I always assumed it was something to do with the heat and high humidity, and air turbulence caused as the wind blows inland up the range to the tablelands above Cairns. It's certainly much, much better inland where it's dryer and cooler.
Looking at diffraction rings, it almost alway looks more like #1-3 on from the animations on
http://uk.geocities.com/dpeach_78/pickering.htm, but they usually look rather more spiky around the outside edge. Could this be something other than seeing?
The scopes an 8" newt btw.
Those Burgess ep's do look nice. hmmm...
