X-ray diffractometer: Eulerian cradle, or four circles diffractometer. The difference between ψ and χ is the zero reference and the orientation: χ = π/2 - φ (rad) or χ = 90 - φ (deg).
Christophe Dang Ngoc Chan (<a href="//commons.wikimedia.org/wiki/User:Cdang" title="User:Cdang">cdang</a>)
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span>