Dear Gerard,
you sound like a sales person for Dectris. Fine slicing is perfectly
fine with CCD detectors - it takes a bit longer because of the step scan
instead of continuous scan. The read noise issue is often overstated
compared to the sample induced scatter background. If for fine slicing
at 0.05 degree or less the diffraction peaks go too close to the read
noise make a longer exposure - signal goes up, ratio signal to
sample-induced-BG less, as for any fine slicing, same read noise.
It would be helpful to analyze the dense spot packing along layer lines
if we knew the wavelength and the sample-to-detector distance (assuming
this is a 300 mm detector) and the rotation width - as you pointed out.
That would help to distinguish between multiple crystals (my guess) and
lattice translocation disorder. Fine slicing is definitely needed to
figure out what the diffraction pattern at 120 degree could tell you in
terms of strong anisotropy .
Best regard.
Gerd
On 13.07.2017 08:20, Gerard Bricogne wrote:
Dear Tang,
I noticed that your diffraction images seem to have been recorded
on a 3x3 CCD detector. With this type of detector, fine slicing is
often discouraged (because of the readout noise), and yet with the two
long cell axes you have, any form of thick (or only semi-fine) slicing
would result in spot overlaps.
What, then, was your image width? Would you have access to a
beamline with a Pilatus detector so that you could collect fine-sliced
data?
I would tend to agree with Herman that your crystals might be
cursed with lattice translocation disorder (LTD), but you might as
well try and put every chance of surviving this on your side by making
sure that you collect fine-sliced data. LTD plus thick slicing would
give you random data along the streaky direction. Use an image width
of at most 0.1 degree (0.05 would be better) on a Pilatus, and use XDS
to process your images.
Good luck!
Gerard
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On Thu, Jul 13, 2017 at 01:21:02PM +0100, Tang Chenjun wrote:
Hi David,
Thanks for your comments. Although the spots become streaky in certain
directions, I have processed the data in HKL3000 and imosflm, which suggested
the C2221 space group (66.59, 246.95 and 210.17). The Rmerge(0.14),
completeness(94.8%), redundancy(4.6) are OK. When I tried to run Balbes with
the solved native structure, the molecular replacement solution was poor. So I
ran Balbes with the split domains of the native structure. Although the
solutions were also poor, I found the MR score of one solution above 35. On the
basis of this solution, I tried to run Buccaneer and the Rfree could be 0.46.
Unfortunately, there are four molecules in the asymmetric unit and it is to
hard for me to reduce the Rfree further.
All best,
Chenjun Tang
