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[Imgcif-l] Questions of reference frames

• To: <imgcif-l@iucr.org>
• Subject: [Imgcif-l] Questions of reference frames
• From: <Graeme.Winter@Diamond.ac.uk>
• Date: Tue, 16 Feb 2010 09:41:41 -0000

Hi Folks,
 
I'm still trying to finish off the details of our CBF template here and
I have hit what feels like a slight conceptual snag... 
 
The X axis is defined as rigidly the direction of the rotation axis,
from the sample. Now, historically the sample has been "small" so this
is pretty clear, but if the sample is "large" (or a normal size, but on
a uFocus beamline) then we can consider different regions of the sample
for exposure. A very real possibility is then to offset the rotation
axis from the direct beam, so that a toroidal region of sample is
exposed. This then brings a conceptual challenge:
 
The origin of the coordinate system is defined as the point where the
rotation axis intersects the direct beam. They do not intersect. Hmmm...
 
Also, if I do wish to translate the rotation axis, how to I report this?
The reference frame is "fixed" to this, so in effect I am moving the
beamline (indeed, the whole world) by the same distance in the opposite
direction. It would be good to record this, so that an analysis program
can determine the use of a toroidal (or corkscrew) data collection for
radiation damage purposes, given the size and placement of the direct
beam.
 
Has anyone else figured this out?
 
Of have I the wrong end of the stick (quite likely!)
 
Many thanks,
 
Graeme
 
Graeme Winter
Software and MX Support Scientist
Diamond Light Source
 
+44 1235 778091 (work)
+44 7786 662784 (work mobile)
 
 



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