A newbee with aRebel XT

[dpab]

Ive recently acquired this camera , takes great shots.
But what Im having a hard time with is ,And Im reading the manual.
I was wondering if anyone knew of a simple ,quick n easy way of setting the exposure time to say 2:30 minutes or 3 minutes or 5 minutes, if you can.
Im really new at this, and have got the adaptor ring to attach the camera to my telescope .
I need the longer exposures for the nebula and galaxies and star clusters.
Any info would be great .
I must be missin somethin here really basic , can get into and do other features no problem