Since writing about my first attempt at creating a star trails image I have carried out a few further experiments, and thought about what went wrong and what I would do differently next time.
Firstly, I mistakenly believed that my interval timer (the Canon TC-80N3; good grief, what a name…) could only be set to take no more than 99 consecutive photos. This is not the case; it can be set to take an unlimited number of images, which in practice means until your camera battery runs out.
You can do this by setting the Interval Timer to, say, five seconds (using my original photo as an example), the Long Exposure setting to 40 seconds, and leaving the Exposure Count set to zero.
You can estimate the maximum exposure for a given focal length before stars begin to leave visible trails in the image by using the following calculation: 600 / focal length = expsoure in seconds.
E.g. 600 / 17 (the shortest focal length of my 17-40mm zoom lens) = 35 seconds.
My latest experience of shooting star trails showed that I was getting about 2hrs 55mins of use from the two freshly-charged batteries in the 5D battery grip. This gave me 478 exposures of 20 seconds each.
That’s the beauty of stacking multiple images to create star trails – even when my batteries died in the middle of the 479th image, I still had 478 images safely stored on the card.
Memory card speed and capacity
My 5D produces raw files of roughly 10-12 MB in size. The transfer speed of my 4Gb SanDisk Extreme III card (according to the card database on robgalbraith.com) is about 7MB/sec, so I would therefore need an interval of two seconds to allow for the image data to be written to the card before the next image is captured.
I would prefer to have just a one second gap between exposures when shooting star trails, so I therefore needed a faster CF card. Luckily, the most recent SanDisk Extreme III cards have transfer speeds of up to 30MB/sec, and they are labelled with this speed so that you can differentiate them from the older, slower Extreme III cards.
Perhaps most importantly, I think that it was also a mistake to set the aperture on my 17mmm f4 lens to 5.6. The most important thing to do is to collect as much light as possible within the exposure period, so for my next photo I will be using a 24mm f2.8 lens, and shooting wide open (i.e. at f2.8). That should allow for much more starlight to be captured, while sacrificing just a small amount of the field of view.
I will publish another (better, I hope) star trails photo as soon as I have the chance to take one. Meanwhile, if you have any technical suggestions or thoughts, please let me know via the Contact page.