Let me explain why a proper resume within the S50 is needed. As many people have reported, mosaic mode will often fail before it has made even one full pass over the selected frame. In my experience it often fails as it's working on the upper left corner of the mosaic frame, and my hypothesis is that because it can take over an hour of real time to work from the middle outward, spiraling out as it goes, when it gets to the upper left corner the sky has rotated at least 15 degrees, and when it's working on the edges and corners it's often imaging only half inside and half outside the frame (the exposure is centered on the edge) so it's having real serious issues aligning the stars, especially because the corners of each exposure are often pretty noisy as well.
So what?
It does me no good to restart with the same framing only to wait another hour for it to fail again in the upper left corner and then try to stack my saved exposures in an external stacker because I'm still lacking actual exposures in that corner of the frame. What I need is to be able to continue (not restart) the mosaic the next night, starting near the same time, but most importantly continue stacking where the first one failed. Without that 15+ degrees of rotation there's a better chance of the star alignment working in the corner, and then it can complete that first pass to fill out the frame, and continue on with multiple passes. The few times I've gotten it to complete a full frame (which means everything you can see, plus filling all the edges/corners with the half on/half off exposures) then subsequent passes seem to go more smoothly (the existing image is less noisy so star alignment works better).
It already knows how to add a single exposure to a larger mosaic. It's doing that with every exposure.
There is no reason that it shouldn't be able to load up an existing mosaic from a FIT file and continue doing that. It needs only to save the existing framing and the last exposure position in the FIT metadata (or another file).
