[Nexus] NXgeometry lacking documentation and others....

[Mark Koennecke]

Hi,

my 2c worths on todays topics:

- NXaperture: IMHO this one of the least well done base classes. The 
general idea, as I recollect it, is that
   you give the shape through NXgeometry/NXshape. The enum on NXshape 
determines the meaning of
   the parameters. For example if the shape is nxbox then size means the 
height and width of the slit, if it
   is a nxcylinder it means the diameter and length of the cylinder.  
Now for the position of the thing you are
   hit by the undocumented fact that you can have the normal distance 
polar_angle etc fields in almost all
   beamline components. This is the first way to position the thing. On 
our reflectometer, I personally use
   the distance as this is the only thing which matters. The other way 
is to use NXtranslation in NXgeometry.
   And, as Pete Jemian correctly mentioned, use the CIF stuff. What I 
personally do not like about NXaperture
   is that  90% of people seem to have rectangular apertures and the 
NXgeometry description is a kind of
   overkill. But I got outvoted....
- CIF coordinates: I updated the coordinate section in the manual a 
while ago. Is that description lacking? If so,
   in what way?
- Coplanar/non coplanar. I had to look this up on the WWW. It really 
means what I call scattering in plane and
   out of plane. The beauty of the polar coordinate system as choosen by 
NeXus is that polar_angle is *always*
   the scattering angle two theta even when working non coplanar. If you 
are coplanar, azimuthal_angle is 0 and
   polar_angle is trivially the setting angle (two theta )of the 
detector. If you work non coplanar, i.e. move the
   detector out of the scattering plane you have the disadvantage that 
you have to calculate polar_angle and
   azimuthal angle from the setting angles.  In my place we call them 
gamma and nu.

Best Regards,

              Mark