shooting technique - How do I set the focus in long exposure night sky shots?

I want to catch a good long exposure (5 - 15 seconds) shot of the International Space Station in the night sky as it passes over my city.

Setting the ISO higher (say, to 100) and the exposure time is straight-forward enough. I'm using an aperture of f/5 but what should the focal length be?

It's difficult to gauge using the naked eye and viewfinder. I'm using a Canon 400D.

Answer

The last time I took picture of the night sky, the following was what I learnt:

• Set to manual focus and used live view to focus until the stars are in focus.


• From the place I live (Kuala Lumpur, Malaysia), shooting at 5secs@85mm will make stars become star trails pills¹. You might want to consider using a shorter shutter speed to ensure a sharp shot (I'm assuming the space station will "move" faster than the star)


• Take a few shots before hand to gauge the exposure.


• You might end up needing to bump up the ISO. Fast lens can help.

edit: ¹ - The pin pricks of stars becomes slightly elongated that they look more like star pills than star trails or dots of stars