Hi Gunther, thanks for the reply.
I am shooting with a Canon 5D mark 3, so I can go relatively high on the ISO, though last time I was shooting at 6400 and the footage was completely unusable - the noise was very noticeable. As a one shot it would have been fine but as a timelapse, since the noise is not uniform and changes from frame to frame, it became very apparent and made the footage useless for that purpose.
When ramping shutter and ISO, there's a catch. Set the shutter too slow and you will introduce motion blur on the stars, so I tend to stay away from shutter speeds longer than 20s for a wide lens. Set the ISO too high and the noise, even on the 5DIII, will be very hard to cope with, so I tend to keep it at about 1600, maybe 2000 if I'm hard pressed.
I didn't understand your comment on ramping aperture introducing flicker - I am shooting in manual mode so changing aperture should be similar to changing shutter in terms of flicker...
So the dilemma is, considering I don't want acceleration and set a constant interval, jittery clouds during the sunset or a not so visible and bright milky way at night. I imagine that if the clouds are moving slow the jitter will not be as apparent.
The thing is, that setting a 15s interval will probably cause jitter on the one hand, and won't be enough light for the milky way on the other hand, as it enforces a 13s shutter to allow for write and process time.
One alternative is to shoot the sunset time lapse at 15s interval and when the ramping gets to 13s as the stars come up, stop the intervals on the intervalometer and change the exposure manually to 20s and lock the shutter button while the camera is in burst mode. Since the exposure during the milky way shoot doesn't change there's no need to ramp it. It will effectively change the interval from 15s to 20s but I think it will not be very apparent as the stars move.
I think I'll try this next time.
If you have suggestions - I'd be happy to hear!
Thanks again,
Sefi