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Re: [Imgcif-l] imgCIF X axis

  • To: James Hester <[email protected]>, The Crystallographic Binary File and its imgCIF application to image data<[email protected]>
  • Subject: Re: [Imgcif-l] imgCIF X axis
  • From: "Herbert J. Bernstein via imgcif-l" <[email protected]>
  • Date: Wed, 15 Jun 2022 07:33:34 -0400
  • Cc: "Herbert J. Bernstein" <[email protected]>
  • In-Reply-To: <CAM+dB2c5Hv=g03gGkC4==awB_ygVL6Hvdp-vf6R86pVOoRSmYQ@mail.gmail.com>
  • References: <CAM+dB2c5Hv=g03gGkC4==awB_ygVL6Hvdp-vf6R86pVOoRSmYQ@mail.gmail.com>
Dear James,
  This is a little tricky.  The relevant rules are:

"Axis 1 (X): The X-axis is aligned to the mechanical axis pointing from
     the sample or specimen along the  principal axis of the goniometer."

  "If the axis involved is a rotation axis, it is right-handed, i.e. as
     one views the object to be rotated from the origin (the tail) of the
     unit vector, the rotation is clockwise.  If a translation axis is
     specified, the direction of the unit vector specifies the sense of
     positive translation"

So, yes the x-axis starts from the sample, but where it point to in the
goniometer depends on where in the goniometer the "principal"
axis of the goniometer points _and_ in which direction the axis
settings increase and whether the principal axis is a rotation
axis or a translation axis.  The only way I have figured out to
do this is to start from the engineering diagrams (or at least
photos) of the goniometer, and even then, I once screwed up
the x-axis for a beamline at DLS (ask Graeme Winter).

  Regards,
    Herbert

On Wed, Jun 15, 2022 at 2:38 AM James H via imgcif-l <[email protected]>
wrote:

> Dear imgCIF experts,
>
> Just to double-check, is it correct to say that the imgCIF X axis always
> points from the specimen towards the goniometer base? Closely parsing the
> dictionary and Volume G suggests that the X axis being "aligned" with the
> principal axis could also allow the X axis to point from the specimen away
> from the goniometer base, particularly if the sense of rotation of the
> principal goniometer axis is taken into account when defining X (is it?).
>
> thanks,
> James.
> --
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