[NeXus-committee] Proposal for NXcrystal and NXmonochromator

[Benjamin Watts]

Hi Eugen & Pete,
    If you want to move the "usage" field into NXmonochromator, then 
maybe we should rethink NXmonochromator and consider a larger overhaul. 
We currently have the "crystal" and "velocity_selector" as optional 
fields for hard X-rays and neutron, respectively, and I want to add 
"grating" to that set of optional fields for the soft X-ray case. For 
soft X-rays, the situation is actually even more complicated as the 
geometry set by entrance and exit slits are also a fundamental part of 
the monochromator (and the entrance slit is also optional, since some 
beamlines simply consider the source size and position stable and small 
enough).
Can you think of a better way to achieve all this? One option would be 
to set it up so that one looks at the "usage" field first and then uses 
its value to decide which optional fields to looks for next. This would 
mean including values such as "grating" and "velocity_selector" in the 
list of possibilities for the "usage" field, which would then make the 
name "type" more appropriate.

Cheers,
Ben


On 09/03/15 21:41, Eugen Wintersberger wrote:
>> The choice of Bragg or Laue orientation is important when calculating
>> the rocking curve profile, whether or not the crystal is used as part of
>> a monochromator.
> I entirely agree with you here. However, this is a particular use case
> for a crystal in connection with a monochromator. Thus, at least in my
> opinion, the "usage" field belongs to NXmonochromator.
>
>> On 03/09/2015 03:14 PM, Eugen Wintersberger wrote:
>>> Hi Pete
>>>
>>> On Sun, 2015-03-08 at 11:48 -0500, Pete Jemian wrote:
>>>> Eugen:
>>>>
>>>> You propose to:
>>>> * rename "usage" field to "type"
>>> I propose to remove the "usage" field entirely from NXcrystal.
>>>
>>>> * add "channel_cut" as another possible value
>>> And I propose to add a "type" field to NXmonochromator. The possible
>>> values for "type" could be "Laue", "Bragg", "Pseudo Channel Cut", and
>>> "Channel Cut". Or something in this way.
>>>
>>>> As I said before, "channel_cut" is not an alternative to "Bragg" or
>>>> "Laue" in its description of crystal orientation.  Most channel cut
>>>> crystals use Bragg orientation.  I suggest a new field could be added to
>>>> describe "channel_cut".  Another similar term could be
>>>> "pseudo_channel_cut" which describes the crystal arrangement of most
>>>> synchrotron X-ray monochromators.
>>> This would be the "type" field for NXmonochromator. It should identify
>>> what type of mono we talk about.
>>>
>>>> Like many things in NeXus, the field name "usage" does not seem to fit
>>>> so well, as you point out.
>>>>
>>>> Rather than disrupt the historical use of the "usage" field in NeXus
>>>> data files and however many data files have been written with this
>>>> field, would it be acceptable to improve the documentation for the
>>>> "usage" field?
>>> In the best case I would declare it as deprecated. At least from my
>>> point of view NXcrystal describes a crystal and not its particular
>>> application. But we can leave it there for compatibility reasons and
>>> declare it as deprecated.
>>>
>>> I am not sure if, what I propose, is really the best approach. But for
>>> the time being I have no better idea how to deal with the situation.
>>>
>>> regards
>>>     Eugen
>>>
>>>
>>>> Pete
>>>>
>>>> On 3/8/2015 10:56 AM, Eugen Wintersberger wrote:
>>>>> Hi folks
>>>>>      as a result of the recent discussion I would like to propose the
>>>>> following changes to NXcrystal:
>>>>>
>>>>> remove the "usage" field from NXcrystal
>>>>>
>>>>> It makes no sense there as the way the crystal is used in a particular
>>>>> application is entirely unimportant for the particular crystal itself.
>>>>>
>>>>>
>>>>> Furthermore I would like to suggest the following change to
>>>>> NXmonochromator
>>>>>
>>>>> add a "type" field to NXmonochromator
>>>>>
>>>>> This field should basically hold the values of "usage" from the current
>>>>> NXcrystal class. I would also suggest to have an additional possible
>>>>> value "channel_cut" available for this field.
>>>>>
>>>>> Best regards
>>>>> Eugen
>>>>>
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