What is your camera? A popular camera with amateur astrophotographers is the Pentax 6x7, and you're pretty much doomed without lockup. That big mirror hits hard. A lot also depends on the actual lens and tripod mounting system you are using and whether or not harmonic vibrations are set
up synchronously with the shutter itself. It's a bit involved if you're get serious about image quality.
The most knowledgable guy I know on this subject is Francis Sakamoto, a former Pentax and Celestron dealer, but still an enthusiastic telephoto photographer himself. Check his website.