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ar345 Program Manual 3 4
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5
. Data Collection
5
.1 Strategies
While some general rules apply how to collect data best, a sensible choice of data collectio
n
parameters depends on individual circumstances, i.e. crystal quality, beam properties, et
c.
The following section gives some hints on how to program a data collection:
D
istance crystal−detector:
The distance crystal−detector must be chosen such that spots don’t overlap. It should b
e
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
d
to the average spot intensity (i.e. most protein crystals) choose relatively small values (0.5 t
o
2.0 deg.). The smaller, the better the integration works, but the more exposures you need t
o
get a complete data set.
N
umber of images:
How many images you need depends on the symmetry of the space group and on the settin
g
of the crystal. As a rule of thumb, orthorhombic crystals require 90.0 deg. of data, monoclin
ic
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
a
collections usually require a wedge of data − say 20 deg. − at another crystal setting becaus
e
of the blind region.
E
xposure time:
The exposure time should be less than 30 minutes, in any case, because of partial decay o
f
the F−centers on the imaging plate. For very weak diffractors which require longer exposur
e
times it is better to scan one exposure after 30 min. and to repeat the same exposure an
d
add the images together later. Otherwise, exposure times should be chosen such that th
e
dynamic range (0:130000) is not exceeded. It is actually best to have maximum intensitie
s
considerably less, e.g. 60000.
N
umber of oscillations:
Best to leave at 1, except when using exposure times larger than 15 minutes.
S
can mode:
Why scan the 345 mm plate if there is not enough diffraction? A 180 mm scan takes half th
e
time to scan, needs 80% less disk space and 80% less processing time!