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[Imgcif-l] _diffrn_scan_axis.angle_range : total change in startingposition or total change in axis position?

  • To: The Crystallographic Binary File and its imgCIF application to image data<imgcif-l@iucr.org>
  • Subject: [Imgcif-l] _diffrn_scan_axis.angle_range : total change in startingposition or total change in axis position?
  • From: James H via imgcif-l <imgcif-l@iucr.org>
  • Date: Tue, 24 May 2022 12:47:58 +1000
  • Cc: James H <jamesrhester@gmail.com>
Dear imgCIF group,

Just a simple question: _diffrn_scan_axis.angle_range is currently
defined as "The range from the starting position for the specified
axis in degrees.". Read in conjunction with the chapter in volume G,
this could be interpreted as the range of starting positions for the
beginning of a step, or the difference between the starting position
and the final position at the end of the final step. I prefer the
latter interpretation, which is supported by Examples 2 and 3 for the
diffrn_scan category. So, looking at line 7695
(https://github.com/yayahjb/cbflib/blob/main/doc/cif_img_1.8.6.dic#L7695)
for a single-frame scan the range is given as 1.0 degree, which is the
total angular range of the scan.

However, Example 1 above the previous example has 201 frames, each
covering 0.1 degrees, and the scan range is given as 20.0. This
suggests that range refers to the range in starting positions for each
step. This cannot be reconciled by supposing that Example 1 involved
no scanning of the axis during data collection as
_diffrn_scan_frame_axis.angle_increment is 0.1

Can I suggest either correcting Example 1 or Examples 2 and 3, and
improving the definition text to clarify which interpretation is
correct?

I have raised this also in the repo as an issue.

thanks,
James.
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