\subsection{Q Motor}

This is an MX pseudomotor driver for the momentum transfer parameter $q$,
which is defined as
$$
q = {{4\pi\sin(\theta)}\over{\lambda}} = {{2\pi}\over{d}}
$$
where $\lambda$ is the wavelength of the incident X-ray, $\theta$ is the
Bragg angle for the analyzer arm and $d$ is the effective crystal 
$d$-spacing that is currently being probed.

\textbf{Warning:}  $\theta$ above must not be set to the
angle of the analyzer arm.  Instead, it must be set to half of that angle
which is the nominal Bragg angle.

