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Re: [Imgcif-l] imgCIF X axis

  • To: "Herbert J. Bernstein" <yayahjb@gmail.com>
  • Subject: Re: [Imgcif-l] imgCIF X axis
  • From: James H via imgcif-l <imgcif-l@iucr.org>
  • Date: Wed, 15 Jun 2022 21:40:10 +1000
  • Cc: James H <jamesrhester@gmail.com>, The Crystallographic Binary File and its imgCIF application to image data<imgcif-l@iucr.org>
  • In-Reply-To: <CABcsX25oGH95vP8RLOET9R=BQZHDoizOu4FDht2ZjbXuYyYw8g@mail.gmail.com>
  • References: <CAM+dB2c5Hv=g03gGkC4==awB_ygVL6Hvdp-vf6R86pVOoRSmYQ@mail.gmail.com><CABcsX25oGH95vP8RLOET9R=BQZHDoizOu4FDht2ZjbXuYyYw8g@mail.gmail.com>
Thanks Herbert. If the sense of rotation of the principal axis is part of
the procedure in determining the direction of X, that is fine and explains
why the reference material is written the way it is.

all the best,
James.

On Wed, 15 Jun 2022 at 21:34, Herbert J. Bernstein <yayahjb@gmail.com>
wrote:

> 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 <imgcif-l@iucr.org>
> 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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