I did a silly thing the other day - I started thinking about flats. In doing so I completely confused myself. Every night i take flats (Thank-you, ACP Scheduler!) A set at dusk and another at dawn. Now my C14 sits on a German Equatorial mount so when it passes from East to West or West to East it does the wonderful equatorial mount dance. So does that mean the flats taken at dawn are "upside" down compared to the dusk flats and hence the dawns and dusks can't be combined into masters? For the life of me I cant visualize the orientation of the camera after the flip! So, do folks keep two sets of masters - one for East images and one for West - as i've been doing? Or is that not necessary ?