We propose to observe the "source" of cometary dust trails, that is,
where the dust trails connect to the coma and nucleus of the comet.  In
previous observations from the central program (JDAVIES.JKDTRAIL), we
observed the trail of P/Kopff at 3 different offsets ahead and behind
the nucleus.  Because the trails are so long (tens of degrees) compared
to the field of view of the ISOCAM (3 arcmin), it is only possible to
observe patches of the trail.  In the pervious proposal we observed more
than 0.5 degrees from the comet, and here we propose to observe the
crucial patches close to the cometary nucleus.  The largest dust
particles, which are weakly coupled to the gas, remain close to the
nucleus longest, so we will observe the largest and most fresh.  The
production rate of trail particles will be best constrained by comparing
the trail brightness near its source to that further away.  We selected
4 comets already planned in the central program for observation 1 degree
from the nucleus.