I am trying to take 180 x 25 second exposures (changed down from 30 second ones) with say a 2 second interval, either with the internal Interval Timer or an external one. How would the menu read? Thanks to the previous reply to my first question regarding the same problem. On the 30 second exposure, the camera fired a shot and the green light flashed every 30 seconds for one hour or 100 shots.
Select shutter speed required in manual or check approximate auto shutter speed. In the menu select interval timing (camera date & time must be set) - go for "now" option it's easier. Select the interval period - this MUST - be longer than the shutter time - set number of intervals required and choose 1 shot per interval (experiment later!) that should work
First set your shutter speed to 25 seconds. Then go to Menu > Shooting Menu > Interval Timer Shooting to set it at 180 for the number for intervals you want. And you are done.
Turn Long Exposure NR off. The camera spends too much time processing the shots between intervals, it screws up the sequence.
Also, what user20657 and coneslayer said apply. Interval should be 2 seconds longer than shutter speed.
I give you a simple instruction, set 2sec for the interval time and then set the other parameters as 9999*9 no need for further calculation