I have been attempting to use the '155 f/7 with the PL39K for three weeks
now. The faster focal ratio, f/7 versus the f/9 of the '180edt, means nearly
a 2x improvement in light gathering rate with a modest increase in image
scale: 1.15 versus 0.87 "/pixel. Finally the weather cooperated on Saturday
night.
I took 4.75 hours of exposure on Sat night using 15 minute subexposures.
the full 39 megapixel resolution on the linked page below is interesting:
there are a lot of stars in there!
http://www.narrowbandimaging.com/m8_...pl39K_page.htm