Camera lenses I am thinking of buying a digital SLR camera. I have a Sirius camera lens for an old Praktica TL 1000. will it fit a new DSLR camera?

If your new DSLR has the ability to be used in some mode with accessories like microscopes, telescopes, and the like (perhaps via T-adapters), you should be able to mount the lens via a screw-mount adapter for your brand. Unless the lens has a switch to make the aperture setting manual, though, or your adapter depresses the aperture pin on the lens, you will only be able to use it wide open. Generally, you should only expect to be able to use a manual exposure mode or perhaps an aperture priority mode.

Further, on some brands of camera (like Nikon), you will not be able to focus out to infinity with the lens with a simple adapter. While there are adapters that include an optical element to allow infinity focus, you should do some research to get one with good optical quality. The one made by Bower appears to do a decent job:

http://www.flickr.com/photos/mureena/2853461109/

Other camera brands may be able to attain infinity focus with a simple adapter.

In general, adapting an older screw-mount lens to a newer body is going to require some constraints that are likely only worth the bother if the lens provides some capability you otherwise do not have in your current selection of lenses or is unique in some fashion. It would be, for example, a good way to obtain the use of a lens like the Sirius 500mm f/8.8 mirror lens, which uses the fixed f/8.8 aperture. It would be doubtful that adapting the Sirius 80-200mm f/3.9 zoom would make you happy, though, when a lens offered within your DSLR brand of similar speed would not cost much and could be used in all modes offered by the camera, and have autofocus as well.