psi constant mode with "PETRA3 P09 EH2" geometry

Conversation with Fred Picca:

> With the "PETRA3 P09 EH2" geometry, how to calculate angles (from h,k,l) when
> keeping psi constant?

Yes you need a dedicated psi_constant_mode in order to expresse the psi_constant constraint.

> Can the "psi_constant_horizontal" mode be added to the "PETRA3 P09 EH2"
> geometry?

I think so, but maybe the best solution would be to create a dedicated feometry for your dedicated diffractometer.
This way we can set the right axes names, sens of rotations of the axes, this way the binoculas-ng
(2D detectors -> h, k, l, qx, qy, qz, q, qpar, qper, etc...) part could be use almost out of the box with your geoemtry.

> Is it common for the engines other than "hkl" to be used for operations? Or are
> these used for calculations only?

Some peoples use the psi pseudo axis in order to rotate around the Q vector.

some pseudo axes are read/write and others are read only.

Fred

No files, tests, or entry points are named. Start by locating the PETRA3 P09 EH2 geometry and existing psi-constant modes, then clarify whether the intended result is a new mode or a dedicated diffractometer geometry; completion should include an agreed implementation scope and verified angle calculations for constant psi.

Written by the indexing model from the issue text.