P1x26Yb4 I can't speak to 20 second exposures but with 10 second, yes 45% reject rate sounds about right. And yes it's something like a rectangular spiral pattern, each side of the spiral it drive in one direction (on both axes) for a while.
I haven't had a chance to do mosaics in EQ mode (all the big winter targets are gone), but I found the key to mosaics in alt/az mode before was to not bother with anything more than about 1.5-1.6x framing so that it has time to work all the way around that spiral and start getting 2nd pass of coverage before the field rotation is too great, meaning inside of about an hour of real time or 25 minutes or so of exposure time. Any longer and it got lost when it got back to the upper left corner.. quite often. I look forward to seeing if mosaics in EQ mode are more stable.
