40D or 450D or 400D and star trail photography

[cropfactorconfusion]

Hi there I own a canon 300D (I think you refer to it as Rebel?) I mainly take landscape pics although am now moving a bit more into portrait photography. I'd really like a 5D but I think it's beyond my budget. I've basically narrowed it down to either the 40D, the 450D, or the 400D.

I'm only interested in image quality and which one produces less noise and hot spots. I presume the answer would be the 450D with the better resolution, with the 40D and 400D being equal, but wasn't sure if it was as simple as that?

Also I'm really taken with the idea of star trail photography using bulb exposures as well. I have a 35mm SLR and read that I'd gain better results using an ISO 100 slide film than a DSLR?

Camera recommendation would be very helpful.
Thanks for your time.

[KeithB]

One nice thing about using a film camera for star trails is that the battery drain on some cameras is very low while the shutter is open so that you can effectively have an infinite exposure time.  While using a DSLR you probably want to turn off the feature that automatically takes a second "dark" portrait to subtract hot pixels, and handle it in photoshop.  (But you can make your own "dark Frame" for subsequent processing.)

For a digital camera, your exposure is limited by battery life.  Also, since the sensors are not cooled (like on the IR microscopes I use at work that have a liquid LN2 dewar to cool the sensor) noise will be an issue after some period of time.

Another factor on battery life is that it is worse in the cold - when you are usually making star trails.

Your best bet is to try some experiments at home to see how long an exposure you can get - simply put the lens cap on the camera and let it sit at the B setting and see how long it takes before you get a low battery warning.