Hi Wei-Hao and agavephoto for the last post of the night,
I tried bracketing + low burst speed with the mirror first engaged and then mirror locked up. All successful and fast without crashing SEM.
Here's the script, run with my camera aimed at a clock with millisecond display.
BKTDATA,9,0.3EV,u0o
EXPMODM,C2,-,00:32.0,D850, , , , , , , ,set camera to manual (M)
CHGCSSL,C2,-,00:31.0,D850, , ,3, , , , ,burst low speed 3 fps
LVPSTART,C2,-,00:30.0,D850, , , , , , , ,mirror up
SETBKT,C2,-,00:28.0,D850, , ,1, , , , ,activate bracketing for the burst
TAKEBST,C2,-,00:27.0,D850,1/100,8,1000,9,RAW,None,N,burst of 9 bracketed pictures
SETBKT,C2,-,00:05.0,D850, , ,0, , , , ,deactivate bracketing
LVPSTOP,C2,-,00:03.0,D850, , , , , , , ,mirror down
Both with mirror engaged (with no delay) and with mirror locked up, the images rolled off with an average interval of 0.33s, determined by the burst speed. This is at least as fast as Xavier's 'deluxe.txt' sample script with the small disadvantage of loss of flexibility in choosing exposures arbitrarily. I see that is a minor inconvenience.
Still to test is (1) the case when the exposure time is longer than the burst interval. Will the camera slow gracefully to accommodate the exposure or will something crash? And (2) how fast a burst speed with this method tolerate if the case of exposure > burst interval is tolerated? I already saw that 6 fps causes frame dropping for the D850 but what about 4 and 5 fps?
In any case, I think that this method is the key to getting HDR-ready images for the very dynamic phases of the eclipse, namely Baily's beads and the appearance of the chromosphere where a bit of HDR will be a big help but only if there is reasonably small change between exposures. Worst case -- HDR doesn't work and a single shot within a span of about 1 second is a good exposure.
So, with Wei-Hao's burst method during the coronal phase and mirror-up bracketing bursts during the dynamic phases and just TAKEPIC for the pre-C2 and post-C3 phases or some simple looping, SEM should provide really good results.
Once again, thank you Cloudy Nights for bringing making this thread possible and to all the contributors.