
long exposure like 5min is not practical over here! light pollution coming earlier than that! plus with that resolution, it won't beat the qualtity of Afocal with digital camera! but if it can do 1min exposure, then sure it will be a good guider!
May be there are some ccd camera capable of doing that, but because most object is quite dim, so it is quite difficult to find a suitable guide star in the same field of view as the object.correct me if i'm wrong, some high end ccd cameras are capable of doing imaging & guiding at the same time. i don't remember the exact details....
The F-number (f-ratio) is the ratio between focal length and aperture (focal length / aperture). A scope with a smaller f-ratio will require less exposure time than a scope with larger f-ratio (both scope with same aperture), this is because a longer focal length will generate a higher magnification, so the image is dimer than a shorter focal length scope (smaller magnification).the apature value is depend on the scope you used which is the F number. sometime bigger scope enable to get more signal than small scope! so it mean more faster than small scope! so if same amount of exposure time on different setup will produce different result.