and high redundancy give the best results.
Our crystals diffract between 7 and 4 A. At low dose I do not check
diffraction really by eye. I collect 200 images (fine slicing), process
and check XDS statistics. (200 images is not much when the dataset
comprises 9000-12000 images). Especially for anomalous data this
strategy was pivotal.
We did also comparisons between CCD and PILATUS. We used one long
crystal and collected on end end a MAD dataset using a CCD and
afterwards at the other end a MAD dataset on PILATUS, same dose per
degree. Results were absolutely clear that the fine slicing give much
better results.
Best,
Guenter
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.
--
***********************************
Priv.Doz.Dr. Guenter Fritz
Fachbereich Biologie
Universitaet Konstanz
http://www.biologie.uni-konstanz.de/fritz
e-mail: guenter.fr...@uni-konstanz.de
e-mail: guenter.fr...@uniklinik-freiburg.de
http://www.uniklinik-freiburg.de/neuropathologie/live/forschung/ag-g-fritz.html
Tel.: +49 761 270 5078
Fax.: +49 761 270 5050
