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I use a similar method depending on cloud cover to the South; as follows
10 sec exposure - star drift?
20 secs - star drift ?
40 secs - star drift ?
60 secs - star drift?
If stars drift? correct in azimuth/altitude and take next exposure, 20, 40, 60 as before and so on as required. Generally, round stars at 60 secs means round at 180 secs, or so as a rule with this method.
It's also easier to interpret drift if the camera square; that is the base of the camera is level.
I guess you could start at any exposure time, but the method above very quickly identifies gross error. Not much point starting at 30 seconds only to find trails 10mm long. These would have been evident at 10 seconds.
Tedious to say the least.
Cheers
Rowland
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