Thanks, Martin and Graham, for useful and interesting advice.
I have been trying to photograph the night sky (with film SLR and a homemade webcam setup) for several years, with varying success. I have already got a copy of “An Introduction to DSLR Photography”, which is where I got my original interest in the Canon EOS1000D: the example pictures looked good value for money. I was unaware that the internal IR filters were so strong at the Balmar Alpha (H alpha) line – surely it’s in the visible part of the spectrum. I will certainly get an appropriate T-adaptor.
I think I have a much better sky at this location than previously so it is worth my while setting up to expose for as long as practicable for diffuse objects, to reduce the effect of readout noise, but 30s to 2 minutes should be good enough to start with.
I do wish to attach the camera to my GEM mount either on its own or piggybacked on the telescope – the specialist Camera GEMs seem to be over the top in terms of expense. If I get a new mount for a new telescope, then the existing mount may become dedicated to the camera – if I can find a way of attaching it, that is. There doesn’t seem to be a suitable mounting ring with a camera screw for my tube.