Comet Holmes 2007
Shot back in 2007 using an EOS 40D and a Canon EF 500/4.5L. Comet Holmes atained naked eye visibility in October of 2007 and reached a maximum apparent size of about 13 arc minutes, almost half the angular size of the moon. This was shot just using a normal tripod, not a tracking mount. This meant that the maximum allowable exposure was around 1.5 seconds. Longer exposures resulted in stars trailing due to the earth's rotation.
Canon EOS 40D