With an alt-az telescope the field rotates as the telescope tracks, and the
instrument is rotated to compensate for this by a two motor instrument
rotator with integral cable wrap. Unlike the Cassegrain rotator the Prime
Focus rotator only has a travel of 358 
. This means that it is
not possible to observe at certain position angles, and that extreme care
must be taken when starting an exposure that the turntable will not run into
a software limit before the exposure ends. Users are recommended to:
(A) use
Chris Mayer's PLANETS program, which will tell you if and when a given
source will reach a wrap limit. PLANETS is an Xwindows application that
runs under VMS and Motif, and is useful also to see if and for how
long a planet
or other source is visible from a particular observing site.
(B) check on the TCS display to see
the time to reach wrap limits.