Hi Siebren,

Ah, I see... I can save you some time there: field rotation
is momentarily zero when an object is at azimuths 90 or 270
(the "east-west meridian").

Find an object in Guide, then set up animation with the
"horizon" radio button set. Put the chart in "zenith up"
mode, simulating the view through an alt/az scope. Make
sure the alt/az is set to be displayed in the legend. Now
start animating with a step size of minutes to hours.

The field will wobble back and forth (that is, it rotates).
You'll see that it pauses in its rotation when the azimuth is
90 or 270 degrees.

-- Bill