Sandy,
That's a beautiful camera and lens combination you've picked up there.
You'll find that it will do well for astrophotography. Although, bear in mind, that it is of the older generation and that the noise characteristics will pale in comparison to even the cheaper entry-level Canon's available today.
However, use it for terrestrial use and it will shine.
As far as my understanding goes, I don't think your claim of it being able to take dark frames whilst capturing lights is true... the sensor would be exposed to photons. Dark frames work by exposing with the shutter closed. I'm no expert on that camera, though, and could be wrong.
H
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