Because the AP mount does not have a command to report whether orIMO this would be a well received addition to the capabilities of the
AP controller. Why? Since this thread originated as an "ACP problem",
I feel it's OK to speak up.
Motion (er, lack of motion) monitoring as a means to detect slew
completion comes with a price: How long do you wait until you decide
that the slew has really stopped? Are there cases where the mount
stops "briefly" only to start moving slowly again towards its final
destination? I'm sure Ray and Ajai have experimented enough to know
the safe wait time. I understand that the AP does a multi-phase slew
for backlash compensation, in which case it probably -does- stop and
start at least once.
One of ACP's main strengths is efficiency. As soon as the shutter
closes, it starts slewing to the next target. Not a big deal for
astro-imagers who change targets zero to a few times a night, but
extremely important for science imagers who collect large numbers of
images of different targets.
The extra time needed to make a safe/reliable decision that a slew has
ended could (and I'm guessing probably does) impact observing
efficiency for science imaging.
A prompt and positive slew-end signal from the mount would eliminate
guesswork/estimation. Seems like a low-cost/low-risk way to allow the
mount to deliver it's true capabilities and efficiency.