ar345 Program Manual 3 4
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. Data Collection
.1 Strategies
While some general rules apply how to collect data best, a sensible choice of data collectio
parameters depends on individual circumstances, i.e. crystal quality, beam properties, et
The following section gives some hints on how to program a data collection:
istance crystal−detector:
The distance crystal−detector must be chosen such that spots don’t overlap. It should b
chosen such that the highest resolution of the spots is close to the edge of the scanned area
i.e. move the detector back as far as possible. This gives a better Intensity/σ ratio!
∆ϕper image:
The
∆ϕmust be chosen such that spots don’t overlap. If the background is large compare
to the average spot intensity (i.e. most protein crystals) choose relatively small values (0.5 t
2.0 deg.). The smaller, the better the integration works, but the more exposures you need t
get a complete data set.
umber of images:
How many images you need depends on the symmetry of the space group and on the settin
of the crystal. As a rule of thumb, orthorhombic crystals require 90.0 deg. of data, monoclin
ones 120 deg., triclinic ones 180 deg., trigonal ones 60.0 deg., tetragonal ones 45.0 deg
hexagonal ones about 30.0 deg. and cubic ones about 25.0 deg.. High resolution dat
collections usually require a wedge of data − say 20 deg. − at another crystal setting becaus
of the blind region.
xposure time:
The exposure time should be less than 30 minutes, in any case, because of partial decay o
the F−centers on the imaging plate. For very weak diffractors which require longer exposur
times it is better to scan one exposure after 30 min. and to repeat the same exposure an
add the images together later. Otherwise, exposure times should be chosen such that th
dynamic range (0:130000) is not exceeded. It is actually best to have maximum intensitie
considerably less, e.g. 60000.
umber of oscillations:
Best to leave at 1, except when using exposure times larger than 15 minutes.
can mode:
Why scan the 345 mm plate if there is not enough diffraction? A 180 mm scan takes half th
time to scan, needs 80% less disk space and 80% less processing time!
