> 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.
Perhaps the reason for the better data was an instance of the
redundancy-vs-long-exposure dilemma. Given, say, 1deg exposures: should one
collect 500x1s exposures or 250x2s exposures? I think this has been
examined, and while it does depend on the details of the parameters, it is
often better to collect 250x2s exposures because there is a "flat rate per
frame" noise level in the detector. I am wondering whether you just
increased the number of "flat rates" in your data sets by increasing the
number of frames while keeping the exposure/degree equal?
JPK
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