Re: CoreCIF revision 2.3

[Howard Flack <Howard.Flack@cryst.unige.ch>]

> _diffrn_source_take-off_angle
> 
> The definition seems a little sparse, but might be adequate. I wondered
> whether it should say "The angle between the normal to the surface of the
> target and the X-ray beam..."? This still assumes that the area of the
> target illuminated by the X-ray beam is perfectly flat. If not, should it
> say further "... and the midline of the X-ray beam..."? Pedantic, perhaps, 
> but it's better to be explicit.

  In most works I remember reading the take-off angle was small, a few
degrees away from zero rather than a few degrees away from 90 degrees.
This seems to be the usage required by the definiton you are
criticisizing (found by searching on the text of all those e-mails I
never read - there were only two concerning
_diffrn_source_take-off_angle one from David and one from George). To
comply with this usage, you need to substitute <<The angle between the
normal>> by <<The complement of the angle between the normal>>. I agree
with you that the initial wording was unsatisfactory. One can not define
an angle with respect to a plane.

> This still assumes that the area of the
> > target illuminated by the X-ray beam is perfectly flat.

  It seems you are also assuming that the electron beam has zero
divergence on the target. Unless the persons who suggested this item
complain, I would leave out the reference to the midline. I'm not sure
what use they are really going to make of the numerical value of
_diffrn_source_take-off_angle. 

H.