The others may be correct but given that your scope is listed in your sig is a dob, it could also be camera movement. For short exposures (less than 30 seconds; any longer and you will need tracking) you would be better to put the camera on a very sturdy tripod, check the focus on a distant object during daylight and note whether the infinity focus setting marker on the lens is correct. Then it's simply a matter of setting the focus point to the same place when it's dark, without having to rely on star images. It may also be worth getting the mirror lock hack(Firmware) as the mirror jumping up and down will also create enough movement to be a problem.
