Does Canon still impose a minimum time between exposures when running via a computer? Used to be 5 seconds - I suppose it avoided a filled image buffer messing up a download.

What was the 18s rationale? Longer and you risk internal damage to the camera? 

Hello Grant, The shooting rate on the 550D, under USB control and writing RAW files, seems in practice to be 0.5 frame/ s. That is with a 64 GB Class 10 SDXC card. The camera has a buffer for six RAW frames, so sets of up to six can be written at 1 frame/ s, particularly if the exposures are short. In testing scripts before going to Spain, requesting exposures at 1 frame/ s starts promisingly but later it does not change to the requested exposures and merely repeats the previous exposure settings for several exposures.

Regarding the 18 s, that came from the example script by Wolfgang Strickling, the author of EclipseDroid. Like him, I had the script display 'Filters off' 20 s before C2. One second later, I had the script turn on Live View mode, to lock up the mirror. Then the first exposure at 18s; although that has a little motion blur - presumably vibration from the mirror moving up. The same sequence was used in reverse for the final exposure, ending of Live View and announcing 'Filters on'.